Post by thread:16C74A problem (dual Vdd,Vss??)

Hi... I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit but this is my first try using the 16C74 with a HS osc at 16Mhz. Problem: I cant even get an led to flash. I know the code is correct. I set the osc to HS. Question: Are the two VDD and VSS pins connected internally??? I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. Is this correct?? Should I connect these pins externally??? Is there something else that could be causing the problem? Any help would be much appreciated. (the frustration's building) Thanks, Stewart Pye

Stewart: You realy must connect those pins together and to supply. I accidentally left one of the VS's off on a '73 once, and the A/D didn't work right at all. A cursory check with a meter saw that pin at ground, or so one would think on a quick glance. Turns out that a few tens of mV will whack the A/D. Connect them through and to supply, with bypass. Try a small hello world by killing the watchdog, interrupts, and all unnecessary calls. Start small and work up! You might just read TMR0 and stuff it onto port C for the fun of it.. Scope should go nuts. Don't forget that the portA is set for analog function by default. Ref ADCON0/1. And that RA4 is an open collector all the time. If you were looking for excitement on one of these RA pins, you could have the wool over your eyes. Stewart Pye wrote: {Quote hidden}> Hi... > > I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit > but this is my first try using the 16C74 with a HS osc at 16Mhz. > > Problem: > I cant even get an led to flash. I know the code is correct. I set the osc > to HS. > > Question: > Are the two VDD and VSS pins connected internally??? > I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. > Is this correct?? > Should I connect these pins externally??? > Is there something else that could be causing the problem? > > Any help would be much appreciated. (the frustration's building) > > Thanks, > Stewart Pye

check ram base address is corect. check output pin is not designated as a special function pin from reset. connect ALL power supply pinz. DON'T activate security bitz, they won't U.V. erase ! regards, Graham. Stewart Pye wrote: {Quote hidden}> > Hi... > > I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit > but this is my first try using the 16C74 with a HS osc at 16Mhz. > > Problem: > I cant even get an led to flash. I know the code is correct. I set the osc > to HS. > > Question: > Are the two VDD and VSS pins connected internally??? > I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. > Is this correct?? > Should I connect these pins externally??? > Is there something else that could be causing the problem? > > Any help would be much appreciated. (the frustration's building) > > Thanks, > Stewart Pye -- Steam engines may be out of fashion, but when you consider that an internal combustion engine would require recovery of waste heat by transfer just before top dead centre then fashion becomes rather redundant, USE STRATIFIED HEAT EXCHANGERS ! and external combustion. You heard it first from: Graham Daniel, managing director of Electronic Product Enhancements. Phone NZ 04 387 4347, Fax NZ 04 3874348, Cellular NZ 021 954 196.

I've connected the two Vss pins together and the two Vdd pins. Still got the same problem. Something weird though.... When I touch the top of the 16C74JW the LED lights up. The LED is active low. ie: connected between RB0 and +5V with 220ohm R. Should this happen?? It seems strange to me. Thanks Stewart Pye At 11:17 PM 8/06/99 -0400, you wrote: >Stewart: > >You realy must connect those pins together and to supply. I accidentally left >one of the VS's off on a '73 once, and the A/D didn't work right at all. A >cursory check with a meter saw that pin at ground, or so one would think on a >quick glance. Turns out that a few tens of mV will whack the A/D. Connect >them through and to supply, with bypass. > >Try a small hello world by killing the watchdog, interrupts, and all >unnecessary calls. Start small and work up! You might just read TMR0 and >stuff it onto port C for the fun of it.. Scope should go nuts. Don't forget >that the portA is set for analog function by default. Ref ADCON0/1. And that {Quote hidden}>RA4 is an open collector all the time. If you were looking for excitement on >one of these RA pins, you could have the wool over your eyes. > >Stewart Pye wrote: > >> Hi... >> >> I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit >> but this is my first try using the 16C74 with a HS osc at 16Mhz. >> >> Problem: >> I cant even get an led to flash. I know the code is correct. I set the osc >> to HS. >> >> Question: >> Are the two VDD and VSS pins connected internally??? >> I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. >> Is this correct?? >> Should I connect these pins externally??? >> Is there something else that could be causing the problem? >> >> Any help would be much appreciated. (the frustration's building) >> >> Thanks, >> Stewart Pye >

