Minimum interval on OutputCompare?

NETMF & Gadgeteer
1

I can’t get OutputCompare to switch any faster than 50us on the Panda. Is that the lower limit for OutputCompare?

I can enter lower values but anything below 50us creates a 50us pulse (measured by my logic analyzer).

( would have searched the forums for this but can’t find the search option )

2

for search go to:

Help --> Search forum.

Yes it’s been pointed out that the search needs to be under Forum :slight_smile:

3

Ah, there it is. Thanks!! (I would have searched the forum for it, but, yeah, couldn’t, um…)

See what happens–I get busy for a few weeks and everything changes when I get back :).

4

OC is meant for low precision stuff, like a fading LED. Use PWM for high res applications.

5

In addition to what Chris said, I guess 50 us is the minimum that can be handled :wink: This is very low and will keep the processor very busy.

6

I was able to do this on Panda using SPI, believe it or not. I used this declaration:

spi = new SPI(new SPI.Configuration((Cpu.Pin)FEZ_Pin.Digital.Di5, false, 0, 0, false, false, 400, spiModule));

(I don’t use CS so Di5 is a spare pin; I only use MOSI.) I had to tweak the SPI parameters to get the timing even: clock edge, hold time, etc, etc. At these timing settings when you send a series of bytes the clock timing is steady and continuous, even between bytes.

At 40kHz, each bit is 2.5us. To control the Christmas lights, each bit is 30us: either 10us high /20us low or 20us high/10us low (0/1 bit). So to send a bit:

1111 1111 0000 0000 sends 20us high, 10us low (and an extra 10us low)

To make the bit and byte boudaries line up, I map two bits into 3 bytes of SPA.

It works remarkably well, except encoding the data at the bit level in managed code is a little sloooow. :slight_smile: