Why is it recommended to use diodes and resistors for UART communication

[sarose]

Why is it recommended to use diodes and resistors for serial communication as shown in the circuit given? It is given in datasheet of SIM800C. Why we cannot directly connect TX and RX?

[T3sl4co1l]

Guessing DTE is 5V TTL or something.

Tim

[Ian.M]

Well that's one of the less dodgy interface circuits in a somewhat crappy datasheet (see page 25). .

The SIM800C runs at a nominal 4.0V, so with 0.6V-0.7V diode Vf drop you'll get pretty close to 3.3V levels if the 3.3V DTE's got CMOS outputs.  On the module side it shifts the UART1_RXD level up by one diode drop.  Note that although the datasheet specifies Voh-min and Vol-max it doesn't specify at what current, so its insufficient for designing a passive level shifter properly!

However, no matter what voltage (between 1.8V & 5.5V) the DTE uses for logic levels, one could simply use a bunch of 74LVC1T45 or 74LVC2T45 dual supply level shifters (depending on how many digital signals in each direction you need), and get much lower quiescent power consumption, and better signal quality with far more drive strength on the digital interface lines.