Help me understand this UART circuit

[Muku]

Hello, everyone!



I found this circuit in the datasheet of a GSM module, SIM800C. I guess this is some kind of level conversion because the data sheet says not to exceed 2.8V on the UART lines. Now I understand using a resistor divider to drop the voltage but I don't understand how the above circuit. Could anyone help me understand this, I might learn something new? How is the UART1_RXD pin going to see the TXD voltage because of the diode? VDD_EXT is an output voltage from module of 2.8v which is guess there for using with level changing circuit.

[ledtester]

Presumably it seems that the "DTE" device operates at 2.8V and the "Module" side operates at 3 or 3.3V.

If the DTE makes TXD high (i.e. 2.8V) then UART1_RXD will also be high being pulled up by the 10K resistor.

Code: [Select]

      3.3V            3.1V           2.8V           
   VDD_EXT --- /\/\/ ---- --|>|- --- TXD
                10K

I say "3.1V" here because some current has to flow through the resistor to create the voltage drop across the diode.

If the DTE pulls TXD low then the other side of the diode will be like 0.6V which hopefully still registers as a low for the module.

Code: [Select]

      3.3V            0.6V           0V           
   VDD_EXT --- /\/\/ ---- --|>|- --- TXD
                10K

Basically the diode raises the high voltage level of the DTE by something like a diode drop.

The other diode lowers the high voltage level of the Module by 0.6V.