UART 5: Powering router for UART data exchange

I don’t think I fully understand the purpose of the voltage selector on the Tigard board.

On lab UART 5: Common Interfaces.
We are asked to connect to the router through UART.
The router has its own power source, why do we need to set Tigard to 3.3V for this to work?
I was under the impression that we only need to set Tigard’s voltage to anything other than VTGT, when powering a chip that doesn’t have its own power source.

  • Note: I’ve tried the same exercise with Tigard set to VTGT but I can’t send/receive data.


The power swtich lets you choose between voltage levels on Tigard. In this case, if we don’t set a voltage, then Tigard’s level shifters will just flatline at 0v.

The tigard readme lists the typical use cases and why. The UART lab is use case #3 listed below.

Let me know if you’d like me to describe in more detail.

Voltage Switch

This switch chooses the reference voltage for the level shifters and the target system:

  • 1V8, 3V3, and 5V all apply a voltage to the VTGT pin.
  • VTGT disconnects the VTGT pin from supplies, and depends on the wire connected to the target to set the level shifter voltage.

This results in 3 distinct use cases:

  1. Target-Powered: Set the switch to VTGT and connect the VTGT wire to the powered target. The target powers the level shifters.
  2. Tigard-Powered: Set the switch to a voltage, and connect the VTGT to the unpowered target. Tigard supplies power to the target.
  3. Self-Powered: Set the switch to a voltage, but do NOT connect the VTGT wire. Tigard powers its own level shifters. Target powers itself.

Thanks for the reply, Joe.

If I understand correctly, the switch (and pins) let you choose between:

  • Powering Tigard but not the target
  • Powering the target but not Tigard
  • Powering both Tigard and the target
  • Not powering any of them.

I thought it was either controlling power on Tigard or on the target but not both. Thanks for explaining.

Yeah, there’s a bit more complexity, and that’s after we whittled it down from the original design.

Originally, we had separated VREF (the reference voltage for the level shifters) and VTGT(which connects on the red wires), plus a switch to connect the two of them. TIMEP, a predecessor to Tigard, had called them ‘Isolated’ and ‘bridged’ mode.
We realized that in the one scenario you would ever use isolated mode, you could just disconnect the VTGT wire and have an easier setup. In case someone does come up with an important use for it, there’s a jumper on the back of tigard labeled “ISO” that you can cut with a knife to separate VREF and VTGT.