[sdiy] lovely 'glitch' puzzle...siel DK600 74HC923 key matrix read

Bob Weigel sounddoctorin at imt.net
Wed Dec 28 02:55:36 CET 2011


Hmm.  Here's the correlation after getting everything *else* working I 
think on this synth. Grounds to front panels seem good.  But that front 
panel board has the 74HC923 on there to code the buttons.  Now... I see 
that if I hit say a waveform button it'll cause other buttons to act 
like they've been hit as well.  The outputs are called A, B, C, D, and E 
however E isn't used it appears.  Not sure why they didn't use the 922 
chip since it would have been find I think..but whatever.  Anyway so I 
notice that all the buttons are being affected that have common 1's in 
the code.  For instance the LFO pitch A and B routing switch is never 
affected since it's code is 01000 EXCEPT when another button with that 
same 1 in the code is being hit.  All responses are somewhat 
intermittent too.  Sometimes you'll see no affect.  Sometimes it's like 
another button got hit twice etc..  The "B" in other words is the only 
output high during the read.  (Data available of the chip goes to 
interrupt the cpu and tell it to do a read on the panel codes coming in 
to a 74HC244 latch that interfaces to the data buss)

The 923 chip has a timing capacitor for a schmidt trigger input inverter 
clock (ie. a f/b resistor charges the capacitor and discharges it when 
it crosses threshhold) and this sets the scan rate.  There is also a 
bounce mask timing capacitor.  But I'm thinking... that wouldn't be 
relevant probably.  I'm having trouble triggering a scope on the signals 
out of this for some reason also so anyone with a thought would be 
appreciated.

Sadly the panel board is a little hard to pull.  And I had a spare and 
tried it but it doesn't work at all it appears so... hoping to trouble 
shoot it w/out ripping it apart.

-Bob

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