Electronic – Binary Counter C Code for pic16f877a

ccountermicrocontrollerpicprogramming

I'm trying to write a code for an up/down binary counter. The task is to light up 8 leds (up counter)and then reverse the lighting order (down counter). I am trying to create an array of leds in my code but I don't know how to do so.Whenever I use PORTB[i] I get and error pointer required and if I use PORTB.i I also get an error how can i access the leds connected to portb ? It's my first experience with microcontrollers in general and I would really appreciate your help. Here's my code:

char LED_Direction at TRISB;
void main() {
int i;
LED_Direction=0x00;
PORTB=0x00;
//sbit LED_Array [8] = {PORTB.0,PORTB.1,PORTB.2,PORTB.3,PORTB.4,PORTB.5,PORTB.6,PORTB.7};
    while(1)
   {
      for (i=0; i<8; i++)
      {
          //LED_Array [i] = 1;
          PORTB[i]=1;
          Delay_ms(1000);
          //LED_Array [i] = 0;
          PORTB.i=1;
      }
      for (i=7; i>-1; i--)
      {
          //LED_Array [i] = 1;
          PORTB.i=1;
          Delay_ms(1000);
          PORTB.i=0;
          //LED_Array [i] = 0;
      }
    }
}

Best Answer

PORTB is neither a structure nor is it an array (or pointer).
Each bit in the value you assign to it directly corresponds to a pin.
Your line of code near the beginning of main() which reads 'PORTB=0x00;' is the correct way to assign values to PORTB.
If you want the least-significant bit high with the rest low, then set it to 0x01 (0b00000001).
If you want the most-significant bit high with the rest low, 0x80 (0b10000000) is your value.
A 0x55 (0b01010101) or 0xAA (0b10101010) will set alternating bits high & low.
You can also use bit-shifting to set a particular bit, so (1 << 3) will set the 3rd bit, and so on.

Related Solutions

Electronic – the reason the PIC16 multitasking RTOS kernel doesn’t work

What you are trying to do is tricky, but very educational (if you are prepared to spend a lot of effort).

First, you must realise that this kind of PC-only (as opposed to PC+SP) task switching (which is the only thing you can do on a plain 12 or 14-bit PIC core) will only work when all the yield() statements in a task are in the same funtion: they can't be in a called function, and the compiler must not have messed with the function structure (as optimization might do).

currentTask->pch = PCLATH;\
currentTask->pcl = PCL + 8;\
asm("goto _taskswitcher");

You seem to assume that PCLATH is the upper bits of the program counter, as PCL is the lower bits. This is NOT the case. When you write to PCL the PCLATH bits are written to the PC, but the upper PC bits are never (implicitly) written to PCLATH. Re-read the relevant section of the datasheet.
Even if PCLATH was the upper bits of the PC, this would get you into trouble when the instruction after the goto is on not on the same 256-instruction 'page' as the first instruction.
the plain goto will not work when _taskswitcher is not in the current PCLATH page, you will need an LGOTO or equivalent.

A solution to your PCLATH problem is to declare a label after the goto, and write the lower and upper bits of that label to your pch and pcl locations. But I am not sure you can declare a 'local' label in inline assembly. You sure can in plain MPASM (Olin will smile).

Lastly, to this kind of context switching you must save and restore ALL context that the compiler might depend on, which might include

indirection register(s)
status flags
scratch memory locations
local variables that might overlap in memory because the compiler does not realise that your tasks must be independent
other things I can't imagine right now but the compiler author might use in the next version of the compiler (they tend to be very imaginative)

The PIC architecture is more problematic in this respect because a lot of resources are loacted all over the memory map, where more traditional architectures have them in registers or on the stack. As a consequence, PIC compilers often do not generate reentrant code, which is what you definitely need to do the things you want (again, Olin will probaly smile and assemble along.)

If you are into this for the joy of writng an task switcher I suggest that you swicth to a CPU that has a more traditional organization, like an ARM or Cortex. If you are stuck with your feet in a concrete plate of PICs, study existing PIC switchers (for instance salvo/pumkin?).

Frequency counter using pic16f877a

There are two different common methods to measure frequency with this PIC. Which one is appropriate depends on things you haven't told us, like the minimum and maximum frequency.

The conceptually simple way is to do a frequency counter. Hardware can assist here in that you can have a external signal increment timer 0 or timer 1. Then the firmware reads the timer at known intervals, and the rest is simple math. Note that you do NOT want to reset the timer after each read. Simply do a unsigned integer subtract of the new reading minus the previous and you get the counts in the last interval.

You have to keep timer overflow in mind. Timer 0 is 8 bits and timer 1 16 bits. If using timer 0, for example, you have to be sure that the sampling period is short enough so that there are less than 256 cycles per period at the highest input frequency. With timer 1 you get a lot more room since it's 16 bits wide. You can therefore allow up to 65535 cycles per sampling period.

The other way to measure frequency is to actually measure the period of individual cycles or groups of cycles. This is best done by using one of the CCP modules in capture mode. Timer 1 is set up as a free running timer, and the CCP module takes a snapshot of it every 1, 4, or 16 periods of the external signal. In the CCP interrupt, you subtract the new capture reading minus the previous to get the time duration of the external period (or 4 or 16 periods), and the rest is math.

The CCP capture method is usually more accurate and responsive, but more tricky to implement and to deal with all the corner cases. With this method you have to worry about timer overflow for low input frequencies, not high. I usually deal with this by grabbing the timer value periodically faster than it can wrap, and effectively extending the timer to a arbitrary number of bits in firmware.

Best Answer

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Electronic – the reason the PIC16 multitasking RTOS kernel doesn’t work

Frequency counter using pic16f877a

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