How to explain peculiar obseravtion on PIC16F84A when file fials to work at 4.5V

[mvs sarma]

Hi, I had a peculiar observation on PIC16F84A.

i modded the code at AN0615 of PIc application note on cheap clock using PIC16C54A, to work with 16F84A and with a small mod eliminating disp going off after 8 sec (it was to conserve battery).

the software works fine at 3VOLTS DC but if i supply 4.5 or 5V , it behaves erratic going to ther subroutines used for maunufacturer check and there afrter goes on incrementing the hrs count and loops around from 00-00 to 24-00. for help i am posting the file here a file named clk8fcont.asm

i am trying to debug , but how to understand the voltage based behaviour?

 

[Sceadwian]

It's a hardware problem not a software one. You need to display the entire circuit.

 

[donniedj]

An increase in volts is also a possible increase in power. Are any of the loads current limited? Are any of the PIC pins overloaded? Are you using proper bypass and coupling caps?

 

[mvs sarma]

It's a hardware problem not a software one. You need to display the entire circuit.

Hi Sceadwian,
thenaks for so fast reply.
the Application note in original meant for 16c54a is attached. i did not change the schematic. Of course, as pointed by Donniedj , perhaps i may have to increse thr sigment and digit drive resistor values- i shall try-- he appears valid. i shall come back on the issue.

Thanks

 

[eng1]

Yes, increase those resistors (220 ohms should be fine).
You could add a npn transistor for each display, with base resitors connected to RA0, RA1, RA2, RA3 and the collectors connected to the cathodes of the displays.

In the datasheet of the PIC16F84A recommended values of C1 and C2 are 68-100 pF when using a LP 32 kHz crystal.

 

[mvs sarma]

Yes, increase those resistors (220 ohms should be fine).
You could add a npn transistor for each display, with base resitors connected to RA0, RA1, RA2, RA3 and the collectors connected to the cathodes of the displays.

In the datasheet of the PIC16F84A recommended values of C1 and C2 are 68-100 pF when using a LP 32 kHz crystal.

Hi, Eng1 and other friends on the subject,

I have just changed the segment resistors to 430 Ohms which i had. Ofcorse, the digit is dirct on RA0...RA4. No improvement.68pF caps are used. but if i apply 3volts it works steady.

 

[eng1]

Bear in mind that the maximum current sunk by any pin of PORTA is 25 mA. I would use the transistors, as I said.

 

[mvs sarma]

Hi Eng1,
i shall implement transistors and be back