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Hi All, I'm in the process of designing an MCU board using a 9S12DP256. I want to use a crystal and generate the clock internally. I have used to recommended values (from Motorola) to set up the PLL filter on the XFC pin, but I currently have an interesting problem: As you all know, the HCS12 can user either a crystal and internal generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). I have the board assembled and it seems to work just fine using a function generator for an external clock. When I change the !XCLKS voltage to use the crystal, the board no longer works. The quirk is that when !XCLKS is set up for internal, applying the external clock will still work, and the board will fully be functional. I have tested this on an Axiom Manufacturing board with the same chip, and when the !XCLKS line is VCC (for internal) the HCS12 will ignore the external clock from the generators. Any help greatly appreciated, Ryan Kruse |
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Trouble With Crystal Clock Generation on 9S12DP256
Started by ●May 1, 2003
Reply by ●May 2, 20032003-05-02
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When you refer to "crystal", do you mean a quartz crystal, or an
external oscillator? 607-656-2597 -----Original Message----- From: rmk1113 [mailto:] Sent: Thursday, May 01, 2003 5:28 PM To: Subject: [68HC12] Trouble With Crystal Clock Generation on 9S12DP256 Hi All, I'm in the process of designing an MCU board using a 9S12DP256. I want to use a crystal and generate the clock internally. I have used to recommended values (from Motorola) to set up the PLL filter on the XFC pin, but I currently have an interesting problem: As you all know, the HCS12 can user either a crystal and internal generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). I have the board assembled and it seems to work just fine using a function generator for an external clock. When I change the !XCLKS voltage to use the crystal, the board no longer works. The quirk is that when !XCLKS is set up for internal, applying the external clock will still work, and the board will fully be functional. I have tested this on an Axiom Manufacturing board with the same chip, and when the !XCLKS line is VCC (for internal) the HCS12 will ignore the external clock from the generators. Any help greatly appreciated, Ryan Kruse -------------------- ">http://docs.yahoo.com/info/terms/ |
Reply by ●May 2, 20032003-05-02
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Is the crystal frequency upper limit for the DP256 different than that of the DG128A? On March 18, 2002 Motorola announced in this forum that there was an erratum in the data sheet for the DG128A that implied that either a 16Mhz crystal or package oscillator could be used. It turns out that for reliable operations, a crystal can only be used for frequencies up to 8Mhz! -----Original Message----- From: Doron Fael [mailto:] Sent: Friday, May 02, 2003 11:11 AM To: Subject: Re: [68HC12] Trouble With Crystal Clock Generation on 9S12DP256 Ryan, There are a few things to notice when designing with a crystal for the 9S12DP256: When PE7/XCLKS is pulled high during Reset, the DP256 expects an external Colpitts crystal configuration to generate its clock. The Colpitts crystal configuration requires different external crystal connections as compared with the more common Pierce crystal configuration used by most micro-processor systems. In the Colpitts configuration, the crystal in mounted between EXTAL and VSSPLL, a capacitor is mounted between EXTAL and XTAL, and another capacitor is mounted between XTAL and VSSPLL. The Colpitts configuration is used to preserve power, as the generated voltage swing with it is only a few hundred mili-volts, around a positive DC voltage level. The down-side of this quality is that it makes the Colpitts oscillator also more susceptible to noise, than the Pierce configuration, and so careful layout of the crystal circuit next to the DP256 is needed, to reduce possible influence of noise and leakage current (Motorola include recommended crystal layout in their data sheets). The Colpits configuration is also limited in the case of the DP256 to use crystals in the range of 0.5MHz to 16MHz. Attempting to use a higher frequency crystal, above 16MHz, will likely lead to a clock failure. An additional complication of the Colpitts configuration is that a DC voltage is applied across the crystal, which may shorten the life expectancy of many crystals, and may also prevent some crystals from starting to oscillate. The way to overcome these problems is by installing a 3rd capacitor in the crystal circuit between the EXTAL pin and the crystal, to block the DC level from reaching the crystal. Some users choose not to use the Colpits crystal configuration due to these complications mentioned above. If you also choose not to use the Colpits configuration you have the following two alternatives: 1. Move to another HCS12 part, that can also work with the Piece crystal configuration and has much less complications. 