Hi, can anyone advise please? I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an external RC network. On the OSC2 pin both processors show a nice clock at 25 KHz (which is correct). However, the 18F1320 seems to be executing code at twice the speed! Have I overlooked something? Thanks in advance for any help? Simon
PIC migration timing problem
Started by ●June 20, 2006
Reply by ●June 20, 20062006-06-20
PigPOg wrote:> Hi, can anyone advise please? > > I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an > external RC network. On the OSC2 pin both processors show a nice clock > at 25 KHz (which is correct). However, the 18F1320 seems to be > executing code at twice the speed! Have I overlooked something? > > Thanks in advance for any help? > > SimonIt's not executing the code twice speed, same C routine may generate fewer instructions on 18Fxxx than 16Cxx and fewer instructions execute faster. Depends on your usage of arrays and pointers but twice the speed sounds just right. Roman
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 07:38:55 -0400, Roman Ziak <news12@ziak.com> wrote:>PigPOg wrote: >> Hi, can anyone advise please? >> >> I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >> external RC network. On the OSC2 pin both processors show a nice clock >> at 25 KHz (which is correct). However, the 18F1320 seems to be >> executing code at twice the speed! Have I overlooked something? >> >> Thanks in advance for any help? >> >> Simon > >It's not executing the code twice speed, same C routine may generate >fewer instructions on 18Fxxx than 16Cxx and fewer instructions execute >faster. Depends on your usage of arrays and pointers but twice the speed >sounds just right. > > RomanThe code is written in assembly language. I can't see that this would make a difference. Thanks for the reply. Simon
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 11:25:07 +0100, PigPOg <simon@capella.co.uk> wrote:>Hi, can anyone advise please? > >I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >external RC network. On the OSC2 pin both processors show a nice clock >at 25 KHz (which is correct). However, the 18F1320 seems to be >executing code at twice the speed! Have I overlooked something?I seem to recall that some of the higher-end PICs execute an instruction in 2 OSC clock cycles rather than the usual 4 clock cycles. Check the data sheet for that. Robert Scott Real-Time Specialties Robert Scott Ypsilanti, Michigan
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 12:00:04 GMT, ---@--- (Robert Scott) wrote:>On Tue, 20 Jun 2006 11:25:07 +0100, PigPOg <simon@capella.co.uk> wrote: > >>Hi, can anyone advise please? >> >>I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >>external RC network. On the OSC2 pin both processors show a nice clock >>at 25 KHz (which is correct). However, the 18F1320 seems to be >>executing code at twice the speed! Have I overlooked something? > >I seem to recall that some of the higher-end PICs execute an instruction in 2 >OSC clock cycles rather than the usual 4 clock cycles. Check the data sheet for >that. > >Robert Scott >Real-Time Specialties > >Robert Scott >Ypsilanti, MichiganThanks Robert, I'll check that out. Simon
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 11:25:07 +0100, the renowned PigPOg <simon@capella.co.uk> wrote:>Hi, can anyone advise please? > >I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >external RC network. On the OSC2 pin both processors show a nice clock >at 25 KHz (which is correct). However, the 18F1320 seems to be >executing code at twice the speed! Have I overlooked something? > >Thanks in advance for any help? > >SimonHow about your layout and bypassing? You're not using a solderless breadboard are you? The RC capacitor lead ('Cext' in Microchip-hua) should go directly back to a Vss pin on the micro and *nowhere else*. At least 0.1uF ceramic in parallel with something like 10uF-100uF electrolytic, with the ceramic especially directly across Vdd-Vss very near the chip. With such a slow clock speed, and some kind of layout or bypassing issue, it's entirely possible you're getting a hiccup of a few hundred ns on one of the clock edges, which is double-clocking the core. The PIC18F series is faster than the older 16C series and will likely be less tolerant of such things. