XTAL Problem in PIC 16F877A - Electronic Circuits Projects Diagrams Free

Hesam Kamalan · 16th July 2006, 03:43 PM

Hi
I have a problem with 16f877a. I build a PCB for this PIC component. After programming (for example I wrote blinking LED), LED is off, but when I touch XTAL, LED turned on continuously. I change XTAL several times but outcome was the same. My XTAL is 20MHz and Caps 33pF.

rtamas · 16th July 2006, 04:04 PM

That's why the XTAL's body must be soldered to GND and ofcourse the PCB's GND must be rastered.I think this method will solve your problem.
P.S. You can use oscillators insted of XTAL.They are more stabile.

mramos1 · 16th July 2006, 04:06 PM

Can you send the start of your code (where you define PIC, OSC, etc)?

Do you have any 20pf caps? Maybe try swapping the 33 for something smaller.

zkt_PiratesDen · 16th July 2006, 04:19 PM

Quote:

P.S. You can use oscillators insted of XTAL.They are more stabile.

Are you sure about this ?

Hesam Kamalan · 16th July 2006, 04:29 PM

Thank you very much for your Helps and advertises, my code is written as follows:
PROCESSOR 16F877A
#INCLUDE "P16F877A.inc"
#INCLUDE "HESAM.INC"

__CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC

CBLOCK 0X20
HICNT
LOCNT
LOOPCNT
ENDC

ORG 0X00
BANK3
MOVLW 0X02
MOVWF ADCON1
BANK1
MOVLW 0X00
MOVWF TRISE
BANK0
MOVLW 0X00
MOVWF PORTE

MAIN:
MOVLW 0X01
MOVWF PORTE
PAUSEMS 500
MOVLW 0X02
MOVWF PORTE
PAUSEMS 500
MOVLW 0X04
MOVWF PORTE
PAUSEMS 500

GOTO MAIN

END

mister_e · 16th July 2006, 04:35 PM

1. you must use HS_OSC
2. Be sure you connect all VSS lines and all VDD line
3.as suggested, try something in between 15-33pF
4. be sure your MCLR pin is tied to VDD via resistor
5. Be sure you'd place capacitor (0.1uF) as close as possible of your PIC. For this one, i suggest 2, one on each side as you have VDD and VSS on both side of it.

**Nigel Goodwin** · 16th July 2006, 05:55 PM

As suggested, use smaller capacitors, check my tutorial hardware at http://www.winpicprog.co.uk/pic_tuto...ain3_board.htm which uses 12pF ones. You don't mention how you're building it?, but you need as short a leads as possible, both to the crystal and from the capacitors to 0V.

As also suggested, it's important that you select HS_OSC, which is for high speed crystals.

Mike, K8LH · 16th July 2006, 07:41 PM

Your problem may be that you have not disabled LVP in the configuration fuses and you have left the PGM pin 'floating'. I've seen these symptoms described many times and easily corrected by reprogramming the PIC with the LVP option in the config' fuses turned off.

Regards, Mike

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16th July 2006, 03:43 PM	#1
Hesam Kamalan	XTAL Problem in PIC 16F877A Hi I have a problem with 16f877a. I build a PCB for this PIC component. After programming (for example I wrote blinking LED), LED is off, but when I touch XTAL, LED turned on continuously. I change XTAL several times but outcome was the same. My XTAL is 20MHz and Caps 33pF.

16th July 2006, 04:04 PM	#2
rtamas	That's why the XTAL's body must be soldered to GND and ofcourse the PCB's GND must be rastered.I think this method will solve your problem. P.S. You can use oscillators insted of XTAL.They are more stabile.

16th July 2006, 04:06 PM	#3
mramos1	Can you send the start of your code (where you define PIC, OSC, etc)? Do you have any 20pf caps? Maybe try swapping the 33 for something smaller.

16th July 2006, 04:29 PM	#5
Hesam Kamalan	Thank you very much for your Helps and advertises, my code is written as follows: PROCESSOR 16F877A #INCLUDE "P16F877A.inc" #INCLUDE "HESAM.INC" __CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC CBLOCK 0X20 HICNT LOCNT LOOPCNT ENDC ORG 0X00 BANK3 MOVLW 0X02 MOVWF ADCON1 BANK1 MOVLW 0X00 MOVWF TRISE BANK0 MOVLW 0X00 MOVWF PORTE MAIN: MOVLW 0X01 MOVWF PORTE PAUSEMS 500 MOVLW 0X02 MOVWF PORTE PAUSEMS 500 MOVLW 0X04 MOVWF PORTE PAUSEMS 500 GOTO MAIN END

16th July 2006, 04:35 PM	#6
mister_e	1. you must use HS_OSC 2. Be sure you connect all VSS lines and all VDD line 3.as suggested, try something in between 15-33pF 4. be sure your MCLR pin is tied to VDD via resistor 5. Be sure you'd place capacitor (0.1uF) as close as possible of your PIC. For this one, i suggest 2, one on each side as you have VDD and VSS on both side of it.

16th July 2006, 05:55 PM	#7
Nigel Goodwin	As suggested, use smaller capacitors, check my tutorial hardware at http://www.winpicprog.co.uk/pic_tuto...ain3_board.htm which uses 12pF ones. You don't mention how you're building it?, but you need as short a leads as possible, both to the crystal and from the capacitors to 0V. As also suggested, it's important that you select HS_OSC, which is for high speed crystals. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

16th July 2006, 07:41 PM	#8
Mike, K8LH	Your problem may be that you have not disabled LVP in the configuration fuses and you have left the PGM pin 'floating'. I've seen these symptoms described many times and easily corrected by reprogramming the PIC with the LVP option in the config' fuses turned off. Regards, Mike Last edited by Mike, K8LH; 16th July 2006 at 08:33 PM.