Computing stuff tied to the physical world

A small improvement to rf12_sendStart()

In Software on Jun 21, 2010 at 00:01

The RF12 driver has a function called rf12_sendStart() which – you guessed it – is used to start sending out a new wireless packet.

Yesterday’s post was about battery savings, which I implemented by going into a power-down state right after sending out a new data packet.

That code included this little detail:

Screen Shot 2010 06 20 at 03.50.41

The reason for this is that the RF12 driver works mostly on an interrupt basis – i.e. in the background. Without the delay, the node would power down before the packet had actually been sent out. Then, when waking up, the transmission would fail due to the transmitter underrun caused by this logic.

By simply waiting a bit, the interrupts can continue to take place, and all the bytes will get sent to the RFM12B and from there into the airwaves.

Totally by chance, an EtherCard software problem was reported on the Jee Labs discussion forum, which turns out to be related to this very same issue. Sure enough, adding the “delay(5);” fixed that problem as well.

This felt a bit unsatisfactory as solution, so I’ve extended the rf12_sendStart() function to support a “synchronous mode”. The change is backwards compatible, i.e. the default behavior of rf12_sendStart() hasn’t changed.

Now, if you pass 1 as fourth argument, rf12_sendStart() will wait for the packet transmission to actually complete before returning to its caller. In general, calling rf12_sendStart() in sync mode is probably a good idea. Sync mode can be used for sending data packets as well as acknowledgement packets.

So the above code can be replaced by this single line:

Screen Shot 2010 06 20 at 04.02.11

This simplifies the bmp085demo.pde sketch slightly, and it also solves the problem that the EtherCard library may occasionally lock out interrupts on the SPI bus a bit too long to keep the RF12 driver running.

The code and documentation have been updated.