2. Use an external clock generator. The external clock generator also has several possible complications that need to be taken care of: It requires a square wave voltage levels of 0V and 2.5V which requires also a 2.5V external voltage regulator on your board. If going into power-down modes and especially STOP power-down, the internal generate VDD and VDDPLL in the DP256 drop significantly bellow 2.5V (to preserve power). If your clock generator continues to supply 2.5V square wave during this period damage may occur to the DP256 internal clock circuit. The two possible ways to overcome this problem, are: A) supply VDD and VDDPLL from an external 2.5V supply that does not drop its voltage when going into the STOP power down mode (and connect VREGEN to GND). B) Connect two Schottky diodes from your external 2.5V regulator to VDD and VDDPLL, to prevent these voltages to drop more than 0.3V bellow the external clock generator square wave. Another thing to note when using an external clock generator is to not generate a 50MHz external clock using it, as its specified rise time and fall time need to be less than 1nSEC, and the duty cycle needs to be 47.5% - 52.5% at this maximum frequency. If a lower frequency is used instead (40MHz or bellow), the rise time, fall time and duty cycle can be much more relaxed and easier to generate at 0V - 2.5V voltage levels. You can then still use the internal PLL to up the clock to the maximum allowed 25MHz bus speed. I hope you find this explanation useful, and am sorry it is not any less complicated. Doron Nohau Corporation HC12 In-Circuit Emulators www.nohau.com/emul12pc.html At 09:28 PM 5/1/2003 +0000, you wrote: >Hi All, > >I'm in the process of designing an MCU board using a 9S12DP256. I >want to use a crystal and generate the clock internally. I have used >to recommended values (from Motorola) to set up the PLL filter on the >XFC pin, but I currently have an interesting problem: > >As you all know, the HCS12 can user either a crystal and internal >generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). > >I have the board assembled and it seems to work just fine using a >function generator for an external clock. When I change the !XCLKS >voltage to use the crystal, the board no longer works. The quirk is >that when !XCLKS is set up for internal, applying the external clock >will still work, and the board will fully be functional. > >I have tested this on an Axiom Manufacturing board with the same >chip, and when the !XCLKS line is VCC (for internal) the HCS12 will >ignore the external clock from the generators. > >Any help greatly appreciated, > >Ryan Kruse -------------------- ">http://docs.yahoo.com/info/terms/ This e-mail and any files transmitted with it ( Message ) are confidential and may contain privileged information. This Message is intended solely for the addressee(s). If you have received this Message in error, please inform us promptly by reply e-mail then delete the Message and destroy any printed copy of it. Any unauthorized use, review, retransmission, dissemination, distribution, printing or copying of this Message or any part thereof is strictly prohibited. E-mails are susceptible to alteration. Neither Technip-Coflexip nor any of its subsidiaries and affiliates shall be liable for the Message if altered, changed or falsified |
Reply by ●May 2, 20032003-05-02
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Ryan, There are a few things to notice when designing with a crystal for the 9S12DP256: When PE7/XCLKS is pulled high during Reset, the DP256 expects an external Colpitts crystal configuration to generate its clock. The Colpitts crystal configuration requires different external crystal connections as compared with the more common Pierce crystal configuration used by most micro-processor systems. In the Colpitts configuration, the crystal in mounted between EXTAL and VSSPLL, a capacitor is mounted between EXTAL and XTAL, and another capacitor is mounted between XTAL and VSSPLL. The Colpitts configuration is used to preserve power, as the generated voltage swing with it is only a few hundred mili-volts, around a positive DC voltage level. The down-side of this quality is that it makes the Colpitts oscillator also more susceptible to noise, than the Pierce configuration, and so careful layout of the