Bit Angle Modulation (BAM) in a PIC

[Mike, K8LH]

Someone asked about the diagonal line on my Charlieplexed display sketch. It's what I refer to as the "float" pin and it's connected to the LEDs that occupy positions in the matrix that would otherwise share a common column and row (RB0/RB0, RB1/RB1, etc.).

I apologize if the diagonal line has thrown you off. It's a remnant from some of my sketches of Charlieplexed 7-segment displays.

Both drawings below are electrically the same but the second drawing below may make more sense. Basically I just slid those LEDs in the bottom row up into the empty spaces in the matrix.

Kind regards, Mike

[Mike, K8LH]

Darrel,

I was just wondering -- is MIBAM even necessary on a multiplexed display due to the long pauses between LED activation. You know, column one LEDs would be BAM'ed then turned off while we sequencially BAM column two LEDs, then BAM column three LEDs, etc.?

[Darrel Taylor]

Hi Mike,

I wholeheartedly agree.
MIBAM with multiplexed display doesn't seem like it's worth it, because you loose so much of the available dutycycle.

Thanks for the explanation of the diagonal line.
I did get your example working, with only 8 leds (2 columns).
I made enough changes to not know if it's actually the way yours works or not.

But I don't see a way to incorporate it into the MIBAM routines (because of the duty cycle losses), so I'm officially tossing Charlie out the window.
Good thing I live at ground level.

I'll complete the modifications for your previous idea (full port updates) and get this thing done.

Thanks for all your assistance Mike, much appreciated.

[Mike, K8LH]

Yes, LED duty cycle is decreased with any multiplexed display and while we normally pump up the "peak" current to get better "average" current (and brightness) we can't do that with a Charlieplexed display (or any other multiplexed display that uses "direct drive" for matrix columns or rows).

Anyway, I thought you might enjoy taking a peek at a multiplexed 8-channel RGB display demo' (24 discrete LEDs) with 256-level BAM driver and a pretty amazing 868-Hz refresh rate. The display is fashioned as a 3x8 matrix (33% duty cycle) so biasing the LED "peak" current to approximately 3x desired "average" current will be necessary. This assembly language program just contains the driver. No animation or anything like that, yet.

Regards, Mike

[Mike, K8LH]

Is there any way I can get temporary access to locked post above to update the program listing? I posted incorrect version which has some bugs. Sorry!

[Mike, K8LH]

Darrel (and gang),

A long overdue assembly language Charlieplexed BAM example for 20 LEDs on a 12F683. Example uses 8-bit 256 step BAM driver with a 1-usec step size. Refresh rate is 781-Hz with a 1280-usec frame rate. LED "0" is animated. That is, it ramps from duty cycle value of 0 up to 127 and then back down to 0. LED "10" through LED "19" are lighted with duty cycle values of 1 through 10 so you can check out "low end" illumination.

You might just hook up a single LED for LED "0" to check out the program. Just connect the LED cathode to GP5 and the LED anode to GP0.

Kind regards, Mike

[Mike, K8LH]

Has anyone tried "gamma" correction to smooth out LED brightness level changes? I just added a 64 level rom "gamma" array to the program in the previous post and it made an amazing difference in the smoothness of the fade.