# Cheapest way to wirelessly synchronize two MCUs

I have two battery powered MCUs about 30cm apart, one will be the master, and one the slave. Each MCU has several LEDs connected which flash in a sequence.

All I want to do is synchronize the two MCUs so that the sequences don't drift out of phase.

Is there some super easy way to send a simple RF pulse from one MCU to the other?

• The sequence lasts for approx 1 second, and repeats forever.
• If the slave misses some sync pulses, it's not the end of the world, because it would take many cycles to drift noticeably out of phase.
• For personal / religious / aesthetic reasons which I am not allowed to discuss, I do not wish to use IR, capacitive, acoustic means to transmit this pulse.
• The range is very short, less than 30cm.
• The signal will need to travel through open air. No concrete or metal in the way.
• RF-like inductive coupling would be OK too.
• Since the signal consists of only a single pulse, I imagine there must be a smaller, cheaper solution than those involving packet transmission (Xbee, etc.)
• Ideally the solution would consist of some kind of PCB trace antenna, and a simple oscillator for the transmitter, and simple circuit (demodulator?) to detect the pulse at the receiver.
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A better decription of your environment would save everyone time and improve answer quality. There are many "obvious" solutions that may not be suitable BUT as you do not explain your al need nobody can tell*. Please explain why RF not suited. NFC may suffice - electromagnetic RF LIKE coupling. Optical may work. Acoustic may work. You do not mention wire connection. Is this unsuitable - if so, say why, as that is the most obvious and easiest method. [[* eg potted in concrete, welsed in a mumetal drum at the bottom of a shallow shark pond ... :-) ]] –  Russell McMahon Mar 26 '12 at 0:57
Unteachable. Intractable. Declines to accept sage advice. D- fail. Bzzzzt. Next please. –  Russell McMahon Mar 26 '12 at 10:10
Tha's pretty rude. Do you not see my extensive and apposite answer with fully fleshed RF solution. Is that not what you asked for? –  Russell McMahon Mar 26 '12 at 10:58
@RM - You asked for an RF or other solution. I provided one possible RF solutions in vast detail + suggestions re searching for others. You have not commented on or shown any sign of having seen my RF solution but keep asking for an RF solution. You say an IR solution will not work but given what you have told me I could have build an IR solution with ease. You say you have disclosed all data relevant to an IR solution and also say it cannot be done. I know [tm] that your statement set as a whole is logically inconsistent, and you are asking for an RF solution but ignoring the one given. –  Russell McMahon Mar 26 '12 at 13:01
So - serious question - How would you summarise the situation from my viewpoint. What have I got wrong. Why can't you see my RF solution? Why do you keep asking for one when there is one laid out in vast detail - even if just to say it doesn't suit, and why. When I, afer too many decades playing this game KNOW I could make IR work given your spec, why do yu say I am wrong, and on what grounds? You say I refuse to answer your question but you seem to be refusing to read my answer. How can I do better? Really? –  Russell McMahon Mar 26 '12 at 13:04

The application SOUNDS so trivially simple that, if it isn't, there is no way to know due to the "death by 1,000 blows" approach to defining the problem. At 30 cm the way to use RF is to use RF. Any RF equipment that I have ever met would work over that range in free air in the absence of metal. You can buy transceivers and transmitter / receiver pairs. With the stated specs, if it transmits and receives RF it will be suitable. If any RF data transfer solution does not work in this context then it is because the spec is inadequate. Simple RF solutions usually have minimal latency once the data stream is synchronised. All that is required is Sync_phase - delay_until - sync_now. QED. Repeat as needed. Processor helps BUT a cheap no processor solution is easy.

More complex RF systems such as Bluetooth will also work but may have variable latency. This can be overcome by some interactive communications that establishes link parameters OR designed around known performance. BUT simple cheap off the shelf one way RF modules will work well enough. "ASK" transmission is probably fine and gives you something to search for.

plus this $4 transmitter or its up frequency cousin will together give you a$9 solution if you are happy to talk one way.
Double that for 2 way.

