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FizzBuzz in PIC16f877A driving me crazy..

madthescientist — Fri, 28 Aug 2026 00:38:46 GMT

I am trying to create a firmware for Pic16f877A, using mplab 6.15 on gnu/linux and pickit 3.

After finding out code examples in C language i could try and test parts to build a working piece of source code for it. However, i am finding myself at odds whenever i do wish to get actual work done.

I do wish to drive some leds on an off in sequences, sometimes turning on one, off other several times and holding them off some minutes to do it back. I could accomplish part of it using something called "fizz buzz" example and reading ticks from Timer 1's prescaler. My C and PIC knowledge is not yet the best.

The major problem i am struggling with is that, i can get 2 leds toggling one after the other, but if i try to get that within a FOR cycle, it either never stops if i do not add any code at FOR's end, or if i add code, it inmediatly jumps to it avoiding to toggle the leds altogether. If i code them off after for() they stay off, if i code them on after for, they stay offf.

The source code i am in trouble with is this one:

[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:1a56b7dc-8464-438d-b459-00b95ecaf22b:type=c_cpp&text=void%20MainProgramme%28void%29%0A%7B%0A%2F%2AEssential%20Variables%20Are%20Declared%20Here%2A%2F%0ALed3%20%3D%200%3B%20%2F%2FSeems%20It%20Has%20To%20Be%20Turned%20Explicitly%20Off%20Before%20Anything...%0A%0Aunsigned%20int%20tick%20%3D%200%3B%20%2F%2FA%20Common%20Variable%20For%20Measuring%20Timer0%20Ticks%0Aunsigned%20int%20activations%20%3D%200%3B%20%2F%2FThis%20Is%20For%20Counting%20Movements.%0Aunsigned%20int%20times%20%3D%200%3B%20%2F%2FThis%20Is%20For%20Counting%20SubFunction%20Executions.%0Aunsigned%20int%20blinks%20%3D%200%3B%2F%2F%20This%20Is%20For%20Programme%20Led%20Blink%20Timer%200%20Overflow%20Counting.%0Aunsigned%20int%20counter%20%3D%201%3B%0A%0A%0A%2F%2AThe%20Function%20Actually%20Starts%20Here%2A%2F%0A%0Awhile%28Button1%20%3D%3D%200%29%20%2F%2FChecks%20If%20Button1%20Is%20Depressed%20Aiming%20Towards%20Turning%20Off%20The%20Machine%0A%7B%0Awhile%28Run%20%3D%3D%200%29%20%2F%2FChecks%20If%20Run%20Button%20Is%20Depressed%20Toggling%20Between%20Work%20Or%20Pause%0A%7B%0A%2F%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2F%0A%2F%2AMain%20Led%20Stays%20Blinking%20At%20Around%202hz%20While%20The%20Procedure%20Takes%20Place%20If%20Not%20Paused%2A%2F%0A%0Atick%2B%2B%3B%0A%0A%2F%2F%20Check%20if%20Timer0%20has%20overflowed%0Aif%28TMR0IF%20%3D%3D%201%29%0A%7B%0ATMR0IF%20%3D%200%3B%20%2F%2F%20Reset%20the%20flag%20immediately%0Ablinks%2B%2B%3B%0A%0A%2F%2F%207%20overflows%20~%20458ms%20%28close%20to%200.5s%20for%201Hz%20toggle%29%0Aif%28blinks%20%3E%3D%20266%29%20%7B%0AMainLed%20%3D%20~MainLed%3B%20%2F%2F%20Toggle%20LED%20on%20RB0%0Ablinks%20%3D%200%3B%20%2F%2F%20Reset%20counter%0A%0A%7D%0A%0A%0A%0A%2F%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2F%0Aif%28Led7%20%3D%3D%200%29%0A%7B%0A%2F%2F--------------------------------------------------------------%0Awhile%28Switch2%20%3D%3D%200%29%0A%7B%0A%0ABlink2%20%3D%200%3B%0ABlink1%20%3D%200%3B%0ALed3%20%3D%200%3B%0A%0A%2F%2F%20Timer%200%20For%20Programme%20Led%20Blinking.