Problem with PIC 16F819

[newbee]

Hi, I have this terrible problem with PIC 16F819, it does not starts up with external or internal oscillators if the supply voltage is 5V regulated. However if I decrease the VDD to 3.4 it starts working aftter about 20 seconds when powered up. I have used PICs 16F84A and 16F628A on the same board they do not have this problem.

Please help.

[mackrackit]

Welcome to the forum.

Can you provide more information?
How is MCLR connected?
How is the chip configured? (Fuses) (config bits)
Code?

[newbee]

Thanks Mack, MCLR is connected to VDD through a 10K resistor and also grounded with a bypass ceramic capacitor of 0.1uf. I also placed a big 1000 uf capacitor between ground and 5V line but oscillator still doesnt start at 5V supply but it does if the VDD is reduced to 3.4V. Its behaviour is same if fuse is set to Internal Oscillator with OSCCON and OSCTUNE registers set properly.
Right now I have loaded the following simplest program for testing:

DEVICE = 16F819

WARNINGS 0

Config HS_OSC, WDT_OFF, PWRTE_OFF, BODEN_OFF, LVP_OFF, CP_OFF, MCLRE_ON
WARNINGS 1
XTAL= 12

ALL_DIGITAL TRUE
OUTPUT PORTB
loop:
PORTB=255
DELAYMS 500
PORTB=0
DELAYMS 500
GOTO loop

[newbee]

Hello again: Here is an update on this strange chip. I placed the 1000 uF Capacitor on the input side of the 5V regulator as it was not letting the ICSP and placed 0.1 uf ceramic directly on the VSS & VDD pins. It still didn't work at 5 V, I gave it jolt by shorting VDD to ground momentarily, Lo and behold it started to work! ( previously I had 10 uf between VSS and VDD and 0.1 uf on both sides of the 5V regulator and one 0.1uf ceramic directly on VDD and VSS, I had even tried to power it with 9 V battery cells, so dont think anything was wrong with my previous circuit as 16F84A and 16F628A worked fine with that arrangement)

So now I must give it a jolt every time at start up to make it run. I have uploaded another program that uses LCD and PORTA.3 & PORTA.4 as input switches, this program also works. Next I will try to add small capacitance to OSC/RA6 and see if its external oscillator stabilizes.

[mackrackit]

Sounds like you are getting a handle on it and know what to look for.

BTW... Are you using Proton? Your code does not look like PicBasic.

[newbee]

Mack: Yes i am using Proton, haven't done any change in the circuit yet but seems I might be able to run it on external oscillator by adjusting capacitors on OSC1 & OSC2 ports. Just realized that the Microchip data sheet says 15 pf on each but I have 22 pf, this maybe the problem but why doesn't it run on internal RC, this has to be figured out.