crystal circuit next to the DP256 is needed, to reduce possible influence of noise and leakage current (Motorola include recommended crystal layout in their data sheets). The Colpits configuration is also limited in the case of the DP256 to use crystals in the range of 0.5MHz to 16MHz. Attempting to use a higher frequency crystal, above 16MHz, will likely lead to a clock failure. An additional complication of the Colpitts configuration is that a DC voltage is applied across the crystal, which may shorten the life expectancy of many crystals, and may also prevent some crystals from starting to oscillate. The way to overcome these problems is by installing a 3rd capacitor in the crystal circuit between the EXTAL pin and the crystal, to block the DC level from reaching the crystal. Some users choose not to use the Colpits crystal configuration due to these complications mentioned above. If you also choose not to use the Colpits configuration you have the following two alternatives: 1. Move to another HCS12 part, that can also work with the Piece crystal configuration and has much less complications. 2. Use an external clock generator. The external clock generator also has several possible complications that need to be taken care of: It requires a square wave voltage levels of 0V and 2.5V which requires also a 2.5V external voltage regulator on your board. If going into power-down modes and especially STOP power-down, the internal generate VDD and VDDPLL in the DP256 drop significantly bellow 2.5V (to preserve power). If your clock generator continues to supply 2.5V square wave during this period damage may occur to the DP256 internal clock circuit. The two possible ways to overcome this problem, are: A) supply VDD and VDDPLL from an external 2.5V supply that does not drop its voltage when going into the STOP power down mode (and connect VREGEN to GND). B) Connect two Schottky diodes from your external 2.5V regulator to VDD and VDDPLL, to prevent these voltages to drop more than 0.3V bellow the external clock generator square wave. Another thing to note when using an external clock generator is to not generate a 50MHz external clock using it, as its specified rise time and fall time need to be less than 1nSEC, and the duty cycle needs to be 47.5% - 52.5% at this maximum frequency. If a lower frequency is used instead (40MHz or bellow), the rise time, fall time and duty cycle can be much more relaxed and easier to generate at 0V - 2.5V voltage levels. You can then still use the internal PLL to up the clock to the maximum allowed 25MHz bus speed. I hope you find this explanation useful, and am sorry it is not any less complicated. Doron Nohau Corporation HC12 In-Circuit Emulators www.nohau.com/emul12pc.html At 09:28 PM 5/1/2003 +0000, you wrote: >Hi All, > >I'm in the process of designing an MCU board using a 9S12DP256. I >want to use a crystal and generate the clock internally. I have used >to recommended values (from Motorola) to set up the PLL filter on the >XFC pin, but I currently have an interesting problem: > >As you all know, the HCS12 can user either a crystal and internal >generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). > >I have the board assembled and it seems to work just fine using a >function generator for an external clock. When I change the !XCLKS >voltage to use the crystal, the board no longer works. The quirk is >that when !XCLKS is set up for internal, applying the external clock >will still work, and the board will fully be functional. > >I have tested this on an Axiom Manufacturing board with the same >chip, and when the !XCLKS line is VCC (for internal) the HCS12 will >ignore the external clock from the generators. > >Any help greatly appreciated, > >Ryan Kruse |
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Reply by ●May 2, 20032003-05-02
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The 9S12DP256 Colpitts crystal circuit is more stable than on the 68HC912D parts. As far as I know, on the 9S12DP256 the Colpitts crystal can really work to 16MHz. With newer HCS12 parts, also the Pierce crystal configuration is possible, which can work to 40MHz. Doron Nohau Corporation HC12 In-Circuit Emulators www.nohau.com/emul12pc.html At 04:31 PM 5/2/2003 +0200, you wrote: >Is the crystal frequency upper limit for the DP256 different than that of >the DG128A? > >On March 18, 2002 Motorola announced in this forum that there was an erratum >in the data sheet for the DG128A that implied that either a 16Mhz crystal or >package oscillator could be used. It turns out that for reliable >operations, a crystal can only be used for frequencies up to 8Mhz! > >-----Original Message----- >From: Doron Fael [mailto:] >Sent: Friday, May 