Computing stuff tied to the physical world

Serial Port on JeeNode Micro

In AVR, Software on Mar 9, 2012 at 00:01

The JeeNode Micro is based on an ATtiny84, which has quite a bit less hardware functionality built-in than an ATmega. There is some rudimentary byte-shifting hardware for sending or receiving a serial bit stream, but that’s already assigned to the SPI-style RFM12B interface.

So how about a serial port, for debugging? Even just serial out would be a big help, after all.

Luckily, the developers of the Arduino-Tiny library have thought of this, and have implemented a software solution. Better still, it’s done in an almost completely compatible way.

Here’s an example test sketch:

Screen Shot 2012 03 08 at 07 52 04

Look familiar? Of course it does: it’s exactly the same code as for a standard ATmega sketch!

The one thing you have to keep in mind, is that only a few baud rates are supported:

  • 9600, 38400, and 115200 baud

The latter is unlikely to work when running on the internal 8 MHz clock, though. It’s all done in software. For an ATtiny84 running at 8 MHz, the serial output appears on PB0. This is pin 2 on the chip and pin 10 on the 10-pin header of the JNĀµ, marked “IOX”.

The code for this software-based serial port is fairly tricky, using embedded assembly code and C++ templates to create just the right timing loops and toggle just the right pin.

Note also that since this is all done in software, interrupts cannot occur while sending out each byte, and sending eats up all the time – the code will resume after the print() and println() calls after all data has been sent.

But apart from these details, you get an excellent debugging facility – even on an ATtiny!

  1. Nice and useful! How would one read this port from another JeeNode?

    • Connect pin 10 and ground to the JeeNode. Or to a USB BUB, if you want to view it from a serial USB port.

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