Input issue on a Pic16F88

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
Hello all i am having some problems i cannot figure out on a project i am building. ill try to describe the best i can.

Pic 16F88 In 300 mil SOIC pkg.
20mhz (citizen crystal) 18pf caps

Port A4 is a input (momentary push button)
Port A0 is a input (Momentary push button)
Port A1 is a input (Momentary push button)
ALL ACTIVE HIGH have 4.7k pulldown resistors attached as well.

Software wise all my routenes are the same for reading the status of these buttons, BUT input A4 is erratic it will not work like it should, In my software i have a configuration area when A4 is help down it enters config, oddly it does this fine but once it reaches the main loop it ceases to function when the button is pressed. If i cycle the power to the ckt off and on really fast a few times the port a4 will work. if i shut it down wait a min and apply power it will not work. I have done a continuity check on all traces and wires and those are all good.Is it possible the Chip is bad or possibly overheated when i soldered it to the pcb damaging pin a4. anyone have any ideas?
 

Markd77

Joined Sep 7, 2009
2,806
RA4 has a Schmitt trigger input buffer but the others are TTL which means RA4 needs a higher voltage to read logic 1 (0.8Vdd) so 4V with a 5V supply.
As described the circuit should work, but maybe increasing the value of the pulldown resistors might help. Also check the voltage at the pin with the button pressed.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
RA4 has a Schmitt trigger input buffer but the others are TTL which means RA4 needs a higher voltage to read logic 1 (0.8Vdd) so 4V with a 5V supply.
As described the circuit should work, but maybe increasing the value of the pulldown resistors might help. Also check the voltage at the pin with the button pressed.
Thanks for the input Mark, i am going to go measure the voltage on the pin right now will report back shortly. What value do you propose i should try on the pulldown in lieu of the 4.7k
 

Markd77

Joined Sep 7, 2009
2,806
4.7K is in the area which should work, but maybe 20K might be worth trying. It's probably not the problem, the button would have to be nearly 1K in it's on position to cause the problem, and even membrane switches are normally only about 100 ohms.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
4.7K is in the area which should work, but maybe 20K might be worth trying. It's probably not the problem, the button would have to be nearly 1K in it's on position to cause the problem, and even membrane switches are normally only about 100 ohms.

I measured 4.844V at the pin with the wire shorted to the 5v reg. I removed the switch and just shorted the pin. I got 4.844v and it still does not work as expected.
 

Markd77

Joined Sep 7, 2009
2,806
It's good to rule things out, the problem must be the PIC or the code. If you post the code someone will probably have a look.
 

crutschow

Joined Mar 14, 2008
23,514
Could switch bounce be a problem? Do you have delay in the loop of about a 100ms or so before it can respond to another switch change?
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
Could switch bounce be a problem? Do you have delay in the loop of about a 100ms or so before it can respond to another switch change?
i wrote the code in flowcode. i am useing the same routenes to read all 3 switches the pins A0-A1 work, A4 does not work for some reason. it is not the switches though cause i removed them and shorted the pin directly to 5v, a0-a1 respond, a4 still does not.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
Whats really odd is if i cycle the power to the ckt (12v applied) to a 5v reg. on off on off really fast and keep it on, Pin a4 will respond. if i let it sit there and just apply power without cycleing it, it does not respond.
 

PaulEE

Joined Dec 23, 2011
423
This sounds like a stupid suggestion, but I have wasted countless hours on a circuit like this before. The reason? I forgot to configure the port I/O as DIGITAL I/O as opposed to ANALOG. Check the ANSEL register to be sure that the power on reset value of the pin you're having issues with #1 doesn't conflict with other default-on modules and #2 is configured as digital and not analog.

Again, I'm sure you've already checked this, but I had to say it because of the countless hours I've spent troubleshooting similar issues.

Also, with reference to the Schmitt Trigger comments...Schmitt Triggers are just buffers with hysteresis to quiet noisy signals.