%20IT%20WORKS%2C%20AVOID%20BREAKING.%0A%2F%2F%20Check%20if%20Timer0%20has%20overflowed%0Aif%28TMR0IF%20%3D%3D%201%29%0A%7B%0ATMR0IF%20%3D%200%3B%20%2F%2F%20Reset%20the%20flag%20immediately%0Ablinks%2B%2B%3B%0A%0A%2F%2F%207%20overflows%20~%20458ms%20%28close%20to%200.5s%20for%201Hz%20toggle%29%0Aif%28blinks%20%3E%3D%20266%29%20%7B%0AMainLed%20%3D%20~MainLed%3B%20%2F%2F%20Toggle%20LED%20on%20RB0%0Ablinks%20%3D%200%3B%20%2F%2F%20Reset%20counter%0A%0A%7D%0A%7D%20Led6%20%3D%201%3B%0A%7D%20Led6%20%3D%200%3B%0A%7D%20%2F%2F%20Main%20Programme%20Led%20Blink%20Ends%20Here.%20AVOID%20TOUCHING%20THIS%20PART%2C%20IT%20WORKS.%0A%2F%2F------------------------------------------------------------------------%0Aif%28Led7%20%3D%3D%201%29%0A%7B%0Awhile%28Switch1%20%3D%3D%200%29%0A%7B%0A%0ABlink2%20%3D%200%3B%0ABlink1%20%3D%200%3B%0ALed3%20%3D%200%3B%0A%0A%2F%2F%20Timer%200%20For%20Programme%20Led%20Blinking%0A%2F%2F%20Check%20if%20Timer0%20has%20overflowed%0Aif%28TMR0IF%20%3D%3D%201%29%0A%7B%0ATMR0IF%20%3D%200%3B%20%2F%2F%20Reset%20the%20flag%20immediately%0Ablinks%2B%2B%3B%0A%0A%2F%2F%207%20overflows%20~%20458ms%20%28close%20to%200.5s%20for%201Hz%20toggle%29%0Aif%28blinks%20%3E%3D%20266%29%20%7B%0AMainLed%20%3D%20~MainLed%3B%20%2F%2F%20Toggle%20LED%20on%20RB0%0Ablinks%20%3D%200%3B%20%2F%2F%20Reset%20counter%0A%0A%7D%0A%7D%0ALed6%20%3D%201%3B%0A%7D%20Led6%20%3D%200%3B%0A%7D%20%2F%2F%20Main%20Programme%20Led%20Blink%20Ends%20Here.%20AVOID%20TOUCHING%20THIS%20PART%2C%20IT%20WORKS.%0A%2F%2F------------------------------------------------------------------------------------------------%0A%0A%0A%0A%0A%2F%2FTimer%201%20For%20Blink%20Led%20Timings%0Awhile%28Switch1%20%3D%3D%201%20%7C%7C%20Switch2%20%3D%3D%201%29%0A%7B%0A%0A%0A%2F%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2A%2F%0A%2F%2AProgramme%20Led%20Seems%20To%20Be%20Blinked%20Locally%2A%2F%0A%2F%2F%20Check%20if%20Timer0%20has%20overflowed%0Aif%28TMR0IF%20%3D%3D%201%29%0A%7B%0ATMR0IF%20%3D%200%3B%20%2F%2F%20Reset%20the%20flag%20immediately%0Ablinks%2B%2B%3B%0A%0A%2F%2F%207%20overflows%20~%20458ms%20%28close%20to%200.5s%20for%201Hz%20toggle%29%0Aif%28blinks%20%3E%3D%20198%29%20%7B%0AMainLed%20%3D%20~MainLed%3B%20%2F%2F%20Toggle%20LED%20on%20RB0%0Ablinks%20%3D%200%3B%20%2F%2F%20Reset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RE: FizzBuzz in PIC16f877A driving me crazy..

beacon_dave — Sat, 29 Aug 2026 14:35:23 GMT

[quote userid="121623" url="~/technologies/embedded/f/pic-micro-forum/57220/fizzbuzz-in-pic16f877a-driving-me-crazy/237959"] is this because you didn't use any or because E14 lost it ?[/quote]

Probably because the Code widget defaults to 'Text'

and needs to be set to 'C'

RE: FizzBuzz in PIC16f877A driving me crazy..

michaelkellett — Sat, 29 Aug 2026 14:13:59 GMT

An unsigned variable can reach zero, but it can't be negative.

