A denser LLAP serial RF data format

Ciseco’s LLAP format is a nice lightweight and PIC microcontroller and Arduino friendly serial protocol. I use their XRF modules for RF communication, these support power-down so they are well-suited to intermittent operation off a battery. Standing current on receive is 23mA so continuous operation is more of a challenge, for instance at the RF to SMS gateway. It has 12 bytes like so:

LLAP Message format

Each message is exactly 12 characters long and in three distinct sections:

  • One start byte
  • Two bytes for the device identifier
  • Nine data bytes, padded by dashes if necessary.

<SB> <ID> <ID> <D> <D> <D> <D> <D> <D> <D> <D> <D>

See Appendix 3 for details of the permissible characters in each field.

Their examples, however, send only one data value per LLAP message, with a descriptive section. Hence

aAATMPA12345

Which is wasteful IMO. A lot of sensors have two data points,for instance temperature difference measurements, or temperature and relative humidity.

Few real world sensors  can justify the precision of using all the digits; I don’t have any with an accuracy of more than three digits. Sensing temperature to an accuracy of 0.1C is unusual – the popular dalas 18B20 is accurate to 0.5C but to do much more implies a piece of laboratory equipment. Useful values of temperature in the UK would be -20 to 120 °C, Relative humidity is 0 to 100 – cheap sensors don’t really justify a .x so allocating four digits covers most bases. Negative values give the ugly -21. as the – takes up a digit but it’s only a machine that sees it. So I can make a double density device as

aAA12.34X5.67

and keep within the spec. I use **** for failed or missing sensors, and the X is replaced by L,M or H for battery status. M and H are operational, L means may be about to switch off in a few cycles. In sensors that support H then M means would still accept charge, H is enough. However I use a simple comparator at about 4.4 V on a PIC 16F628 so I can only show L and M.

This saves me precious power, and allows me to consolidate two temperature sensors to one radio saving cost of the radio and aggravation of maintaining batteries.

 

two-sensor PIC and XRF device

two-sensor PIC and XRF device

I couldn’t use JAL for this because I laid out the board to use the 16F628’s internal oscillator that runs at 4MHz and the JAL one-wire lib wants to run at 20MHz. So I had to code it in assembler 🙁 Next time I’ll leave space for a 20MHz resonator on the board that will save me all that grief.

I now get to read two temp sensor and the battery status, all in one LLAP message 🙂

 

 

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