How to get more than one uart interface

Question

Ok, I have one uart interface (TXD GPIO 14, RXD GPIO 15). I want at least one more uart interface.

Possible solutions:

• Bit banging: Use two unrelated spare GPIOs. I understand that timing is a problem on a standard linux. Would it be reliable with very low baudrate?

• Switching: RPI decides when to talk to which device. Using e.g. CD4066BC.

• spi to 2 x uart bridge: Didn't find a suitable part (availability, price, dil package)

• usb to uart: Expensive

Are there other options? I'm inclined to switching, if it can be done. What would you advise?

Answer 1

A USB UART, such as FTDI, isn't really expensive. All your other options sound like they will cost you more in parts and time than the ~$13 it might cost you, and be unreliable or slow. Just go for the fast and trouble-free option, such as:

http://www.dfrobot.com/index.php?route=product/product&product_id=147#.UOamLG-sh8E

Sparkfun sells one too. In fact, maybe you can just pull one out of some old USB device or buy one from a junk store that doesn't know what it does.

I messed with an SPI to UART adapter for an Arduino project, there was no existing library so I wrote my own. In the end it worked ok, but if I could have just dropped in a $15 part I would have. In fact given the time it cost me I should have just got a mega with 4 serial ports.

Alternatively if you want lots of serial ports you could look at RS485 serial, which is similar to 232 (though not compatible), which supports multi-drop, ie several interfaces over the one line.

Answer 2

I found what you are looking for: an I2C/SPI slave to UART/IrDA/GPIO bridge.

They come in single and dual version (so 1 or 2 extra UARTs). They (NXP) also (for the other side in case needed) have I2C/SPI master to UART/IrDA/GPIO bridges.

More information can be found about these chips here and the master counterpart.

Maxim also has chips that do the exact same thing.

Answer 3

A microcontroller like a Picaxe could take serial data in on one pin and output serial data on a certain pin appropriately. This would effectively give you more serial outputs if you were prepared to have to tell the Picaxe which pin to output on. It could also do the same, but in reverse, so it could receive serial data from multiple devices and send it to the Raspberry Pi. Another option could be to make connected devices require a qualifier. This means that Device 1 would have to receive the data 'd1', for example before it will listen to further data on the serial line. Device 2 could have 'd2' as its qualifier. This would mean that to say 'hello' to Device 1, you'd just have to send 'd1hello' on the UART line.

Picaxes are fairly cheap, you can get them over at http://www.techsupplies.co.uk/ and they come in numerous sizes with different numbers of pins and so on.