Bit Angle Modulation (BAM) in a PIC

[Darrel Taylor]

What's the processor utilization per MHz per LED?

Utilization is less than 6% of total.

The limiting factor is the Least Significant Bit in the dutycycle. It's so short that it doesn't give much time to do anything. The mirror image helped some, because I was able to combine the LSB of both halves of the mirror for a longer period, but it's still only 68 instructions (worst case).

It has to get into the interrupt handler, save context, reload the Timer, service all the LEDs dutycycles, keep track of the bit sequencing, restore context and exit the interrupt within those 68 instructions.

Each PIN uses 6 instructions, and the rest of the handler uses about 40.
So @ 4 mhz you can only get 4 LED's (40 + 24 = 64).
Refresh rate @ 4mhz = 80 hz with 4 LEDs.

With higher OSC freqs, refresh rates can be as high as 650hz, depends on how many LEDs there are.
The rate can be set lower with a DEFINE if needed.

Warnings from the assembler will indicate if your setup is out of range.
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[JEC]

Utilization is less than 3% of total.

The limiting factor is the Least Significant Bit in the dutycycle. It's so short that it doesn't give much time to do anything. The mirror image helped some, because I was able to combine the LSB of both halves of the mirror, for a longer period, but it's still only 68 instructions (worst case).

It has to get into the interrupt handler, save context, reload the Timer, service all the LEDs dutycycles, keep track of the bit sequencing, restore context and exit the interrupt within those 68 instructions.

Each PIN uses 6 instructions, and the rest of the handler uses about 40.
So @ 4 mhz you can only get 4 LED's (40 + 24 = 64).
Refresh rate @ 4mhz = 80 hz with 4 LEDs.

With higher OSC freqs, refresh rates can be as high as 650hz, depends on how many LEDs there are.
The rate can be set lower with a DEFINE if needed.

Warnings from the assembler will indicate if your setup is out of range.
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How flickery is 80 Hz x 4 @ 4 MHz?

[Darrel Taylor]

How flickery is 80 Hz x 4 @ 4 MHz?

Even though it's 80hz, the ducycycle is put out to the pin twice, so it's visually better than 80hz PWM would be.

I don't notice any flicker.
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[PicLearner]

Bam sounds great. Does anybody have a sample code? Thank you in advance.

[gmglickman]

I played with a very simple form of BAM a while back, with at least 24 LEDs on a 20MHz 16F877A. This is the main loop:

     BAM:                        'each pass is one cycle, equiv to one PWM period
delay_index=%00000001
for i=0 to 7                        'time slots, 0 shortest, 7 longest
    PORTA.1=LED1>>i               
    PORTA.2=LED2>>i          
    PORTA.3=LED3>>i 
    .
    .
    .
    PORTA.n=LEDn>>1
    for j=1 to delay_index      'empty delay loop for each time slot, total 255
    next j
    delay_index=delay_index<<1
next i  
GOTO BAM

where LED1, LED2 are intensity values that are updated when needed by interrupt. Refresh is totally dependent on clock speed and number of LEDs driven, but 100Hz is achievable.

I came across a scheme to reorder bits to avoid phase shift glitches but didn't get that far myself. See http://www.tenmilesquare.com/library

I'd love to see Darrel's code...

Gary

[Darrel Taylor]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=3157&stc=1&d=123328553 8" />

Had that come up first, I woud have tried it. Looks interesting.

But the mirror image seems to work great, so no reason to change horses just yet.

There's 3 of our regular members looking at it now.
I like knowing they can set CONFIGs and disable A/D on their own.

Barring any problems, I'll post it soon.
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[Darrel Taylor]

After realizing that Pebbles is probably 60-70 years old by now ...
I'm not so interested in BAMBAM anymore.

The name has been changed to MIBAM. (Mirror Imaged Bit Angle Modulation)

And the code for PicBasic Pro is now available here ...
http://www.picbasic.co.uk/forum/showthread.php?t=10564

Cheers,