02, 2003 11:11 AM >To: >Subject: Re: [68HC12] Trouble With Crystal Clock Generation on 9S12DP256 >Ryan, > >There are a few things to notice when designing with a crystal for the >9S12DP256: > >When PE7/XCLKS is pulled high during Reset, the DP256 expects an external >Colpitts crystal configuration to generate its clock. The Colpitts crystal >configuration requires different external crystal connections as compared >with the more common Pierce crystal configuration used by most >micro-processor systems. In the Colpitts configuration, the crystal in >mounted between EXTAL and VSSPLL, a capacitor is mounted between EXTAL and >XTAL, and another capacitor is mounted between XTAL and VSSPLL. The >Colpitts configuration is used to preserve power, as the generated voltage >swing with it is only a few hundred mili-volts, around a positive DC >voltage level. The down-side of this quality is that it makes the Colpitts >oscillator also more susceptible to noise, than the Pierce configuration, >and so careful layout of the crystal circuit next to the DP256 is needed, >to reduce possible influence of noise and leakage current (Motorola include >recommended crystal layout in their data sheets). The Colpits configuration >is also limited in the case of the DP256 to use crystals in the range of >0.5MHz to 16MHz. Attempting to use a higher frequency crystal, above 16MHz, >will likely lead to a clock failure. An additional complication of the >Colpitts configuration is that a DC voltage is applied across the crystal, >which may shorten the life expectancy of many crystals, and may also >prevent some crystals from starting to oscillate. The way to overcome these >problems is by installing a 3rd capacitor in the crystal circuit between >the EXTAL pin and the crystal, to block the DC level from reaching the >crystal. > >Some users choose not to use the Colpits crystal configuration due to these >complications mentioned above. If you also choose not to use the Colpits >configuration you have the following two alternatives: > >1. Move to another HCS12 part, that can also work with the Piece crystal >configuration and has much less complications. > >2. Use an external clock generator. >The external clock generator also has several possible complications that >need to be taken care of: >It requires a square wave voltage levels of 0V and 2.5V which requires also >a 2.5V external voltage regulator on your board. If going into power-down >modes and especially STOP power-down, the internal generate VDD and VDDPLL >in the DP256 drop significantly bellow 2.5V (to preserve power). If your >clock generator continues to supply 2.5V square wave during this period >damage may occur to the DP256 internal clock circuit. The two possible ways >to overcome this problem, are: A) supply VDD and VDDPLL from an external >2.5V supply that does not drop its voltage when going into the STOP power >down mode (and connect VREGEN to GND). B) Connect two Schottky diodes from >your external 2.5V regulator to VDD and VDDPLL, to prevent these voltages >to drop more than 0.3V bellow the external clock generator square wave. >Another thing to note when using an external clock generator is to not >generate a 50MHz external clock using it, as its specified rise time and >fall time need to be less than 1nSEC, and the duty cycle needs to be 47.5% >- 52.5% at this maximum frequency. If a lower frequency is used instead >(40MHz or bellow), the rise time, fall time and duty cycle can be much more >relaxed and easier to generate at 0V - 2.5V voltage levels. You can then >still use the internal PLL to up the clock to the maximum allowed 25MHz bus >speed. > >I hope you find this explanation useful, and am sorry it is not any less >complicated. > >Doron >Nohau Corporation >HC12 In-Circuit Emulators >www.nohau.com/emul12pc.html > >At 09:28 PM 5/1/2003 +0000, you wrote: > >Hi All, > > > >I'm in the process of designing an MCU board using a 9S12DP256. I > >want to use a crystal and generate the clock internally. I have used > >to recommended values (from Motorola) to set up the PLL filter on the > >XFC pin, but I currently have an interesting problem: > > > >As you all know, the HCS12 can user either a crystal and internal > >generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). > > > >I have the board assembled and it seems to work just fine using a > >function generator for an external clock. When I change the !XCLKS > >voltage to use the crystal, the board no longer works. The quirk is > >that when !XCLKS is set up for internal, applying the external clock > >will still work, and the board will fully be functional. > > > >I have tested this on an Axiom Manufacturing board with the same > >chip, and when the !XCLKS line is VCC (for internal) the HCS12 will > >ignore the external clock from the generators. > > > >Any help greatly appreciated, > > > >Ryan Kruse |
