Beaglebone Black Rev C UART0 Serial Port Tri-State

Hello All,

On the BBB Rev C schematic the tri-state buffer for UART0, U15, is wired with a pull-down for the 1A input (pin 2 on the device). For TTL level serial I/O, the high state is idle and ground is the start bit. So it seems to me the pin should have been pulled-up, not down, so that the input looks idle on start-up. What am I missing?

The reason I ask is I have BBB’s mysteriously while performing a ‘reboot’ (no power cycle). When I attach a UART0 console to track it down the problem seems to go away, which makes me wonder if it isn’t the UART itself.


When had issues with noise when the pin was tied low and no cable attached. Pulling hi solved that issue.