Settling Time after Reset?

Hi all,

Just for practice in learning GC Basic, I whipped up a circuit to light the elements of a ten-element LED bar display, sweeping them back and forth. Note that I am doing this in dot mode (only one element lit at a time) thus keeping the current under the max ratings of the PIC16F88. All ten cathodes are ganged and terminate through a single 330 ohm resistor to ground. Port lines B.0 through B.7 and A.0 and A.1 directly drive the display. Here's the code:

;A program to sweep 10 elements of an LED bar graph.
;In this direct connection circuit, only one LED
;element may be lit at a time to stay under max current.
;********************* Configuration **********************
#chip 16F88, 4            ;PIC16F88 running at 4 MHz
#config mclr=off          ;reset handled internally
#config osc=int           ;use internal clock
;************************ Constants ************************
#define bitsLow PortB     ;low eight bits of display here
#define bitsHigh PortA    ;high two bits of display here
;************************  Program *************************
pgm:
  dim pattern as word alias patternHigh, patternLow ;LED pattern
  wait 1 ms                       ;let the PIC settle down
  do
    pattern = 1                   ;begin with 1st LED
    
    for i = 1 to 10               ;ten LEDs in display unit
      bitsLow = patternLow        ;set two parts separately
      bitsHigh = patternHigh      
      
      wait 100 ms                 ;pause a bit
      rotate pattern left simple  ;get set for the next one
    next 
    
    pattern = 256                 ;now sweep backwards
    
    for i = 9 to 2 step -1        ;9th to 2nd element
    bitsLow = patternLow
    bitsHigh = patternHigh        ;set two parts separately
      
    wait 100 ms                   ;pause a bit
    rotate pattern right simple   ;get set for the next one
    next 
  loop                            ;repeat in perpetuity
end

It's not terribly interesting, but it was a chance to learn about DIM, WORD and ALIAS. I was delighted to see that all of these work, and also that GC Basic automatically set the ports for digital output.

But look at the attached code, if you would. Notice the WAIT 1 ms command at the outset. After several hours of poking around, I found I had to have this. Without that line, the project started up in a mixed-up manner. Sometimes all LEDs were lit, or perhaps a mixture. And then after a slight pause the circuit began operating as expected.

I'm conjecturing that at power-on reset a slight delay is needed (1 ms was what I found worked) for Ports A and B to settle down or for the ports or registers to stabilize.

Does that sound reasonable?

Note that I'm using the internal !MCLR and internal clock. I'm still slogging through the PIC16F88 data sheet but was hoping some experienced person here had ever noticed this.

Again, I'm guessing the internal oscillator needs this brief delay to come up to speed, or perhaps the ports need to settle.

What do you think?

Thomas Henry

You could try turning on the power up timer in your config section - that will hold the chip in reset for about 72ms while the supply voltage stabilises. It defaults to off according to the data sheet.

At the moment the chip is probably executing code as soon as there is sufficient voltage for the core to work but maybe not enough for the peripherals to function or not enough time for everything to initialise.

Hi,

Thanks for the suggestion which was a good one. I played with the power-up timer flag, but with no joy. After a couple more hours, I figured out that what I had observed was a red herring. I believe the answer lies elsewhere. In particular, my experiments suggest that the problem may lie with the GC Basic routine which handles the automatic pin direction setting. Accordingly, I've moved this problem to a new thread.

Thanks again,

Thomas Henry