Or

Two of these $7 transceivers - 434 MHz will give you a bidirectional$14 solution. ~ 16mm x 16mm x 4.2mm.
That size is known OK as the spec did not specifiy otherwise when all details were provided.
That cost is known OK as the spec did not specifiy otherwise when all details were provided.

Two of these give you a WiFi link - over over over kill.

• BUT

In the absence of any real clues as to why the following are not suitable, the following may be suitable:

• IR may work well and easily and cheaply.
Unless you are an IR expert, your reasons for excluding it may well be unsound.
With what you have told us so far, IR would be the candidate solution of choice.
Low cost, easy, low power. ie
Better, cheaper, faster - choose any three!
Tell us what you want and we'll tell you what you need :-)

• Electromagnetic "RF LIKE" "near field" coupling may or may not suit better than RF. .

• Optical

• Acoustic

• Capacitive

• ... GSM ... Iridium ... Inmarsat ... [just joking] :-) .

Pleased explain clearly & completely the environment and relevant constraints. eg:
Potted in concrete, welded in a mumetal drum at the bottom of a shallow shark pond ... :-).
Indoors, outdoors, ... .

There are many "obvious" solutions that may or may be suitable BUT as you do not explain your real need nobody can tell*.

Again: Tell us what you want and we'll tell you what you need :-)

IR sounds good to me, so far.

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Er, what? I didn't say that I excluded RF. I said I ruled out IR. Why can't you just trust me that I'm sure RF is the way to go? –  Rocketmagnet Mar 26 '12 at 3:15
I did not mention a wire connection, but I did clearly say WIRELESS. –  Rocketmagnet Mar 26 '12 at 3:21
@Rocketmagnet: I agree with Russell that you haven't really given a good reason for why non RF methods are inappropriate. Part of the purpose of this site is to make sure you're asking the right question and getting the right answer, not just to get a random answer. –  W5VO Mar 26 '12 at 5:27
Do you guys need to have my whole life story before answering the question? Is it too much to assume that I have already considered the various other options, and would have done something easier like IR if it was appropriate? –  Rocketmagnet Mar 26 '12 at 9:28
@Rocket: Yes, it is too much to assume that you have considered all the options. We don't know your competence level and of course have no idea what you think all the options includes. Often enough the right answer to a question here is to go back and think about the problem differently. If you knew all the answers, you wouldn't be here asking. Your judgement of supposed implementation is of no relevance here since your competency has not been established yet. If you are getting answers that you don't like but meet the requirements, then you provided incomplete requirements. –  Olin Lathrop Mar 26 '12 at 13:08

Please provide information on your budget constraints (power consumption, cost of parts, cost of your time, or hard time constraints (such as, I have one week to make this work))

Many of the RF modules listed have reviews posted about noise problems, which will require more software development or systems integration work to filter out the noise and end up with a usable signal

In addition to the RF modules listed above, there are Zigbee modules in the \$20 price range available from several sources. They have the advantage of having a standard interface, and ZigBee was intended to be used in "sensor networks" and other systems with low power availability (you mentioned battery power).

If you are already Arduino-friendly, there are some time savings to be had with a Zigbee/Xbee setup.

Good luck!

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There is no deadline, but I would rather not spend 1 year tuning the system to make it work. I am aware of things like Zigbee, but since my application requires no data to be transmitted, I was hoping there would be an even simpler, cheaper, smaller solution. –  Rocketmagnet Mar 26 '12 at 10:47
@Rocketmagnet smaller than the 3rd picture is hard...give a look to Jennic, it's embedded micro+RF and has a very simple interface –  clabacchio Mar 26 '12 at 10:55
@The Programmer - At a range of 300mm, and needed time to deliver one time coordinated data bit of hours, I think the modules listed are up to the job [tm]. Modules that HAVE formal protocols are at a disadvantage here - BUT see the documentation I referenced. –  Russell McMahon Mar 26 '12 at 12:54