One-wire interface between PC COM-Port and MSP430

Development Boards MicroMod
1

I tried something simple. It worked. But please help me and tell me if

this seems insane.

The GPIO pins of MSP430 use +3.3V/0V CMOS logic levels, while the PC

COM-Port uses +12V/-12V RS232 levels. Here is the interface hardware I

used:

a) PC COM-Port Gnd <==> MSP430 Gnd

b) PC COM-Port TxD ==> 33K ==> MSP430 GPIO pin Px.y

c) PC COM-Port RxD <== Same MSP430 GPIO pin Px.y

(Yes, I know, there are 4 wires and a resistor. But “one-wire” sounds

better :wink:

The MSP430 I used has no hardware UART. Here is the bit-banging method

I used in firmware:

a) MSP430 initializes PxSEL:BITy=0 (as GPIO); PxDIR:BITy=0 (as input);

PxOUT:BITy=1 (no immediate effect, handy for sending 1).

b) When PC is sending TxD, MSP430 just read PxIN:BITy to receive.

c) When PC is receiving, MSP430 sets PxDIR:BITy=1 to pull RxD high, or

clears PxDIR:BITy=0 so that RxD is pulled low by TxD.