Do I need a bidirectional logic shifter for UART

Question

I need to interface my Raspberry Pi to my Uno hence I need to logic shift 5 V logic to 3.3 V. I will be sending data from the Uno to Pi and from the Pi to the Uno. My understanding is UART uses two pins one for sending and one for receiving. I have built myself one of these

I have only built myself one of these because I wasn't confident I can build it. I was expecting it would blow up or won't work. Anyway I was wondering do I need two of the above. Can I get anyway with just two of the following?

It would be very handy if I can because then I don't have to use more transistors and it's my understanding the first circuit is essentially the second circuit doubled up.

Answer 1

The following is a rough-and-ready technique for experiments, and if you check all the voltages carefully, might be suitable for some designs.

One of the simplest methods, suitable for UARTs and short distances, is simply divide down the higher volts with a resistor divider. In this particular case, there's not any real noise margin, but for an experiment on the bench you can try this.

You have to check the exact details of the given systems

• UNO Input: With a ATMega328P at 5V (such as in your Arduino Uno), the 3.3V signal is adequate as a high signal, where a the high threshold is 0.6 V_CC, ie 3.0 V. The minimum promised logic high voltage, V_OH min, on some models Raspberry is also 3.0V (ref), so it will work on the bench but you'd probably not want to do this in production. On some models it's only 2.4V, which may well not be enough to drive the ATMega.
• Raspberry Pi Input In the other direction, you should check on your exact model in the Raspberry Pi datasheets, but on a 3.3V system you have assume that V_IH max is no higher than V_CC, ie 3.3V. For the RPi4 these values are unhelpfully given "TBD", though say "VDD_IO is the GPIO bank voltage which is tied to the on-board 3.3V supply." This appears to be the same as the CM4, where there is a voltage reference for GPIO, V_{GPIO_VREF}, which can be 1.8V or 3.3V. On the CM4IO board, it's 3.3V, giving V_IL min is 2.0V and max is V_{GPIO_VREF}, ie V_CC, ie 3.3V. See table 4 of CM4 datasheet, V_{GPIO_VREF} is set by R5 on Fig. 5, of CM4IO datasheet. Some more values are given in the overview documentation.

^{simulate this circuit – Schematic created using CircuitLab}

Calculating the resistors:
5 V * 33000 / (18000 + 33000) = 3.24V

ATMega328P

Note that the reset and crystal pins have different thresholds.

From ATMega328P datasheet

Answer 2

Can I get anyway with just two of the following?

Note that the two sub-circuits in the first figure are different. You need one for 3-to-5V translation, another for 5-to-3V.

Since both the 3V and 5V output are driven all the time, there's no need for pull-ups. So only then do the two circuits become identical:

^{simulate this circuit – Schematic created using CircuitLab}