[???] Jetson TK1 J3A2 UART2_TXD & RXD protocols

[???] Jetson TK1 J3A2 UART2_TXD & RXD protocols

I want to use the UART2 on the J3A2 expansion port, have few questions:

  1. Protocols is it TTL? RS232? RS485?

  2. voltage range?

  3. is it enable by default? or do I need to set some registers? if so how and where?

I measured the UART2_TXD (J3A2 pin 68) on mine and found it to be 1.8V (VDD_IO), which is definitely not compatible with the range of voltages which might be found on RS232 or RS485. The schematic for Jetson does not show TXD/RXD physically connected to any devices other than J3A2 pins, so functionality is strictly determined by how the Tegra124 chip itself is configured. You should look at the TRM chapter 32 “UART and VFIR CONTROLLER”, which starts by saying this is configurable to 16450 or 16550 behavior (but defaults to 16450).

The TegraK1 docs specify that UART2 is multiplexed (search for UART2 in the TRM doc). You may have to enable or disable related functionality via pinmux. The controller seems to be referenced in some places as UART-B (system address map) or UART-2 (other places). Probably of interest is that the pinmux controls whether this function is enabled or disabled, and whether it is “normal” or “tristate”. Since I was able to measure 1.8V it must be “normal” by default under L4T. How it is pinmuxed I don’t know.