Change

for(times = 10; times >= 0; times --)

for(times = 10; times > 0; times --)

or better to

for(times = 0u; times < 10u; times++)

(The u tells the C compiler that the 10 (and the 0) are unsigned - arguably pedantic and it will work without in this case)

(and when you have a free choice make your 'for' loops increment because it's much more common and will help you avoid the occasional silly error, and you never get caught by the >=0 trap)

I find your code almost impossible to follow because there is no indentation - is this because you didn't use any or because E14 lost it ?

It would be good if you could repost it with indentation if you can:

(I did this in Notepad++ set for C language)

The indentation style is based on https://barrgroup.com/sites/default/files/barr_c_coding_standard_2018.pdf

I'll take another look then and see if I can work out what is happening.

RE: FizzBuzz in PIC16f877A driving me crazy..

madthescientist — Sat, 29 Aug 2026 12:55:43 GMT

I named it unsigned since most examples i found in the web uses unsigned, i did not know that it prevents it to reach zero.

RE: FizzBuzz in PIC16f877A driving me crazy..

madthescientist — Sat, 29 Aug 2026 12:55:00 GMT

Yes, even pins have their names on them. Watchdog is off.

RE: FizzBuzz in PIC16f877A driving me crazy..

Andrew J — Fri, 28 Aug 2026 14:36:46 GMT

Do you have a routine that sets up the input/output pins and the timers?

RE: FizzBuzz in PIC16f877A driving me crazy..

vmate — Fri, 28 Aug 2026 05:14:12 GMT

I don't quite fully understand what you did so far and what the goal would be, but I can see several problems:

The for loop you made is an infinite loop, because 'times' is unsigned, aka. it can never be lower than 0, and the condition for the for loop to finish is 'times' being lower than 0.

Also, the 'if (counter % 3 == 0)' part increments 'times', and the for loop decrements it, so they are actively fighting one another. You might've intended for that to happen, but this is not generally a good idea to do: the rule of thumb is to use a for loop if you already know how many times you want the loop to run, or use a while loop if you don't.

Another problematic part is the speed of execution. You're checking the counter's value in the for loop in multiple places, but that loop will execute many thousands of times faster than the counter's increment speed.

If I were you, I'd probably start from scratch, and ignore the hardware timer for now. Have a big while(1) loop with a __delay_ms(10); at the end, and just increment your own counter somewhere. Getting an Arduino, and going through the Arduino IDE's examples(like 'Blink without delay()') to learn the logic and program flow for such a microcontroller would probably be the ideal path, as the learning curve is much gentler there.

RE: Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

James123 — Thu, 07 Nov 2024 06:39:53 GMT

ADC input signals are really sensitive to source impedance, and it's generally recommended to keep the maximum impedance of your analog signal source around 10 kΩ. If the source impedance is too high, it can lead to longer sampling times, which may mess up your conversion results. This could be one of the issues you're facing when using an internally generated negative voltage reference.
To avoid problems, make sure your circuit design doesn't introduce extra impedance that could affect the ADC's performance. For more detailed information, check out the PIC16F18877 datasheet, especially the sections on ADC configuration and operation.

Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

Mila_G — Tue, 28 Nov 2023 23:41:38 GMT

Hello,

please can you help me with the wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877?

I would like to use ADC with references 0.416 V and Vdd (5 V).
It works perfectly when I connect 0.416 V to the Vref- pin (RA2) from an external source.

