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[SOLVED] SammichFM LED issue


ExplodingLemur
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I'm having some weird LED issues on a sammichFM I just built.  Both rows of LEDs light up or flicker when I pass my finger near the pins linking the display board to the mainboard, over the traces, or over the 74HC595 shift register that runs the LEDs.  The LCD, buttons, and encoder all operate properly.  Any idea what might be the cause?

Edited by ExplodingLemur
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I don't seem to be able to get any normal LED function.  I've got a scope and can probe the signals going to the UI board.  I see RC, SC, SI, SO.  If I'm reading the docs correctly then RC is the latch enable, SC is the shift clock, and SO is the serial data?  SC clocks 8 bits into this shift register, SO is either 0 or 1 on each SC rising edge, then RC is sent high to latch the new 8 bits in the register.  Lather, rinse, repeat.  Since there are 10 LEDs I assume some kind of multiplexing is taking place.  What's the expected behavior I should be getting on these pins, and what is a known button press I can do from the main screen that's guaranteed to toggle an LED state?

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1 hour ago, ExplodingLemur said:

Since there are 10 LEDs I assume some kind of multiplexing is taking place

Yes they are multiplexed using the two transistors T1 and T2 both connected to the same 595 shift register, 5 pins for leds columns, 2 pins for rows via the transistors.
http://www.midibox.org/dokuwiki/lib/exe/fetch.php?media=sammichfm:sammichfm_cs_pcb_rev_1.pdf

Could you post some pictures of the board?

Edited by Antichambre
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Pics are a bit blurry, but it looks like the LEDs have the correct orientation. You should be able to scope a (probably) 1/8 pulse on the LED cathodes  anodes if that column's LED is illuminated. Likewise, you should see an alternating (but up to 1/8) pulse on the transistor base. Do the transistors follow the expected pinout?

When uploading the hex file did you specifically choose the sammich one or "normal" MIDIbox FM?

Edited by latigid on
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I pulled all the shift registers from the input board.  RC and SC are good, but SO is 0V no matter what.  Installing IC1 (74HC165) and then IC2 (other '165) results in no change in behavior.  Installing IC3 (74HC595) gives me 1V on SO again regardless of it being pulled high or low.  Checking the '595 socket now to make sure I've got continuity where I should and opens where I shouldn't.

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