Reply by ●May 4, 20032003-05-04
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Doron, Thanks for the information. I am using a 4Mhz citizen crystal on the board, and the Colpitts design from the motorola data books. I think that the crystal circuit is OK, it is just this weird problem with the HCS12 not seeming to "acknowledge the voltage" on XCLKS, and only use the external. Any idea why this might be? Ryan --- In , Doron Fael <doronf@n...> wrote: > Ryan, > > There are a few things to notice when designing with a crystal for the > 9S12DP256: > > When PE7/XCLKS is pulled high during Reset, the DP256 expects an external > Colpitts crystal configuration to generate its clock. The Colpitts crystal > configuration requires different external crystal connections as compared > with the more common Pierce crystal configuration used by most > micro-processor systems. In the Colpitts configuration, the crystal in > mounted between EXTAL and VSSPLL, a capacitor is mounted between EXTAL and > XTAL, and another capacitor is mounted between XTAL and VSSPLL. The > Colpitts configuration is used to preserve power, as the generated voltage > swing with it is only a few hundred mili-volts, around a positive DC > voltage level. The down-side of this quality is that it makes the Colpitts > oscillator also more susceptible to noise, than the Pierce configuration, > and so careful layout of the crystal circuit next to the DP256 is needed, > to reduce possible influence of noise and leakage current (Motorola include > recommended crystal layout in their data sheets). The Colpits configuration > is also limited in the case of the DP256 to use crystals in the range of > 0.5MHz to 16MHz. Attempting to use a higher frequency crystal, above 16MHz, > will likely lead to a clock failure. An additional complication of the > Colpitts configuration is that a DC voltage is applied across the crystal, > which may shorten the life expectancy of many crystals, and may also > prevent some crystals from starting to oscillate. The way to overcome these > problems is by installing a 3rd capacitor in the crystal circuit between > the EXTAL pin and the crystal, to block the DC level from reaching the crystal. > > Some users choose not to use the Colpits crystal configuration due to these > complications mentioned above. If you also choose not to use the Colpits > configuration you have the following two alternatives: > > 1. Move to another HCS12 part, that can also work with the Piece crystal > configuration and has much less complications. > > 2. Use an external clock generator. > The external clock generator also has several possible complications that > need to be taken care of: > It requires a square wave voltage levels of 0V and 2.5V which requires also > a 2.5V external voltage regulator on your board. If going into power-down > modes and especially STOP power-down, the internal generate VDD and VDDPLL > in the DP256 drop significantly bellow 2.5V (to preserve power). If your > clock generator continues to supply 2.5V square wave during this period > damage may occur to the DP256 internal clock circuit. The two possible ways > to overcome this problem, are: A) supply VDD and VDDPLL from an external > 2.5V supply that does not drop its voltage when going into the STOP power > down mode (and connect VREGEN to GND). B) Connect two Schottky diodes from > your external 2.5V regulator to VDD and VDDPLL, to prevent these voltages > to drop more than 0.3V bellow the external clock generator square wave. > Another thing to note when using an external clock generator is to not > generate a 50MHz external clock using it, as its specified rise time and > fall time need to be less than 1nSEC, and the duty cycle needs to be 47.5% > - 52.5% at this maximum frequency. If a lower frequency is used instead > (40MHz or bellow), the rise time, fall time and duty cycle can be much more > relaxed and easier to generate at 0V - 2.5V voltage levels. You can then > still use the internal PLL to up the clock to the maximum allowed 25MHz bus > speed. > > I hope you find this explanation useful, and am sorry it is not any less > complicated. > > Doron > Nohau Corporation > HC12 In-Circuit Emulators > www.nohau.com/emul12pc.html > > At 09:28 PM 5/1/2003 +0000, you wrote: > >Hi All, > > > >I'm in the process of designing an MCU board using a 9S12DP256. I > >want to use a crystal and generate the clock internally. I have used > >to recommended values (from Motorola) to set up the PLL filter on the > >XFC pin, but I currently have an interesting