Buspirate Logic Analyzer Mode Issue

Question

I am new to the logic analyzer mode of Bus Pirate, so to play with it i connected channel 3(MOSI) line to the output of an TSOP1708 IR decoder to see if the capture came through.

However when I complete the capture all i get is a flat line @ HIGH. and because of this, trigerring does not work obviously since the signal does not seem to change at all.

I know my setup is working with the TSOP1708 output connected to a LED, which flashes when I press the remote.

Any idea what i might be doing wrong ??

Answer 1

Got it. I had forgotten all about the bus pirates small sampling size (4096 samples). So at a sampling rate of 1Mhz, trying to capture IR signals (approx 38KHz), the sampling was complete even before the IR signal had a chance to change !

Thank you all for help.

Answer 2

Your logic levels may not be set right for the BusPirate or the carrier frequency is not matched within a few %. 38KHz ? The BPF has a Q of 10 or in other words the 3dB BW is 10% of the IR Rx.

You need to supply 10K resistor between 5V and RX-pin for debug adapter input. Choose an SMT part and solder across the plug exposed wires. This doesn't explain why they are always high so make sure you have adequate cap across IR detector and check your logic threshold is not too low in the BusPirate and check that the ground levels are common with a DMM. 1.2V for TTL, 2.5V for HCMOS on 5V logic.

Mfg suggested filter is below.