Then I generate 0.416 V on RA2 from FVR and D/A converter as follows:
FVRCON = 0x84;
DAC1CON0 = 0xA8;
DAC1CON1 = 0x0D;
The DAC also works perfectly and there is about 0.416 V on the Vref- pin (RA2).

But now ADC works only if the negative reference is Gnd. If I switch this from Gnd to Vref- (just by changing ADREF from 0 to 0x10), a failure occurs. The voltage on the RA2 pin changes to 2.1 V (and of course the A/D gives bad results).

Workaround attempts:
- 0.416 V output from DAC to RB7, directly conneted to RA2 via a wire, works just as badly as above.
- After interconnecting via an op amp voltage follower instead of the wire, the A/D starts to work properly.

I assumed the PIC analog circuits were designed for internal connections from the VRef- pin, without any external workaround. Please, advise.

Many thanks.

Mila.

MAX96717F is not recognized by the GMSL SerDes Gui

bugtech — Wed, 18 Sep 2024 09:23:25 GMT

Hi.

I am using MAX96717F for a task. It worked fine for a week but now suddenly the device is not getting identified by the GMSL SerDes Gui. Although when i configure the CFG Pin, it shows that Serializer Detected.

1: Tried with different firmware (4.55, 4.56, 4.59)

2: Tried with connections (Complete setup, camera to serializer, serializer to deserializer, deserializer to RPi4)

3: Tried without the connection (Only serializer connected by the USB)

4: Tried reninstalling the GUI multiple times. No help!

5: Tried on a different PC. No help!

Stuck here. Unable to understand what can be the issue.

Any help here?

Thanks

RE: MAX96717F is not recognized by the GMSL SerDes Gui

James123 — Wed, 06 Nov 2024 02:06:21 GMT

It seems like there might be a problem with the MAX96717F hardware itself.

RE: PIC18F452 with GPS NEO6M UART

ArthurHEL — Mon, 11 Dec 2023 08:47:31 GMT

Hello again,

Your reply made me think a lot and after a few tests, I found out what was wrong and it's quit simple ahah.

When I come out of my first loop, I have to disable communication (probably to avoid reading unwanted data) and re-enable it when I've finished displaying my various data!
I've displayed the time and one of the two GPS coordinates and it's working perfectly!

Thanks for your help, have a nice day!

PIC18F452 with GPS NEO6M UART

ArthurHEL — Sun, 10 Dec 2023 14:02:14 GMT

Hello,

I'm an electronics student and for my final project I'm trying to display data from a GPS (nemo 6m) operating via RS232 communication on an LCD screen with my PIC18F452. I had made a code for the LCD without a library, which worked without a hitch and which I used in this code.

The code for displaying data from an RS232 communication is relatively simple and works very well with a TTL to USB card.

The code below for displaying GPS data on the LCD works, but only once. When I run the program, it displays the time correctly, but I have to press the reset button to refresh. It seems that the code is stuck somewhere....

After many tests, I don't understand why and that's why I'm coming to you.
Thanks in advance for taking a look.

Technical specification:
16MHz quartz

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RE: PIC18F452 with GPS NEO6M UART

JWx — Sun, 10 Dec 2023 19:50:16 GMT

it seems that it generates buffer overflow - what happens when (inside second loop) y > 95? Loop reads data beyond tabGPS end... Question is if it can hang the code? Maybe you can put some debug (cycle some led or print on uart console) inside while loop to check if it iterates or stops?

RE: Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

scottiebabe — Wed, 29 Nov 2023 16:42:54 GMT

You may consider doing the operation digitally. Your increasing the sensitivity 8% or 0.1 bits changing the FSR from 5V to 4.6V. You could subtract 416 mV in software. Sounds like a neat project, best of luck.

RE: Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

shabaz — Wed, 29 Nov 2023 16:36:29 GMT

You explicitly mention in your workaround attempts:

Workaround attempts:
- 0.416 V output from DAC to RB7, directly conneted to RA2 via a wire, works just as badly as above.

I'm merely explaining the pointlessness of doing that, since the datasheet makes it clear that would not be possible.

You have a solution, which is a buffer.

