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Trying to program Evil Mad Scientist's Peggy 2 board without their Library. The board drives a LED Matrix with

  • Source 74HC154 Decoder
  • Sink STP16CPSO5 LED Sink Driver

Schematic

What has me confused is the 74HC154 Outputs can only have one output set to Low at at time. See table below. This means all but one row is "on". Isn't this backwards? Shouldn't all outputs be low except one that is high? Selecting what row you want on, rather than selecting what row you want off. It looks more like the 74HC154 is a Sink. Thanks.

Output Table for 74HC154

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If you look into the datasheet of 74HC154, you can see that each output has an Active Low State. Also in the schematic, the leds row part use a pnp transistor (see page two). As so, they will be active if the input is low.

This means all but one row is "on". Isn't this backwards?

Since they are PNP, the only active row is the one with Low value and the current of that row is sink through the selected column(s) in the STP16CPS05

  • Ahh!!! Makes perfect sense! Low turns PNP on! Thank you brtiberio! – RickH Apr 21 '18 at 16:21

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