Pull-up resistors on Verdin carrier boards level-shifters


I am drawing up the schematic for a custom Verdin carrier board. I noticed that on the Dahlia and Verdin Development Board circuits, that pull-up, and sometimes pull-down, resistors have been placed on all the inputs to the 74AVC4T774 level shifters. I am familiar with the need to stabilize un-used or floating inputs in general, but in many cases on these two boards these inputs do not appear to be either unused or floating.

Conversely, all the 74AVC4T774 devices on these boards have RC networks on their OE pins, which means that their logic output pins will be floating for a period after power up, thus possibly presenting un-stable signals to whatever inputs they are feeding. However, there are no pull-ups or pull-downs and these outputs. I also notice that no pull-up/down resistors are shown on the example circuits in the Verdin Carrier Board Design Guide.

I would be very grateful for any explanation and/or recommendations regarding this.

Thanks in advance.


Hi @bcpberry , the RC circuit connected to SN74AVC4T774 OE pin is a RC differentiator. There will be a pulse when power is applied and this pulse disables the output. The pull-up/pull-down resistors can be added according to output requirements. From SN74AVC4T774 datasheet:

The input circuitry on both A and B ports is always active and must have a logic HIGH or LOW level applied to prevent excess ICC and ICCZ