RF Wakeup - RFID without the ID

[timpa04]

I'm creating a "keyless" entry system using an nRF24L01+ and an AVR on both sides.
The base units power consumption isn't a concern, however the keys power consumption is.

The key will be powered by a CR2032 battery, and I would like to have an average consumption <14 µA.
I've played around with duty cycles, but I can't get the consumption low enough since every message takes about
25 ms from wakeup until the message bounces back.

The only solution I can think of is having another RF module attached which would handle nothing but wakeup.
It would be perfect if there is something like a passive receiver with a range of 1m-3m which would send an IRQ
when it receives a signal.

Do you have any suggestions?

 

[FvM]

The average power consumption is mainly a matter of duty cycle. Your description isn't informative in this regard. A more detailed explanation of the intended system operation would be helpful.

 

[timpa04]

ON consumption: 17mA
OFF consumption: 4 µA
Lowest on-time / transmitted message: 25ms

If my calculations are correct, the average consumption would be around 88µA if the key
sleeps for 5s between messages (5s is a long time, I would like it to be around 2s).
The lifetime on a 230mA battery would realistically be around 75 days?
400 days is a minimum!

 

[FvM]

My first doubt is about 25 ms on-time. nRF24L01 datasheet specifies 1.5 ms startup time from power down, so there should be a way to design the communication more effectively.

The general question is if 2.4 GHz is the suitable band for your application. A purely passive receiver which 3 m range isn't easily achievable with permitted 2.4 GHz power levels. Particularly the receiver might be triggrered by other equipment using the band

 

[timpa04]

I've had the same doubts about the message round-time, haven't found a solution yet.
However, even if I was able to cut down the on-time, it would have to come down to 4ms or pref lower which might be impossible.

Do I have to limit myself to 2.4 GHz? How about using the 2.4 GHz band for the handshaking coupled with another receiver (using another band) to wake the main system up?