Guess number one is some kind of intermittent. Hunch number two.. Are you using two small capacitors from each leg of your crystal to ground?? If not, see manufacturers data. Typical values are 18-22pF . Stewart Pye wrote: {Quote hidden}> I've connected the two Vss pins together and the two Vdd pins. Still got > the same problem. Something weird though.... > > When I touch the top of the 16C74JW the LED lights up. The LED is active > low. ie: connected between RB0 and +5V with 220ohm R. Should this happen?? > It seems strange to me. > > Thanks > > Stewart Pye > > At 11:17 PM 8/06/99 -0400, you wrote: > >Stewart: > > > >You realy must connect those pins together and to supply. I accidentally > left > >one of the VS's off on a '73 once, and the A/D didn't work right at all. A > >cursory check with a meter saw that pin at ground, or so one would think on a > >quick glance. Turns out that a few tens of mV will whack the A/D. Connect > >them through and to supply, with bypass. > > > >Try a small hello world by killing the watchdog, interrupts, and all > >unnecessary calls. Start small and work up! You might just read TMR0 and > >stuff it onto port C for the fun of it.. Scope should go nuts. Don't forget > >that the portA is set for analog function by default. Ref ADCON0/1. And > that > >RA4 is an open collector all the time. If you were looking for excitement on > >one of these RA pins, you could have the wool over your eyes. > > > >Stewart Pye wrote: > > > >> Hi... > >> > >> I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit > >> but this is my first try using the 16C74 with a HS osc at 16Mhz. > >> > >> Problem: > >> I cant even get an led to flash. I know the code is correct. I set the osc > >> to HS. > >> > >> Question: > >> Are the two VDD and VSS pins connected internally??? > >> I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. > >> Is this correct?? > >> Should I connect these pins externally??? > >> Is there something else that could be causing the problem? > >> > >> Any help would be much appreciated. (the frustration's building) > >> > >> Thanks, > >> Stewart Pye > >

Stewart Pye wrote: > When I touch the top of the 16C74JW the LED lights up. The LED is active > low. ie: connected between RB0 and +5V with 220ohm R. Should this happen?? > It seems strange to me. That's probably a good clue. It sounds like you have floating inputs that may be causing your problems. Try making any unused port pins Outputs, and make sure that pins connected as inputs are not tied to open circuit switches etc without a pullup(down) of some kind. It happens on my 16C74's. If you touch a spare pin (which is just a floating input) with a finger, the PIC crashes straight away. -- Best regards Tony 'The Engine' - Design your own programmer. http://www.picnpoke.com Email spam_OUTpicnpokeTakeThisOuTcdi.com.au

Try clearing all memory locations at startup. Joe At 14:05 09/06/99 +1000, you wrote: {Quote hidden}>I've connected the two Vss pins together and the two Vdd pins. Still got >the same problem. Something weird though.... > >When I touch the top of the 16C74JW the LED lights up. The LED is active >low. ie: connected between RB0 and +5V with 220ohm R. Should this happen?? >It seems strange to me. > >Thanks > >Stewart Pye > >At 11:17 PM 8/06/99 -0400, you wrote: >>Stewart: >> >>You realy must connect those pins together and to supply. I accidentally >left >>one of the VS's off on a '73 once, and the A/D didn't work right at all. A >>cursory check with a meter saw that pin at ground, or so one would think on a >>quick glance. Turns out that a few tens of mV will whack the A/D. Connect >>them through and to supply, with bypass. >> >>Try a small hello world by killing the watchdog, interrupts, and all >>unnecessary calls. Start small and work up! You might just read TMR0 and >>stuff it onto port C for the fun of it.. Scope should go nuts. Don't forget >>that the portA is set for analog function by default. Ref ADCON0/1. And >that >>RA4 is an open collector all the time. If you were looking for excitement on {Quote hidden}>>one of these RA pins, you could have the wool over your eyes. >> >>Stewart Pye wrote: >> >>> Hi... >>> >>> I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit >>> but this is my first try using the 16C74 with a HS osc at 16Mhz. >>> >>> Problem: >>> I cant even get an led to flash. I know the code is correct. I set the osc >>> to HS. >>> >>> Question: >>> Are the two VDD and VSS pins connected internally??? >>> I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. {Quote hidden}>>> Is this correct?? >>> Should I connect these pins externally??? >>> Is there something else that could be causing the problem? >>> >>> Any help would be much appreciated. (the frustration's building) >>> >>> Thanks, >>> Stewart Pye >> > >