problem: > > > >As you all know, the HCS12 can user either a crystal and internal > >generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). > > > >I have the board assembled and it seems to work just fine using a > >function generator for an external clock. When I change the !XCLKS > >voltage to use the crystal, the board no longer works. The quirk is > >that when !XCLKS is set up for internal, applying the external clock > >will still work, and the board will fully be functional. > > > >I have tested this on an Axiom Manufacturing board with the same > >chip, and when the !XCLKS line is VCC (for internal) the HCS12 will > >ignore the external clock from the generators. > > > >Any help greatly appreciated, > > > >Ryan Kruse > > |
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Reply by ●May 5, 20032003-05-05
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Ryan, In my experience, the HCS12 will work fine if you give it an external clock generator square wave in the range of 0.5MHz - 16MHz, and wrongly select the Colpitts configuration by puling XCLKS high during reset. This is a normal behavior in my experience, and doesn't mean the HCS12 doesn't expects a Colpitts crystal rather than an external clock generator. It could help if a status bit would exist in an internal HCS12 register and would tell what was the sampled state of XCLKS during Reset. I looked in the manual for such a bit but couldn't find one. I guess after verifying you indeed pull PE7/XCLKS high during Reset (note that high needs to be higher than 0.65 * VDDX = 3.25V typically in the case of the HCS12), you need to believe the Colpitts crystal configuration is selected. In your case, I would suspect especially the DC blocking capacitor. What value are you using? Try changing its value and see if it makes a difference. If this doesn't help try using a different crystal from another manufacturer. We have had many customers that had problems with getting the Colpitts crystal to work. Some replaced crystals and capacitors and got it working eventually, and some gave up and switched to a Pierce crystal configuration with one of the newer HCS12 devices. Hope this helps, Doron Nohau Corporation HC12 In-Circuit Emulators www.nohau.com/emul12pc.html At 01:18 AM 5/5/2003 +0000, you wrote: >Doron, > >Thanks for the information. > >I am using a 4Mhz citizen crystal on the board, and the Colpitts >design from the motorola data books. I think that the crystal >circuit is OK, it is just this weird problem with the HCS12 not >seeming to "acknowledge the voltage" on XCLKS, and only use the >external. Any idea why this might be? > >Ryan >--- In , Doron Fael <doronf@n...> wrote: > > Ryan, > > > > There are a few things to notice when designing with a crystal for >the > > 9S12DP256: > > > > When PE7/XCLKS is pulled high during Reset, the DP256 expects an >external > > Colpitts crystal configuration to generate its clock. The Colpitts >crystal > > configuration requires different external crystal connections as >compared > > with the more common Pierce crystal configuration used by most > > micro-processor systems. In the Colpitts configuration, the crystal >in > > mounted between EXTAL and VSSPLL, a capacitor is mounted between >EXTAL and > > XTAL, and another capacitor is mounted between XTAL and VSSPLL. The > > Colpitts configuration is used to preserve power, as the generated >voltage > > swing with it is only a few hundred mili-volts, around a positive >DC > > voltage level. The down-side of this quality is that it makes the >Colpitts > > oscillator also more susceptible to noise, than the Pierce >configuration, > > and so careful layout of the crystal circuit next to the DP256 is >needed, > > to reduce possible influence of noise and leakage current (Motorola >include > > recommended crystal layout in their data sheets). The Colpits >configuration > > is also limited in the case of the DP256 to use crystals in the >range of > > 0.5MHz to 16MHz. Attempting to use a higher frequency crystal, >above 16MHz, > > will likely lead to a clock failure. An additional complication of >the > > Colpitts configuration is that a DC voltage is applied across the >crystal, > > which may shorten the life expectancy of many crystals, and may >also > > prevent some crystals from starting to oscillate. The way to >overcome these > > problems is by installing a 3rd capacitor in the crystal circuit >between > > the EXTAL pin and the crystal, to block the DC level from reaching >the crystal. > > > > Some users choose not to use the Colpits crystal configuration due >to these > > complications mentioned above. If you also choose not to use the >Colpits > > configuration you have the following two