Experience with BERs and PERs on the nRF24L01

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Experience with BERs and PERs on the nRF24L01

Postby threepointone » Sat Nov 24, 2012 1:37 am

So I've got a somewhat nonstandard (I think?) application right now where I have a unidirectional, high reliability link. The reason for this is that all the receivers must be dirt cheap in the final application after all my prototyping, while I'm going to add a PA to the transmitter to get the necessary range. Thus I can't do the full ACK functionality of Enhanced Shockburst. I'm wondering if anyone has any experience with the BERs and PERs of the receiver at closer distances. I'm using Enhanced Shockburst packets with an 8-byte payload and I'm getting essentially ~5-10% PER, even at close range. I've already stared at just about everything in the logic analyzer, and it's clear that the lost packets are simply never received at all (and always transmitted)--the chip itself is failing to receive these packets successfully (IRQ doesn't go low when these packets are sent, and continuous polling similarly shows no change in status flags). I'm transmitting at 250kbps data rates. CE is on all the time in the transmitter, but I always wait long enough to ensure it falls back to Standby-II mode before retransmitting, and I force at least a 1ms interval before transmitting again so that the receivers have started up again. The receiver turns off CE to receive so that there's no weird FIFO issues.

Is this error rate typical, or maybe the spectrum in my area particularly bad (college campus, several WiFi networks broadcasting over multiple channels, especially the way campus WiFi works). My NRF modules are about a foot or so away from each other. Or maybe I have some sort of power supply noise issue? If this is really what I should be expecting with the nRF (it does look like the BER specified, but only at the minimum receive sensitivity), then I was planning on manually using the retransmit feature and the PID fields of enhanced shockburst to improve my effective PER.
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Re: Experience with BERs and PERs on the nRF24L01

Postby brennen » Sun Nov 25, 2012 5:14 am

I would indeed say this is a fairly typical error rate. You could always put your VCC line on an oscilloscope while you're transmitting/receiving, but it doesn't really sound like a power supply problem to me. If missing packets isn't acceptable in your application, you will definitely have to add some code to handle missed packets and retransmission/acknowledgements yourself. The power amplifier will complicate this, though.
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