STM32 and UART reverse polarity.

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1

Hello,

I’m using a PIC MCU in a project and consider switching to a more powerful STM32F20X MCU.

It seems the peripherals are more than sufficient for what I need, BUT …

I need an UART RX channel with reserved polarity. It is easy with the PIC as there is a bit to set for reversed polarity, but I can’t find if it’s possible with the STM32 (seems not, in fact).

So here are my 2 questions :

1 - Do I miss something, is it possible to reverse the UART RX polarity in hardware ?

2 - If it’s not possible, what solutions do I have ?

I thought about using an input pin, connect it to an interrupt on level change, and set an output pin with reversed polarity, that’s connected to UART RX.

Spending 2 IO pins for this should not that difficult (I will use 64 pin STM32), but this solution seems really ugly to me.

Regards,

Thomas.

2

A simple transistor, NPN or FET, would be ok.

Angelo

3

Hello,

The problem is I use sometime this pin for PWM input, sometime for UART RX.

And I have absolutely no room left even for an SOT-23 FET :wink:

Regards,

Thomas.

4

As far as I can see, the STM32 USART has no support for reversed polarity.

I was looking at soft uarts a year or so ago (before I began experimenting with ST parts). There was a simple example posted on the Keil site, suitable for 9600 baud. There was also an app note on the NXP site that would work for at least 19200. I might be able to track down links if you’re interested.

The problem is that soft uarts eat up a lot of processing power, so that the amount you’re left over with may not be much improvement over the PIC. And if you’re looking at 57600 or higher, forget it.