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Discussion Groups | 68HC12 | Good Oscillator for 9S12Dp256
Join our technical discussions about Freescale Microcontrollers: M68HC12. (Freescale Semiconductor is a Subsidiary of Motorola).
Good Oscillator for 9S12Dp256 - Ryan M. Kruse - May 19 12:43:00 2003
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Hey all, I'm going to redesign my MCU board for a 9S12DP256 to incorporate an oscillator, instead of using a crystal for clock generation. What is a good oscillator for this? Ryan Kruse |
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Re: Good Oscillator for 9S12Dp256 - Doron Fael - May 19 15:56:00 2003
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Ryan, The selection of a good oscillator to use to drive the HCS12 depends on what is the frequency you would like this oscillator to generate. For low frequencies (to 16MHz) a 5V CMOS oscillator with resistor divider to divide down to 2.5V may be appropriate. For higher frequencies, a 5V oscillator and a translation buffer to 2.5V may be more appropriate. Lets us know the frequency and we can hopefully be more specific. Hope this helps, Doron Nohau Corporation HC12 In-Circuit Emulators www.nohau.com/emul12pc.html At 11:43 AM 5/19/2003 -0600, you wrote: >Hey all, > >I'm going to redesign my MCU board for a 9S12DP256 to incorporate an >oscillator, instead of using a crystal for clock generation. What is a good >oscillator for this? > >Ryan Kruse > >-------------------------------------------------------- >To unsubscribe from this group, send an email to: >To learn more about Motorola Microcontrollers, please visit >http://www.motorola.com/mcu [Non-text portions of this message have been removed] |
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Re: Good Oscillator for 9S12Dp256 - Ryan M. Kruse - May 19 19:07:00 2003
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The frequency will be 4MHz. Ryan ----- Original Message ----- From: "Doron Fael" <> To: <> Sent: Monday, May 19, 2003 2:56 PM Subject: Re: [68HC12] Good Oscillator for 9S12Dp256 > Ryan, > > The selection of a good oscillator to use to drive the HCS12 depends on > what is the frequency you would like this oscillator to generate. For low > frequencies (to 16MHz) a 5V CMOS oscillator with resistor divider to divide > down to 2.5V may be appropriate. For higher frequencies, a 5V oscillator > and a translation buffer to 2.5V may be more appropriate. > > Lets us know the frequency and we can hopefully be more specific. > > Hope this helps, > > Doron > Nohau Corporation > HC12 In-Circuit Emulators > www.nohau.com/emul12pc.html > > At 11:43 AM 5/19/2003 -0600, you wrote: > >Hey all, > > > >I'm going to redesign my MCU board for a 9S12DP256 to incorporate an > >oscillator, instead of using a crystal for clock generation. What is a good > >oscillator for this? > > > >Ryan Kruse > > > > > > > >-------------------------------------------------------- > >To unsubscribe from this group, send an email to: > > > > > >To learn more about Motorola Microcontrollers, please visit > >http://www.motorola.com/mcu > > > > > > > [Non-text portions of this message have been removed] > > -------------------------------------------------------- > To unsubscribe from this group, send an email to: > To learn more about Motorola Microcontrollers, please visit > http://www.motorola.com/mcu |
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Re: Good Oscillator for 9S12Dp256 - Oliver Betz - May 20 2:51:00 2003
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Ryan M. Kruse wrote: > The frequency will be 4MHz. at 4MHz you likely will use the PLL. Why don't you want to use a crystal? At 4MHz, the design is pretty robust and cheap. Oliver -- Oliver Betz, Muenchen |
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Re: Good Oscillator for 9S12Dp256 - Doron Fael - May 20 3:20:00 2003