The 16f88 datasheet specifies a logic low on a Schmitt Trigger as 0.2*Vdd (1 V for Vdd = 5v) and 0.8 * Vdd = 4v for logic high. BUT, these are MAXIMUM values..in other words, the upper limit of the logic low/high are 1.0v and 4.0v, that doesn't imply that they have to be those values. Technically, logic high could be anywhere above 1.0v to 4.0v to be valid.

I have a few 16f88s I plan on doing a few tutorials on sitting right here with me...nice little chips. The 12f683 is also excellent.

Hope this helps!
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
i did a quick search in the C outputted from flowcode and here is what it says.

//Initialisation
ansel = 0;
cmcon = 0x07;

//Interrupt initialisation code
option_reg = 0xC0;
 

PaulEE

Joined Dec 23, 2011
423
i did a quick search in the C outputted from flowcode and here is what it says.

//Initialisation
ansel = 0;
cmcon = 0x07;

//Interrupt initialisation code
option_reg = 0xC0;
You're good. ANSEL = 0'b 00000000 means all I/O on pins that have ANx labels are set as digital I/O. Had to check!
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
You're good. ANSEL = 0'b 00000000 means all I/O on pins that have ANx labels are set as digital I/O. Had to check!

What else can i check that could be a problem. This problem is driving me nuts! I am useing the same routene to check the status of the switches. the other ones work just a4 dont. im at a loss as to what it could be!
 

PaulEE

Joined Dec 23, 2011
423
What else can i check that could be a problem. This problem is driving me nuts! I am useing the same routene to check the status of the switches. the other ones work just a4 dont. im at a loss as to what it could be!
I, personally, am not familiar with "flow code".

When I set out to do a PIC project, I come up with a plan and then go through each pin and decide whether or not any internal modules are on by default and am sure to shut each one off in code. (The ones that will interfere with my I/O on that port/pin(s). )

The datasheet, with reference to "I/O Ports" - "PORTA and TRISA", lists all the different functions of PORTA.B4. The datasheet claims "Input/output, analog input or TMR0 external input or comparator input".

Translated by my brain: Set as digital I/O in ANSEL, shut off ADC module, CCP module, and check that TMR0 interrupt isn't messing with your code either. Look at the interrupt structure and make sure that one is disabled. This may or may not explain the sporadic behavior with power cycling and time going by. Also, CMCON, ADCON0, ADCON1, CCP1CON, ANSEL, CMCON0 and a few other registers are work checking out. Most of those were O.K. on power-on reset, though.

You're positive that it's RA4 that's giving you issues, not RA5, right? Had to ask.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
I, personally, am not familiar with "flow code".

When I set out to do a PIC project, I come up with a plan and then go through each pin and decide whether or not any internal modules are on by default and am sure to shut each one off in code. (The ones that will interfere with my I/O on that port/pin(s). )

The datasheet, with reference to "I/O Ports" - "PORTA and TRISA", lists all the different functions of PORTA.B4. The datasheet claims "Input/output, analog input or TMR0 external input or comparator input".

Translated by my brain: Set as digital I/O in ANSEL, shut off ADC module, CCP module, and check that TMR0 interrupt isn't messing with your code either. Look at the interrupt structure and make sure that one is disabled. This may or may not explain the sporadic behavior with power cycling and time going by. Also, CMCON, ADCON0, ADCON1, CCP1CON, ANSEL, CMCON0 and a few other registers are work checking out. Most of those were O.K. on power-on reset, though.

You're positive that it's RA4 that's giving you issues, not RA5, right? Had to ask.
Yes im positive its A.4 giving me the problem. ;/ lol damn bugs!
 

PaulEE

Joined Dec 23, 2011
423
Can you send me the entire output of the flow code? Perhaps I can spot an issue better looking directly at the C that it spits out...
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
i have sent it to your inbox. I will be leaving for a little bit. i will be back in about 2 hours. let me know if you find anything wrong. at quick glance.
 
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