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IR remote system design

We've been experiencing problem with a number of remote receivers over the years using with RCMM standard in a TV type product. We started with Vishay parts and through at least 3 cost reduction effort have found that production parts are never the same as approval samples from the lower priced competitors.

It seems the timing accuracy of output against optical input is a significant variable, suppliers keep going on about increasing our software limits so their parts will work but many are going past half way points and they just seem to want all the tolerance for themselves.

So I need to define tolerances for each part of the system, like many things much of these are not documented or specified anywhere. Any opinion on the following would be useful.

  1. Remote transmitter tolerance - what timing accuracy can I expect say on burst length and cycle length. Remotes seem to use a 4Mhz resonator, and pulse cycles are 444, 611, 777, 944us for each of the 4 levels in RCMM. The remote supplier we are using now has copied an older remote we had but done this inaccurately. So a sample I have measured is 2% out from ideal, but seems fairly stable, but they do need to operate from 0C to say 40C.

  1. What tolerance on the remote receiver and how to specify? RCMM has a varying tolerance that reduces from around 20% to 9% absolute maximum. So should tolerance be specified as time or %. The main problem seems to me to be jitter on the burst start detection, so is jitter a better specification?

  2. Receiving Processor/software limits- Current we operate a no-mans land of 25us either side of the midway point. This account for processor timing variations although I think the processsor can manage better than 1us accuracy, so how close should one go?

At the moment I am thinking 3% for remote control 5% for receivers and remainder for processor (+/- 2us no mans land minimum, probably 4us).

I have gotten the impression RCMM is a difficult protocol to meet and I might try and move away from it eventually but we are stuck with it now.

-- Tony

Reply to
Tony
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Accurate timing on the processor usually costs nothing to manufacture, just better software so it would seem silly to squander budget here if other areas are marginal.

Having done quite a few IR implementations I'd never spec a system that used more than 2 pulse lengths (except maybe a long pulse as start/sync marker) , as pulse length varies significantly with link quality, ambient light etc. IR receiver performance is also dependent on the range of pulse lengths. I've never heard of RCMM, but all the common European consumer protocols I've encountered (RC5,RC6,Sky) have essentially 2 pulse lengths, so this is what most IR receivers are designed to work with. I only use Vishay receivers, but this is mainly as my stuff is often very low-power and the Vishay has a very fast and consistent power-up to 'ready' time and available in 3v versions.

Reply to
Mike Harrison

better software so it

Yes that's what I thought, although it not so much better as more tolerant. Mistaken are worse than non received ones IMO. It just I am not 100% sure what the accuracy is, its a VCXO which varies for various reasons, but the swing and clock accuracy have been included in the 1us. Having said that the Vishay works fine with RCMM with the larger no-mans land.

more than 2 pulse

varies significantly with

the range of pulse

The IR burst length does vary in reception as you would expect (and software has a large margin for this), but cycle should be more accurate, however optical power is still a big variable for pulse length. The software triggers on cycle, but since the receiver varies so much on trigger time (or jitter on the start) with optical power it can affect the pulse length.

encountered

receivers are designed to

low-power and the Vishay

versions.

RCMM was designed for PC wireless stuff like keyboards and mice according to SBprojects.coms, why we every chose it I do not know, its not like we need the bandwidth.

The Vishay stuff is good, it also works better in sunlight, although in my testing I have noticed the burst length does occasionally go outside our software limits.

-- Tony

Reply to
Tony

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