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I am currently using two nRF24L01+ (2.4 GHz RF transceivers) together with two AVRs to create a keyless entry system. My problem is battery lifetime when using a CR2032 battery to power the key.

The key will sleep until a signal is received from the other unit, then wake up and respond.

The RF modules have a power-down mode in which they consume about 900 nA which would be ideal. The problem is that when the module is sleeping it won't listen for any signal and therefore won't send the necessary IRQ to wake up the AVR.

If I put the module in Standby-mode it might trigger the IRQ, but unfortunately it consumes about 26 µA which is to much.

Is there any RF receiver capable of listening for a message and sending an IRQ while consuming <10µA? The data rate doesn't really matter since it could be used only to wake up the AVR, and let the nRF do the rest. Range should be at least 3m (9 ft).

Perhaps someone has some inside knowledge of how car manufacturers have solved this?

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  • \$\begingroup\$ Car keys are usually one direction only. \$\endgroup\$ – Lior Bilia Jun 25 '14 at 9:55
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The short answer is no, there is no such radio module. Note also that the two standby modes available cannot receive anything, they simply provide a faster start-up than is available from sleep mode.

To detect incoming packets you need to be in RX mode, so minimum 12.6mA. Clearly you can't stay in that mode all the time. You need to poll periodically. You could wake up once every two seconds for a few milliseconds. The transmitter would need to send a 2 second preamble before each packet. Even then the battery won't last very long.

Car keyless entry systems use two separate radios to overcome this. The first is an very low frequency (e.g. 145KHz) radio transmitter built into the car that works a bit like NFC/RFID contactless systems. When the user touches the door handle a burst is sent by the car that actually powers the receiver in the key, and thus the key doesn't need to continually power anything, or can at least do so at ultra low currents. Once this wake-up signal is received the car and key switch over to an ISM band radio, typically 433MHz, and do whatever security checks they need to do.

Replicating this over 3m is going to be a real stretch. Car systems are deliberately very short range, so people can't steal your car while you are filling it up.

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  • \$\begingroup\$ My problem exactly, the awake time of the key is up to 25 ms per message (measured very crudely by timing a couple of pings sent from the master unit) which will burn the battery rather quickly (if my consumption calculations are correct). \$\endgroup\$ – Tim Jun 25 '14 at 21:18
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The keyless entry system should send out a preamble/code at regular bursts, which the key wakes up and listens for. The key's RF module should be in power down mode for most of the time, along with the processor.

For example, the keyless entry systems send a 'ping' every 1ms, the key can wake up every 200ms and listen for the signal for 2ms, and if hears the ping, reply. You can play around with the periods to get the best battery life vs responsiveness that you like. With those numbers alone, the period the key is now on has been reduced to 1%. Have it wake every 1000ms instead and the key is on for 0.2%, but it takes at least 1 second for the key to work.

Obviously you'd need to use some form of low power counter on your micro, does it have a low power RTT or RTC? Or can you use the WDT to wake it up, check for radio contact and then go back to sleep?

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  • \$\begingroup\$ Unfortunately the time needed to wake up and send a message is quite long, around 25 ms (measured very crudely by timing a couple of pings sent from the master unit) which puts the average consumption around 0,2 mA if my calculations are correct. With a 230 mA battery that is somewhere around 30 days realistically? \$\endgroup\$ – Tim Jun 25 '14 at 21:32
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CC1101 radio should consume 0.5 µA when in Wake on Radio (WOR) sleep mode.

See section 4.1 of the datasheet.

Thanks to Chris Stratton I realized that I missed the radio needs to periodically wake up, which makes CC2500 to consume depending on conditions 6.3 µA to 544.5 µA, see Current Consumption for a Polling Receiver.

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    \$\begingroup\$ Could this answer be expanded? Links to datasheets and an explanation of your rationale could make this a good answer! \$\endgroup\$ – Hearth Oct 26 '19 at 15:10
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    \$\begingroup\$ Incorrect. While you say "CC1101 radio", your claim only applies to the processor core, not the radio. You forgot to consider the time average cost of running the radio itself during the periodic signal checks. While this technique can result in low consumption, the actual consumption will be higher than what you claim here. ti.com/lit/an/swra207a/swra207a.pdf shows that typical figures will be at least an order of magnitude higher, and in many cases 2 to 3 orders of magnitude. Some of those cases work on a coin cell, some do not. \$\endgroup\$ – Chris Stratton Oct 26 '19 at 18:07
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You can use a Schottky diode (HSMS-285 or similar) based RF energy harvester to wake up your MCU. Problem is, at 2.4GHz, you will get lots of false signals.

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