DIN sync in x0x boxes (was RE: [sdiy] Help-Blacet midi-din sync chip problems!)

[thescum at surfree.com]

> This shows the interrupt routine is also responsible for polling the DIN
> Clock input.
> A pulse shorter than the interrupt period can start and finish in
> between 2 interrupts, and not be seen by the CPU.
> If the main program loop was polling for the clock input, it wouldn't
> lose sync if it was being interrupted less than usual.

I can offer some evidence in support of this theory.

Kyle Jarger's MIDI-CV boxes ruen incoming MIDI clock messages into very short pulses on the DIN clock output...something like 5 mS wide, not related to the tempo.

He had several reports of TR808s not following his sync output, so we tried it with mine, and indeed, it didn't sync reliably...it lagged behind, and got worse the longer we let a sequence run.  The tells me that those sync pulses were getting lost somewhere, and it was independent of the tempo( slow sequences got as lost as fast sequences).  If the sync input were polled by the high speed interrupt, then there's a chance the pulse might be missed.

What's the polling frequency in an 606?  I don't have the schems handy, but 6.2 mS rings a bell.

Byron Jacquot