ZL70550 – raw packet mode

Posted on by crackingcontraptions_3voi0v

According to my dubious expectations the major current drain is going to be the WUP radio which has to listen, or duty-cycle listen, continuously and thus consumes most of the energy.

I read through the Datasheet.  One interesting bit I discovered is that it is possible to transmit raw bits.  This means that the exact contents of TX buffer are transmitted without preamble, frame sync pattern, FEC, CRC or anything.  Now this makes the TX times short, but it doesn’t really help much.

Also, all of these extra bits serve an important purpose in reliability, but if I just wanted to transmit a 32-bit WUP packet, then it ought to be possible.

  • In RX mode it consumes 2.4(sniff) – 3.2 mA(rx).  “typical” rather than worst case values.

So with the simplest protocol the listener would consume 3.2mA @ 3.6V = 8.64mW.

For reference a CR2032 can produce a constant 55mW and has a capactity of 240mAh.  With a nice constant drain rate of 2.9V at 0.19mA it would last for about 800 hours = ~1 month.
But even ignoring the different voltages, buck-boost inefficiencies, smoothing capacitors etc., that seems to mean that the radio would have to duty-cycle listening at 3.2/0.19 = ~20
Duty-cycling is a mixed blessing because it simply moves the load from the listener to the transmitter.  e.g Duty cycling the listener at a ratio of 20:1 requires the transmitter to TX 20 times more often.
If the transmitter consumes more than the receiver (2.7 – 5.3mA) then the transmitter can’t use a CR2032.
This calculation assumes that the listener can sleep with approximately zero current drawer.
  1. In deep sleep with just a few essential registers retained the XL70550 consumes 10nA
  2. Crystal oscillator takes 1ms to start up and then it consumes 200µA

So what kinds of battery can produce a steady 8,64mW, last an eon, but occupy no space.

LiPo

LiPo has an energy density of 250–730 WattHours/L. So worst case, the WUP would run for 250/0.00864 = 28935 hours, or ~1200 days, or ~40 months ,or ~3 years on a 1 liter LiPo.

Bottom Line

Doing the math the other way, to last a month the battery would have to be 1000/40 cc = 25cc 

which is ~3x3x3 cms!

—- David
A small battery powered tag that doesn’t need frequent charging and that listens constantly can’t be made using currently-available radios. It will need either to duty cycle or an ultra-low power wake-up radio has to emerge. So your analysis is correct.

Maybe it’s time to talk to BlinkSight?
—- Mik
The alternative is to use a more chance-based discovery method.   Peers listen and/or transmit at random intervals around some mean.  The chance that A hears B = Pb receiving.  If B listens for 5s every 100s the probability that it is listening when A transmits is 5%.  On average A will have to randomly 20 times at ~5s intervals, so the average latency is 100s.
The alternative is to rely on always-on base-stations, that have access to more power and can stay on all the time.

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