got the output check pulse thing working (I think)
diff --git a/bbb_cape/src/cape/data_struct.h b/bbb_cape/src/cape/data_struct.h
index 6d747bb..5815a34 100644
--- a/bbb_cape/src/cape/data_struct.h
+++ b/bbb_cape/src/cape/data_struct.h
@@ -86,6 +86,7 @@
uint32_t ultrasonic_pulse_length;
// The length of the pulse from the sidecar PWM output in 10MHz ticks.
+ // Goes to non-0 exactly once for each pulse received.
uint32_t output_check_pulse_length;
int32_t shooter_position, pusher_distal_posedge_position,