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 10:59:07 -0400, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Tue, 20 Jun 2006 11:25:07 +0100, the renowned PigPOg ><simon@capella.co.uk> wrote: > >>Hi, can anyone advise please? >> >>I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >>external RC network. On the OSC2 pin both processors show a nice clock >>at 25 KHz (which is correct). However, the 18F1320 seems to be >>executing code at twice the speed! Have I overlooked something? >> >>Thanks in advance for any help? >> >>Simon > >How about your layout and bypassing? You're not using a solderless >breadboard are you? The RC capacitor lead ('Cext' in Microchip-hua) >should go directly back to a Vss pin on the micro and *nowhere else*. >At least 0.1uF ceramic in parallel with something like 10uF-100uF >electrolytic, with the ceramic especially directly across Vdd-Vss very >near the chip. > >With such a slow clock speed, and some kind of layout or bypassing >issue, it's entirely possible you're getting a hiccup of a few hundred >ns on one of the clock edges, which is double-clocking the core. The >PIC18F series is faster than the older 16C series and will likely be >less tolerant of such things. > > >Best regards, >Spehro PefhanySpehro The micro is on a SMD populated 4-layer PCB and Cext is connected directly to VSS. In addition, there is a 100nF cap close to the micro supply pins. I'm still investigating... Regards, Simon
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 16:19:30 +0100, the renowned PigPOg <simon@capella.co.uk> wrote:>On Tue, 20 Jun 2006 10:59:07 -0400, Spehro Pefhany ><speffSNIP@interlogDOTyou.knowwhat> wrote: > >>On Tue, 20 Jun 2006 11:25:07 +0100, the renowned PigPOg >><simon@capella.co.uk> wrote: >> >>>Hi, can anyone advise please? >>> >>>I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >>>external RC network. On the OSC2 pin both processors show a nice clock >>>at 25 KHz (which is correct). However, the 18F1320 seems to be >>>executing code at twice the speed! Have I overlooked something? >>> >>>Thanks in advance for any help? >>> >>>Simon >> >>How about your layout and bypassing? You're not using a solderless >>breadboard are you? The RC capacitor lead ('Cext' in Microchip-hua) >>should go directly back to a Vss pin on the micro and *nowhere else*. >>At least 0.1uF ceramic in parallel with something like 10uF-100uF >>electrolytic, with the ceramic especially directly across Vdd-Vss very >>near the chip. >> >>With such a slow clock speed, and some kind of layout or bypassing >>issue, it's entirely possible you're getting a hiccup of a few hundred >>ns on one of the clock edges, which is double-clocking the core. The >>PIC18F series is faster than the older 16C series and will likely be >>less tolerant of such things. >> >> >>Best regards, >>Spehro Pefhany > >Spehro > >The micro is on a SMD populated 4-layer PCB and Cext is connected >directly to VSS. In addition, there is a 100nF cap close to the micro >supply pins. I'm still investigating... > >Regards, >SimonOkay, we can certainly rule that out. If you're not making a mistake in your evaluation of the execution speed, it's almost looking like a silicon problem. I doubt ext. RC clock is very often used with the 18F series. ...when you have eliminated the impossible, whatever remains, however improbable, must be the truth.. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●June 20, 20062006-06-20
On Tue, 20 Jun 2006 14:56:42 +0100, PigPOg <simon@capella.co.uk> wrote:>On Tue, 20 Jun 2006 12:00:04 GMT, ---@--- (Robert Scott) wrote: > >>On Tue, 20 Jun 2006 11:25:07 +0100, PigPOg <simon@capella.co.uk> wrote: >> >>>Hi, can anyone advise please? >>> >>>I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >>>external RC network. On the OSC2 pin both processors show a nice clock >>>at 25 KHz (which is correct). However, the 18F1320 seems to be >>>executing code at twice the speed! Have I overlooked something? >> >>I seem to recall that some of the higher-end PICs execute an instruction in 2 >>OSC clock cycles rather than the usual 4 clock cycles. Check the data sheet for >>that. >> >> > >Thanks Robert, I'll check that out.I take it back. The PIC24H series is the one with the 2x clock. I think all PIC18s have a 4x clock. Robert Scott Ypsilanti, Michigan
Reply by ●June 20, 20062006-06-20
PigPOg wrote:> On Tue, 20 Jun 2006 07:38:55 -0400, Roman Ziak <news12@ziak.com> > wrote: > >> PigPOg wrote: >>> Hi, can anyone advise please? >>> >>> I've migrated code running on a PIC16C54 to a PIC18F1320. Both use an >>> external RC network. On the OSC2 pin both processors show a nice clock >>> at 25 KHz (which is correct). However, the 18F1320 seems to be >>> executing code at twice the speed! Have I overlooked something? >>> >>> Thanks in advance for any help? >>> >>> Simon >> It's not executing the code twice speed, same C routine may generate >> fewer instructions on 18Fxxx than 16Cxx and fewer instructions execute >> faster. Depends on your usage of arrays and pointers but twice the speed >> sounds just right. >> >> Roman > > The code is written in assembly language. I can't see that this would > make a difference. > > Thanks for the reply. > Simon > >Just curious, hos did you determine it was 2x speed ? It is exactly twice or approximate ? Btw. check errata for this chip, it seems it has several problems with internal RC oscillator. http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2057&ty=&dty=Data+Sheets§ion=Data+Sheets&ssUserText=PIC18F1320 Roman