If you are unwilling to accept a workaround, then I'm no expert here but it's clear from the datasheet that what you're suggesting is not possible.

Check the diagram below. The negative reference input to the ADC comes from either a pin, or from ground. You can see it in red.

The DAC output (described as a DAC reference output in the datasheet) in usable internally , as an input to the channel (shown in blue below), not as a input to the ADC negative reference.

If you provided more information about your actual use-case (what you're measuring for instance) then maybe there are other solutions, but trying to make an internal connection that doesn't exist won't work.

RE: Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

Mila_G — Wed, 29 Nov 2023 12:49:27 GMT

I have read the PIC specs carefully: "A buffer must be used on the DAC reference output for external connections to the DAC1OUT1/2 pins.", see above. But I would like to use internal connection! No user external circuitry conneted to the RA2 pin, no wiring, but only internal connection from RA2 to the ADC ref. input inside the MCU.

Many thanks. Mila.

RE: Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

shabaz — Wed, 29 Nov 2023 01:50:21 GMT

I didn't read it all, and I have not used a PIC for many years, but looking here:

If I've understood your requirement right (I might not have - I'm not used to these registers and other detail you mention) it's not a surprise if wiring the DAC to anything requires a buffer. The DAC appears to be composed of a ladder of resistors with no buffer from what I can tell, so yes, you'd need to add your own buffer (e.g. op-amp) when realistically connecting that externally to anything.

If you mention your actual end requirement (including what you're actually trying to connect the ADC to), and why the external buffer is not usable, then perhaps a different or better solution could be provided.

RE: And the PIC Fans go wild - PIC18 in Tesla Roadster BMS

michaelkellett — Thu, 23 Nov 2023 08:43:06 GMT

Kudos to Elon and Tesla for actually open sourcing this kind of stuff.

I can't think of any other major car maker to do so.

Just a pity Elon can't be more consistently admirable 🙁

And the PIC Fans go wild - PIC18 in Tesla Roadster BMS

scottiebabe — Wed, 22 Nov 2023 19:33:48 GMT

https://service.tesla.com/roadster

RE: And the PIC Fans go wild - PIC18 in Tesla Roadster BMS

Fred27 — Thu, 23 Nov 2023 08:27:05 GMT

*Starts rummaging through parts bin looking for a PIC18 and a Lotus Elise*

RE: And the PIC Fans go wild - PIC18 in Tesla Roadster BMS

scottiebabe — Wed, 22 Nov 2023 20:09:10 GMT

LOL 😭 That;s too funny! Lots of standby legends BAT54, 1n4148, LP2985

RE: And the PIC Fans go wild - PIC18 in Tesla Roadster BMS

Jan Cumps — Wed, 22 Nov 2023 20:05:04 GMT

checking for a 555 timer ...

PIC10F microcontroller output pin tolerance to overriding voltages

davebullockmbe — Mon, 06 Nov 2023 09:44:42 GMT

I am using PIC10F microcontrollers in a wireless project that has evolved into needing to use an 'in circuit' programmable radio IC.

The problem is that the GP2 pin of the PIC would be connected to the DATA input (and programming) pin3 of the radio IC.
The whole circuit is running off 3v7 lipo battery.
My question is will the PIC GP2 output be destroyed when programming the radio IC. ?

Thanks in anticipation....

Dave

RE: PIC10F microcontroller output pin tolerance to overriding voltages

michaelkellett — Tue, 07 Nov 2023 09:17:26 GMT

You can set GP2 as an input to prevent it from loading the radio pin during programming:

From the Microchip data sheet.

5.2 TRIS Registers
The Output Driver Control register is loaded with the
contents of the W register by executing the TRIS f
instruction. A ‘1’ from a TRIS register bit puts the corresponding
output driver in a High-impedance mode. A
‘0’ puts the contents of the output data latch on the
selected pins, enabling the output buffer. The exceptions
are GP3, which is input only and the GP2/T0CKI/
COUT/FOSC4 pin, which may be controlled by various
registers. See Table 5-1