Odd Issue with PIC18F45K20

Thread Starter

Stuntman

Joined Mar 28, 2011
213
This is a simple problem I recently ran into in programming a PIC18F45K20.

The simple problem, if I set one of the outputs (read, one bit in PORTA), at least some of the other bits are cleared. Here is some test code I wrote to show the problem:

#include <htc.h>
#include <stdio.h>

__CONFIG(1, FCMEN_OFF & FOSC_INTIO7);
__CONFIG(2, PWRT_ON & BOREN_OFF & WDTEN_OFF);
__CONFIG(3, PBADEN_OFF & MCLRE_OFF);
__CONFIG(4, LVP_OFF);


void init(void);

void main(void)
{
init();

int i;

RA0=1;
RA1=1;

for(i=1; i<=1000; i++) //Delay for visual
{
_delay(1000);
}

RA0=0;
RA1=0;

for(i=1; i<=1000; i++) //Delay for visual
{
_delay(1000);
}

}


void init(void)
{
// port directions: 1=input, 0=output

TRISA = 0b00000000;
TRISB = 0b00000000;
TRISC = 0b11111111;
TRISD = 0b00000000;

//Set Oscillator to 16MHz
IRCF0=1;
IRCF1=1;
IRCF2=1;

}
If I run this code, only pin RA1 toggles high and low. RA0 stays low (except for the 1 execution step it is high before RA1).

If I switch this code to use pins RB0 and RB1, the pins toggle simultaneously as they should.

I have not used the PIC18F series extensively, but haven't found a reason in the datasheet or errata.

Ideas?

Thanks
 

hexreader

Joined Apr 16, 2011
377
Instead of RA0, use LATA0

Always use LATx register for output, PORTx register for input.

Google Read-Modify-Write or RMW for the reason why.

Also check that analogue is disabled for PORTA.
The datasheet I/O Ports section should explain what is needed.
Probably something like ANSEL = 0;

Hope this helps
 

Thread Starter

Stuntman

Joined Mar 28, 2011
213
hexreader-

Very sharp!

Not knowing about this RMW I looked into this issues to find this:

http://www.cornerstonerobotics.org/curriculum/lessons_year2/erii_rmw_problem.pdf

I modified the code with LATx and voila, did all it was supposed to.

But I couldn't help but investigate why these pins were not reading correctly. In the actual code, this activation of the PORTA pin that cleared the others is far away from direct succession of the previous PORTA operation, making timing a non-issue.

Just in case, I tried timing things out again, and no change. I tried pullups on the pins with no change.

Your idea about the ANSEL byte was one my first thoughts on this problem, but was disregarded as the datasheet stated the ANSEL bit does not affect digital output. To humor myself, I actually set the ANSEL and ANSELH bits high. Now, when running the test, RA0 stayed high while RA1 flashed. So I put a small 10 cycle delay between RA0=0 and RA1=0. This solved the problem using PORTA functions.

Great tip, and some interesting findings on this chip.
 

ErnieM

Joined Apr 24, 2011
8,006
Your idea about the ANSEL byte was one my first thoughts on this problem, but was disregarded as the datasheet stated the ANSEL bit does not affect digital output.
READ-modify-Write starts with a READ.

With ANSEL set to make the I/O analog ALL reads return zero.

When you execute RA1=1; you read PORTA, set bit 1, then write it back. However, if it reads zero for RA0 then it immediately resets this pin when RA1 is set.
 
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