Is it stable to change I/O direction on microcontroller repeatedly?

Question

I'm new to microcontroller programming and I have interfaced my microcontroller board to another device that provides a status based on the command send to it but, this status is provided on the same I/O pin that is used to provide data. So basically, I have an 8-bit data line that is used as an output from the microcontroller, but for certain commands I get a status back on one of the data lines if I choose to read it. So I would be required to change the direction of this one line to read the status thus converting this line as an ouput to an input and then back to an output. Is this acceptable programming or will this changing of the I/O pin this frequently cause instability?

Thanks.

Answer 1

I've done this on digital I/O pins without any problems but I'm very far from an expert on this. It probably depends entirely on which microcontroller you are using though.

Answer 2

There should not be any problem with changing the direction of the I/O line to read the status returned by the peripheral provided that you change the state of the line to an input before the peripheral starts to drive the line and then do not try to drive the line as an output until the peripheral stops driving it. What you must try to avoid is contention between the two driver devices, i.e. having the two ends being driven to opposite states by the processor and peripheral. This would result in, at best a large spike in the power consumption or worse blown pin driver circuitry in the processor, peripheral or both.

You do not say what the processor or peripheral are so I cannot tell whether there are any control bits in the interface that enable the remote device to output the status so that you can know whether the peripheral is driving the line at any time.

Answer 3

It`s important to remember that any IO switching in high speed generates EMI. Depending of switching frequency, board layout and devices sensibilities, this EMI can affect performance and reliability of your application.

If you are having problems in your application use an oscilloscope to check for irradiated EMI in your board lanes.

Answer 4

Yes, it's perfectly fine to change I/O direction on microcontroller repeatedly. That's the standard method of communicating over open-collector buses such as I2C and the iButton. (see PICList: busses for links to assembly-language code examples).

• transmit 0 bit: set output LATx bit to 0, and then set TRISx bit to OUTPUT.
• transmit 1 bit: keep output LATx bit at 0, and set TRIS bit to INPUT (let external resistor pull-up line to high)
• listen for response from peripheral: keep output LATx bit at 0, and set TRIS bit to INPUT. Let external resistor pull-up line to high when peripheral is transmitting a 1, or let the peripheral pull the line low when peripheral is transmitting a 0. Read the bit from the PORTx pin.

If both ends of the bus correctly follow this protocol (in particular, if neither end actively drives the line to high), then you never have to worry about contention or current spikes.