Electronic – Trying to understand this schematic



This board is for level shifting 3.3v RPi GPIO output to 5v. There are 3 outputs; half of the pins of each output are unique to that output and the other half are common outputs.

Of course, the unique outputs are separately buffered; however, some of the common output pins share one buffer vs. other common output pins being separately buffered – particularly:

The strobe, OE and clock signals are separately buffered for each connector.

Is there any benefit to separately buffering a common pin for each output?

Best Answer

The board is designed to drive various LED matrix panels, many of which do not have datasheets or interface specs. Therefore it needs to have strong enough outputs to handle unknown loading and timing requirements.

Since the panels may be connected through long cables the capacitive loading could be quite high, causing slow transition times. The strobe, OE and clock signals are probably more critical than the address lines, so they are separately buffered for each output.

The github readme says:-

RPi output drivers have trouble driving longer cables, in particular with faster Raspberry Pis Version 2. This results in artifacts like randomly showing up pixels, color fringes, or parts of the panel showing 'static'.

This suggests that capacitive loading can be a problem if the drivers are not strong enough.

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