alternatives: > > > > 1. Move to another HCS12 part, that can also work with the Piece >crystal > > configuration and has much less complications. > > > > 2. Use an external clock generator. > > The external clock generator also has several possible >complications that > > need to be taken care of: > > It requires a square wave voltage levels of 0V and 2.5V which >requires also > > a 2.5V external voltage regulator on your board. If going into >power-down > > modes and especially STOP power-down, the internal generate VDD and >VDDPLL > > in the DP256 drop significantly bellow 2.5V (to preserve power). If >your > > clock generator continues to supply 2.5V square wave during this >period > > damage may occur to the DP256 internal clock circuit. The two >possible ways > > to overcome this problem, are: A) supply VDD and VDDPLL from an >external > > 2.5V supply that does not drop its voltage when going into the STOP >power > > down mode (and connect VREGEN to GND). B) Connect two Schottky >diodes from > > your external 2.5V regulator to VDD and VDDPLL, to prevent these >voltages > > to drop more than 0.3V bellow the external clock generator square >wave. > > Another thing to note when using an external clock generator is to >not > > generate a 50MHz external clock using it, as its specified rise >time and > > fall time need to be less than 1nSEC, and the duty cycle needs to >be 47.5% > > - 52.5% at this maximum frequency. If a lower frequency is used >instead > > (40MHz or bellow), the rise time, fall time and duty cycle can be >much more > > relaxed and easier to generate at 0V - 2.5V voltage levels. You can >then > > still use the internal PLL to up the clock to the maximum allowed >25MHz bus > > speed. > > > > I hope you find this explanation useful, and am sorry it is not any >less > > complicated. > > > > Doron > > Nohau Corporation > > HC12 In-Circuit Emulators > > www.nohau.com/emul12pc.html > > > > At 09:28 PM 5/1/2003 +0000, you wrote: > > >Hi All, > > > > > >I'm in the process of designing an MCU board using a 9S12DP256. I > > >want to use a crystal and generate the clock internally. I have >used > > >to recommended values (from Motorola) to set up the PLL filter on >the > > >XFC pin, but I currently have an interesting problem: > > > > > >As you all know, the HCS12 can user either a crystal and internal > > >generator (!XCLKS line = VCC) or an external clock (!XLCKS = GND). > > > > > >I have the board assembled and it seems to work just fine using a > > >function generator for an external clock. When I change the !XCLKS > > >voltage to use the crystal, the board no longer works. The quirk >is > > >that when !XCLKS is set up for internal, applying the external >clock > > >will still work, and the board will fully be functional. > > > > > >I have tested this on an Axiom Manufacturing board with the same > > >chip, and when the !XCLKS line is VCC (for internal) the HCS12 will > > >ignore the external clock from the generators. > > > > > >Any help greatly appreciated, > > > > > >Ryan Kruse |
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Reply by ●May 5, 20032003-05-05
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At 08:49 AM 5/5/2003 +0200, you wrote: >We have had many customers that had problems with getting the Colpitts >crystal to work. Some replaced crystals and capacitors and got it working >eventually, and some gave up and switched to a Pierce crystal configuration >with one of the newer HCS12 devices. We finally gave up on the Colpitts with the D60A. Just too unreliable on startup over temperature. Finally added an external osc. - even though it cost us more money and power. IMHO, the Colpitts was not a good design change. The D60C is supposed to have a better crystal driver design, haven't tested it yet. And the new D60P (with the Pierce driver) doesn't seem to be readily available. Like you said, we decided to switch rather than fight. jmk ----------- James M. Knox TriSoft ph 512-385-0316 1109-A Shady Lane fax 512-366-4331 Austin, Tx 78721 ----------- |
Reply by ●May 6, 20032003-05-06
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At 05:11 PM 5/2/2003 +0200, you wrote: >2. Use an external clock generator. >The external clock generator also has several possible complications that >need to be taken care of: >It requires a square wave voltage levels of 0V and 2.5V which requires also >a 2.5V external voltage regulator on your board. Is that 2.5V restriction unique to the 9S12DP256? I looked at other datasheets (like the D60A) and it looks like a 5V drive signal (basically, anything up to Vdd) is fine. jmk ----------- James M. Knox TriSoft ph 512-385-0316 1109-A Shady Lane fax 512-366-4331 Austin, Tx 78721 ----------- |
