pseudo random number between X and Y? [Embedded]

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From: ChrisQ on 17 Nov 2009 09:12

Go Rf wrote:
> Hello all
>
>
>
> I need a function, when called returns a pseudo random number between X and
> Y.
>
>
>
> I have many boards that periodically broadcasts their board addresses on an
> RF link. All boards use the same RF-channel frequency, so data collisions is
> going to happen. So I need to randomly delay between those broadcasts to be
> able to avoid collisions with other boards broadcasts.
>
>
>
> I want to make a pseudo random delay (X ms - Y ms) before sending the
> package again.
>
>

You could have a look at the Aloha radio network or ethernet spec, to
see how it's been done before...

Regards,

Chris

From: ChrisQ on 17 Nov 2009 09:21

Go Rf wrote:
> "Wil Taphoorn" <wil(a)nogo.wtms.nl> wrote in message
> news:WTMSB29E82BE(a)wtms.nl...
>
>> The boards use different addresses I assume. Should a delay calculated
>> from (part of) the board's address not suffice?
>
> The boards use a 32-bit address that is unique. So I should be able to use
> it. If I for example take 8-bits from theboard address to specify a delay.
> And the next time I take the next 8 bits.
>
> For example
>
> delay = bit(7 to 0) of address
> delay = bit(8 to 1) of address
> delay = bit(9 to 2) of address
> ...
> delay = bit(31 to 24) of address
>
> and then start again at:
>
> delay = bit(7 to 0) of address
>
> But some combinations of addresses works better than others. And it gets bad
> if i enumerate all boards starting at address 1 and the 2, 3, 4, 5 etc.
>

That assumes that all the address bits are distributed fairly evenly,
which may be unlikely. A better way might be to use a hash of the
address and use this to create a backoff table in memory.

Dunno, I would still have a look at Aloha first, which solved exactly
that problem and was the forerunner of ethernet...

Regards,

Chris (Who should be getting on with some work today :-)

From: Mel on 17 Nov 2009 09:23

From: Vladimir Vassilevsky on 17 Nov 2009 09:33

Go Rf wrote:

> Hello all
> I need a function, when called returns a pseudo random number between X and
> Y.

Oh, dear.

> I have many boards that periodically broadcasts their board addresses on an
> RF link. All boards use the same RF-channel frequency, so data collisions is
> going to happen. So I need to randomly delay between those broadcasts to be
> able to avoid collisions with other boards broadcasts.
> I want to make a pseudo random delay (X ms - Y ms) before sending the
> package again.

Abandon this task. It is not for the sorrow programmers like you.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

From: Tim Wescott on 17 Nov 2009 10:02

On Tue, 17 Nov 2009 14:06:07 +0100, Go Rf wrote:

> Hello all
>
>
>
> I need a function, when called returns a pseudo random number between X
> and Y.
>
>
>
> I have many boards that periodically broadcasts their board addresses on
> an RF link. All boards use the same RF-channel frequency, so data
> collisions is going to happen. So I need to randomly delay between those
> broadcasts to be able to avoid collisions with other boards broadcasts.
>
>
>
> I want to make a pseudo random delay (X ms - Y ms) before sending the
> package again.

Did you do a web search for "random number generator"?

For your purposes a linear shift register ought to work, particularly if
you seed it with your board address. There's a nifty way to go through a
linear shift register N bits at a time; it requires a table of register
states 2^N words long and as wide as your register but the code is short
-- depending on your program storage space you could use that and get a
really big change from one number to the next. Otherwise just iterating
the thing once and taking the bottom bits would probably be random enough.

--
www.wescottdesign.com

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