jenswilly.dk


IR codes and storage

Here's some notes on how to store IR codes for the IR controller project:

If I were using pre-programmed codes in a few known formats (like RC-5 or NEC1) I could store the commands using just the two or three bytes that actually make up the sequence. For example, a NEC sequence consists of one address byte and one command byte (although it is sent as a lead-in followed by four bytes).

But mostly I will need to record IR signals and store those. The proper way of doing it would probably be to parse the recorded signal, identify the protocol and store the required few bytes. But that sounds like way too much work. So instead I'll just be storing the time intervals for on-off pairs. I would like to use one byte for each time code and if I use 100 µs resolution I will get from 0.1 ms to 25.5 ms in .1 ms intervals. I think that might be good enough – but time will tell :)

Storage

Firstly, an ATmega328P has only 1 KB of EEPROM storage. And most likely, that will not be enough. So I'll be using an external EEPROM chip. The "24C" series seems to be very widely used and uses a simple I2C protocol. So something like Microchip's 25LC512 seems suitable (512 Kbits, 3.3 V and is available in both DIP and SOIC packages) and will give me an extra 64 KB of non-volatile memory. I'm thinking that will be enough.

The IR codes will have differing lengths. And the correct way of storing those in the EEPROM would involve some kind of allocation table and stuff in order not to waste memory space. But instead of messing around with all that I've decided to go for a super-simple solution there every IR code uses a fixed size (like 128 bytes – something that is a whole multiple of the EEPROM chip's page size). That way I can easily access the code for a specific index.

The data will be in raw time-on, time-off format and terminated by a 0 value (since a 0 ms pulse will never occur).

Thus, for my LG television which uses the NEC1 protocol, an "off" command (address 0x04, command 0xC5) can be stored like this:

90, 45,
06, 06, 06, 06, 06, 17, 06, 06, 06, 06, 06, 06, 06, 06, 06, 06,
06, 17, 06, 17, 06, 06, 06, 17, 06, 17, 06, 17, 06, 17, 06, 17,
06, 17, 06, 06, 06, 17, 06, 06, 06, 06, 06, 06, 06, 17, 06, 17,
06, 06, 06, 17, 06, 06, 06, 17, 06, 17, 06, 17, 06, 06, 06, 06, 00

For a total of 67 bytes for a code that basically consists of two bytes! Way inefficient, I know, but it's easy to use.

Using a size of 128 bytes for one IR code will accomodate codes of up to 64 IR pulses and I will be able to fit 512 codes into the EEPROM – and that should be more buttons that I'll ever need. (Maybe I will need to up the size to 256 bytes – we'll see.)


One comment on "IR codes and storage"
Manolis says:

256 bytes for one (!) command should be ok with Hitachi's split airconditioner IR codes ...

Leave a Reply