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Ryan, You can use a 5V CMOS 4MHz oscillator. Along with it, you can use a resistor voltage divider to divide the 5V output of the oscillator to the required 2.5V clock input to the 9S12DP256. The values of the resistors you choose should represent a trade off between the current consumed from the 5V Oscillator (for a logic 1 drive), and the time constant generated by the resistors and the parasitic capacitance of the EXTAL input, which will affect the rise/fall time of the clock signal on the EXTAL pin. The smaller the resistors are, the higher the consumed current is, but also the shorter the time constant and rise/fall time become, and vice-versa. I believe a good trade off for a 4MHz oscillator may be a voltage divider of two 1.3Kohm resistors. For these resistor values, the current consumption from the Clock oscillator for a logic 1 drive will be: 5V / (1.3K + 1.3K) = 1.92mA and assuming a 50% clock duty-cycle, the average power consumption will be one half - 0.96mA. Assuming a parasitic capacitance of 10pF on the EXTAL pin, the Tau time constant will be: 10pF * ( 1.3K / 2 ) = 6.5nSEC (Note that the equivalent resistor for this calculation is the two resistors in parallel) For a 4MHz clock, the high-time and low-time of the clock are 125nSEC each (assuming a 50% duty cycle). Thus there is a ratio of about 20 between the clock high/low time and the achieved Tau time constant. This is a good ratio that allows a charging time (to 98% the final voltage) within 5 time-constants which are 32.5nSEC, and 1/4 of the clock high/low time. The alternative to using such a voltage divider, is to use a 5V to 2.5V translation buffer. The disadvantage of this alternative is that you will need an additional 2.5V voltage regulator, and a translation buffer that will be powered by 2.5V and have 5V tolerant inputs. If you are willing to invest the additional components, I can offer good components for both voltage regulator and translation buffer. Hope this helps, Doron Nohau Corporation HC12 In-Circuit Emulators www.nohau.com/emul12pc.html At 06:07 PM 5/19/2003 -0600, you wrote: >The frequency will be 4MHz. > >Ryan >----- Original Message ----- >From: "Doron Fael" <> >To: <> >Sent: Monday, May 19, 2003 2:56 PM >Subject: Re: [68HC12] Good Oscillator for 9S12Dp256 > > Ryan, > > > > The selection of a good oscillator to use to drive the HCS12 depends on > > what is the frequency you would like this oscillator to generate. For low > > frequencies (to 16MHz) a 5V CMOS oscillator with resistor divider to >divide > > down to 2.5V may be appropriate. For higher frequencies, a 5V oscillator > > and a translation buffer to 2.5V may be more appropriate. > > > > Lets us know the frequency and we can hopefully be more specific. > > > > Hope this helps, > > > > Doron > > Nohau Corporation > > HC12 In-Circuit Emulators > > www.nohau.com/emul12pc.html > > > > At 11:43 AM 5/19/2003 -0600, you wrote: > > >Hey all, > > > > > >I'm going to redesign my MCU board for a 9S12DP256 to incorporate an > > >oscillator, instead of using a crystal for clock generation. What is a >good > > >oscillator for this? > > > > > >Ryan Kruse > > [Non-text portions of this message have been removed] |
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Re: Good Oscillator for 9S12Dp256 - Ryan M. Kruse - May 20 10:13:00 2003
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Oliver, Actually, my preference is to use a crystal for clock generation. One the first prototype board that we made, we could not get the crystal (4MHz Citizen Electronics SMD) to work (but were able to generate the clock with a signal generator and the board worked fine), hence the questions about using an oscillator. For the new prototype, we are going to put on both a crystal and oscillator and a way to select between the two. Hopefully, this will allow us to troubleshoot the clocks better and decide on a final clock design. Ryan ----- Original Message ----- From: "Oliver Betz" <> To: <> Sent: Tuesday, May 20, 2003 1:51 AM Subject: Re: [68HC12] Good Oscillator for 9S12Dp256 > Ryan M. Kruse wrote: > > > The frequency will be 4MHz. > > at 4MHz you likely will use the PLL. Why don't you want to use a > crystal? At 4MHz, the design is pretty robust and cheap. > > Oliver > -- > Oliver Betz, Muenchen > > -------------------------------------------------------- > To unsubscribe from this group, send an email to: > To learn more about Motorola Microcontrollers, please visit > http://www.motorola.com/mcu |
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Re: Good Oscillator for 9S12Dp256 - Ryan M. Kruse - May 20 15:28:00 2003
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Thanks for the help so far. One final question: I'm trying to choose between two oscillators for the DP256. 1: ECS. Inc, ECS-3951M-040-BN-TR, 4MHz 5V 2: CTS Frequency Products, CB3-3C-4.0000-T, 4MHz 5V Both are available from Digikey and are about the same cost. Anybody have experience with either of these? Ryan [Non-text portions of this message have been removed] |
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