How can a Switching diode act as a level shifter?

I am following a schematic about a one-way logic 3.3->2.8 level shifting circuit.

The hardware in question is a MCU output that drives 3.3V and the input of the UART interface of a SIM800 GSM Module. Im trying to simulate the circuit but can't figure out why it's supposed to work connected as it is. The Hardware design document of the input module doesnt specify what kind of input it is. And gives other circuits as recomendation:

The specific questions are why do the circuits work like this? How can I emulate the internal components to simulate and check the proper behaviour?

The design reference comes from Adafruit's FONA and Feather FONA boards.

If the TxD output is high the diode is reverse biased, i.e. it blocks. The voltage at RxD is given by the 10k pull-up resistor connected to $V_{DD}$ (the lower positive supply voltage).

If TxD output is low the diode is forward biased, i.e. it is conducting and the voltage at RxD is ca. $0V + V_{forward}$ (about 0.6V for normal diodes or 0.3V for Schottky diodes).

• OK with Tony's and your anwser I understood the recomended design with the ressistor conected to VDD of input. But what about the one used by Adafruit WITHOUT the conection to VDD?, the only thing between the two pins is the Diode. – St Casimir Sep 17 '17 at 16:22
• @St Casimir: I don't know those specific boards; but microcontrollers often have integrated pull-up resistors at inputs (can be activated optionally if needed) maybe that's the case. – Curd Sep 17 '17 at 16:38

UART uses negative logic, It can be Diode OR low and receive Tx from multiple sources. or as a level shifter. The threshold is near Vdd/2. The UART

RS232 inverts Rx Tx and uses positive logic so the diode is reversed and pulled down. 0V is adequate for "0" but V- is better for immunity. The threshold is near 1.5V.

Although this circuit would use a 5V pullup , it allows a 0~3.3V signal to be converted 0.7~4.3V signal with a threshold near 2.5V.

Conversely a diode can be used to convert 5V to 0.7V to 3.3V for Rx with a threshold of 1.67 but only a noise margin of 0.5V , so connection must be short A Schottky Diode would be better if the wire is more than a foot and should be twisted pair.