Nordic FOB Query!!

Hello one and all!

First, a bit of background.

I’m wanting to use the Nordic FOB to arm/disarm an alarm. However, I’d like it to be a bit more complicated than the example firmware.

Basically, I’d like it to be two way for a start. For example:-

  1. Both FOB and receiving device(ATmega168/328 (Arduino based)) have a prestored key which is never transmitted.

  2. FOB key is pressed to arm the alarm.

  3. Receiver sends back a randomly generated key.

  4. FOB receives key and acknowledges receipt.

  5. When coming to disarm the alarm, the FOB uses a combination of the prestored key and the received key to produce a new key by way of an algorithm.

  6. Receiver reverses the key sent by FOB using the same algorithm, but in reverse, and if the key is correct, disarms the alarm.

By using this constantly shifting key system, along side the prestored key, it should make it very difficult to defeat the alarm by sniffing the RF signals.

So, finally, the queries:-

  1. The example FOB firmware only details the FOB sending data. As the RF module is a transceiver and is controled over SPI, am I right in thinking that the FOB could be programed to receive data as well?

  2. Which brings me onto the next question. I looked at the ATtiny24 product card and noticed it only has 128 bytes of RAM and 2K of programmable flash. How much of each of these is left free by the example firmware? Would there be enough space to shoehorn in what I’m proposing, or would it be better to throw in an ATtiny44? (256 bytes/4K respectivly).

The size of the EEPROM in the ATtiny 24 would be ample to store both the prestored key and the randomly generated key so isn’t an issue.

Sooooo…any help would be apreciated!!!

Regards,

The Cageybee