Sounds like you are having problems with light getting in the window and messing with your ram locations. I bet your problem will go away if you put a sticker o n the window. I have had this problem a few times although I normally leave the window uncovered and it is O.K. almost all te time. Stewart Pye wrote: {Quote hidden}> I've connected the two Vss pins together and the two Vdd pins. Still got > the same problem. Something weird though.... > > When I touch the top of the 16C74JW the LED lights up. The LED is active > low. ie: connected between RB0 and +5V with 220ohm R. Should this happen?? > It seems strange to me. > > Thanks > > Stewart Pye > > At 11:17 PM 8/06/99 -0400, you wrote: > >Stewart: > > > >You realy must connect those pins together and to supply. I accidentally > left > >one of the VS's off on a '73 once, and the A/D didn't work right at all. A > >cursory check with a meter saw that pin at ground, or so one would think on a > >quick glance. Turns out that a few tens of mV will whack the A/D. Connect > >them through and to supply, with bypass. > > > >Try a small hello world by killing the watchdog, interrupts, and all > >unnecessary calls. Start small and work up! You might just read TMR0 and > >stuff it onto port C for the fun of it.. Scope should go nuts. Don't forget > >that the portA is set for analog function by default. Ref ADCON0/1. And > that > >RA4 is an open collector all the time. If you were looking for excitement on > >one of these RA pins, you could have the wool over your eyes. > > > >Stewart Pye wrote: > > > >> Hi... > >> > >> I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit > >> but this is my first try using the 16C74 with a HS osc at 16Mhz. > >> > >> Problem: > >> I cant even get an led to flash. I know the code is correct. I set the osc > >> to HS. > >> > >> Question: > >> Are the two VDD and VSS pins connected internally??? > >> I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. > >> Is this correct?? > >> Should I connect these pins externally??? > >> Is there something else that could be causing the problem? > >> > >> Any help would be much appreciated. (the frustration's building) > >> > >> Thanks, > >> Stewart Pye > >

Dear Stewart, I think you can find the answer from the book "Design with PIC Microcontrollers" by John B. Peatman or try to email to .....john.peatmanKILLspam@spam@ece.gatech.edu Regards, Samuel Lau Stewart Pye wrote: {Quote hidden}> Hi... > > I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit > but this is my first try using the 16C74 with a HS osc at 16Mhz. > > Problem: > I cant even get an led to flash. I know the code is correct. I set the osc > to HS. > > Question: > Are the two VDD and VSS pins connected internally??? > I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. > Is this correct?? > Should I connect these pins externally??? > Is there something else that could be causing the problem? > > Any help would be much appreciated. (the frustration's building) > > Thanks, > Stewart Pye

Connect all the power and ground pins to insure proper operation. Sometimes the oscillator will not start and run reliably and you need to adjust the caps slightly for reliable startup and oscillation. Also check the type of crystal you are using. You may be overdriving it and need a resistor to the crystal. At 01:10 PM 6/9/99 +1000, you wrote: {Quote hidden}>Hi... > >I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit >but this is my first try using the 16C74 with a HS osc at 16Mhz. > >Problem: >I cant even get an led to flash. I know the code is correct. I set the osc >to HS. > >Question: >Are the two VDD and VSS pins connected internally??? >I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. >Is this correct?? >Should I connect these pins externally??? >Is there something else that could be causing the problem? > >Any help would be much appreciated. (the frustration's building) > >Thanks, >Stewart Pye > > Larry G. Nelson Sr. L.NelsonKILLspamieee.org http://www.ultranet.com/~nr

Are you using any of the Analog pins? You must convert them to digital I/O first with this code. BSF STATUS,RP0 MOVLW 07H MOVWF 01FH BCF STATUS,RP0 Maybe this might help. -Jim {Quote hidden}> > >At 01:10 PM 6/9/99 +1000, you wrote: >>Hi... >> >>I'm testing a project that uses a 16C74A. I've used the 16F84 quite a bit >>but this is my first try using the 16C74 with a HS osc at 16Mhz. >> >>Problem: >>I cant even get an led to flash. I know the code is correct. I set the osc >>to HS. >> >>Question: >>Are the two VDD and VSS pins connected internally??? >>I got 12ohms between the 2 Vss pins but several Mohms between the Vdd pins. >>Is this correct?? >>Should I connect these pins externally??? >>Is there something else that could be causing the problem? >> >>Any help would be much appreciated. (the frustration's building) >> >>Thanks, >>Stewart Pye >> >> >Larry G. Nelson Sr. >.....L.NelsonKILLspam.....ieee.org >http://www.ultranet.com/~nr > Regards, Jim Robertson NEWFOUND ELECTRONICS ________________________________________ Email: EraseMEnewfoundspam_OUTTakeThisOuTpipeline.com.au http://www.new-elect.com MPLAB compatible PIC programmers. ________________________________________