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The Rookie
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| Output_High() |
 Posted: Mon Jun 21, 2004 11:31 am |
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When I use the following code:
output_high(PIN_A0);
output_high(PIN_A1);
the A0 goes high and as soon as A1 goes high A0 goes low.
Basically the output_high for A0 does not stick.
Any suggestion on how I can keep A0 high |
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alexbilo
Joined: 01 Jun 2004
Posts: 39
Location: Trois-Rivi�res
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| Posted: Mon Jun 21, 2004 11:38 am |
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If you use a PIC with an ADC module, did you disable the analog inputs for the pins A0 and A1?
| Code: | | setup_adc_ports(NO_ANALOG) |
Just an idea...
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Alex |
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The Rookie
Guest
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| Output_High |
| Posted: Mon Jun 21, 2004 11:41 am |
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I am not using ADC
But I think it might be caused by the INT being enabled. |
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The Rookie
Guest
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| Output_High() |
| Posted: Mon Jun 21, 2004 12:28 pm |
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| The INT did not help. I still can not keep the A0 high once I use output_high() again. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jun 21, 2004 12:33 pm |
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You need to tell us the PIC that you're using, and
the version of the CCS compiler, and what external
circuits (if any) you have connected to pins A0 and A1. |
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The Rookie
Guest
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| Output_High() |
| Posted: Mon Jun 21, 2004 12:48 pm |
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I am using 12F629. And I do not have any circuits connected to it. I am looking at the pins via Logic Analyzer.
It almost seems like once Output_High() is called again it releases the state of the previous pin |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jun 21, 2004 1:10 pm |
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| What is your version of the compiler ? |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Mon Jun 21, 2004 1:57 pm |
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Could it be a read-modify-write problem? Try putting a delay-us(1) between the output statements.
The problem occurs because of the way the PIC bit setting operation occurs and the way instructions are pipelined. Basically the pin-a1 command starts before the pin-a0 command is done so the second command overwrites the output of the first.
It is a long standing quirk of PIC processors.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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Guest
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| Posted: Mon Jun 21, 2004 5:02 pm |
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| version 3.185 |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jun 21, 2004 5:49 pm |
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You need to turn off the comparators, as shown below.
I looked at the LST file, and that function should work OK
on your version of the compiler.
| Code: | void main()
{
setup_comparator(NC_NC_NC_NC); // Turn off comparators
output_high(PIN_A0);
output_high(PIN_A1);
while(1); // Prevent PIC from going to Sleep
} |
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Ttelmah
Guest
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| Re: Output_High() |
| Posted: Tue Jun 22, 2004 2:05 am |
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| The Rookie wrote: | When I use the following code:
output_high(PIN_A0);
output_high(PIN_A1);
the A0 goes high and as soon as A1 goes high A0 goes low.
Basically the output_high for A0 does not stick.
Any suggestion on how I can keep A0 high |
Others have mentioned the ADC, and comparator, but the other possibility is that you are suffering from the 'read_modify_write' problem.
Basically, whenever you do I/O on a bit of the port, the whole port is read, the bit is modified, and the result written back. Now the first instruction sets pin A0 'high', but the pin itself does not respond immediately. It starts to drive high at the end of the instruction, and the tme taken for the signal level to actually get to the 'high' detection point, will depend on the nature of the pin (whether it has Schmidt inputs), the capacitance and resistive load attached. Now the next instruction, reads the port as the first clock cycle of the instruction, then modifies the bit from what it has read, and outputs the whole byte at the end of th instruction. The problem is that it is common for pins with 'normal' loads, to take quite a significant time to actually 'read' as high, and when the two instructions are 'nose to tail', this may not yet have happened. This is mentioned both in the Microchip data sheets in various places, and in the CCS manual, but they don't really make it clear, just how long the timescales may be.
If this is the problem, there are three possible solutions:
1) If the chip involved is one of the latter '18' family chips, then instead of using output_high, consider writing directly to the LATB register. This is available on the latter chips to avoid this problem.
2) Try simply adding a delay between the first and second write.
3) Alternatively, keep a global variable containing the value you want to appear on port B, and set the bits on this, then output this as a whole byte using 'output_b'.
Best Wishes |
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