Using a Max232 to add an extra input?

[Peter Truman]

Hi All - I find myself in a pickle with a project that will need an additional input over what I have already provided.

PIC 18F25K22

This project normally has 1 digital and 1 analogue input, because that is generally all it needs (PCB's already made). However, I have come across a situation where I need an additional input (from a panel switch). I don't want to hack the hardware too badly if I can avoid it, at the same time, the additional input needs to be quite robust and immune to glitches.

The PCB has a MAX232 with an external 3.5mm socket which is used at the commissioning stage just to set a few parameter - beyond that it is redundant.

Here's my thinking. Sticking with the current hardware, I could just look for a serial input for the first few seconds after power up (bit like a bootloader) - if I don't see any activity on the UART I can assume there is no configuration changes required. From then on I would like to use the RX2 pin as an input for the panel switch (on the RS232 side).

So I would just make up a cable with a 3.5mm plug on one end and a panel switch on the other end. The panel switch would be either floating (or pulled high) or grounded. So I would expect to see 5v on RX2 or 0v depending on the state of the panel switch.

I realise I probably need to do some experimenting here but, I thought I would check to see if this seems sensible or not.

The basic question is - once I setup the UART to receive data on RX2 can I subsequently abandon that (after the first few seconds from power up) and use that pin as a simple digital input?

If that seems possible - does anyone know what I should expect on ROUT1 with RIN1 disconnected? (I could just add a pullup/down but I prefer not to if I can avoid adding any parts on the board.

Any advice is appreciated.

[tumbleweed]

Something like that is workable... you can disable the UART (or just read the state of the input pin) when you want to use it that way.

The MAX232 has a 5K pulldown to GND on the RINx input (3K-7K ohms per the datasheet), so with the inversion that provides a logic high output on ROUTx pin with nothing connected to RINx. That's the idle state for a UART input/output. If you power up and enable the UART, if the PIC sees a low on the RX input pin that is the same as a serial BREAK condition.

Since the switch is on the RS232 side, you'll have to have it supply a high to tell the difference between open/closed.

If you leave the UART enabled the IO pin will be in Schmitt-trigger mode, so you'll need to supply > 0.8xVDD to be seen as a logic high, so you'll need a pretty stiff pullup, something < 750 ohms or so should do since the RIN pulldown could be as low as 3K.

I suppose you could just switch in 5V and skip the pullup, but I never like doing that.

[Peter Truman]

That is marvelous - easily achievable in this scenario.

Thanks very much for your help

[tumbleweed]

You can forget that part about the PIC IO pin voltage... don't know what I was thinking when I wrote that, but since you'll be going into the MAX232 RIN pin, it only needs >2.4V to be seen as a logic high.

Since the MAX232 RIN pulldown could be as low as 3K, that means that at 5V you could use anything less than about 3K for a pullup value (1K-2K would give you plenty of margin).