Re: Timing in Synch Comm.


"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:aFyIi.695$6p6.209@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Vladimir Vassilevsky wrote:



Jon Slaughter wrote:

How critical is keeping the timing relatively stable in synchronous
communications? Obviously the slave is synched to the master's clock
but is
there ever any problems if the clock timing is extremely unstable? Say
varying up to 10% on average but even peaking to 100% or more in rare
circumstances.

I'm writing a windows app which sends data to the parallel port but
because its pre-emptive there can be extreme latency in the timing. The
data is always synched with the clock so there is no issue with that but
just of the clock's frequency varying a great deal. I imagine since its
synchronous comm. that it should matter but just wondering if there are
instances it could?

Lets say, for example, that its a clock at 1Mhz but then stops
completely for several ms. Could this cause any problem with any device?


Yes, it can cause problems in some cases.

AVR Mega644 is indeed sensitive to the SPI clock irregularities while
being programmed. It causes the programming to fail. I suppose the other
new AVRs are sensitive to this, too. The older AVRs don't have this
problem; you can interrupt the process at any moment for any long time.


I know its the nature of synchronous devices only to send/recv data on a
clock transition and it would seem that frequency variations wouldn't
matter but I just want to be sure that its not going to be an issue.


This can be the issue - it is a fact. We can only speculate why it
happens; perhaps, the internal state machines may have a timeout
protection or something. I didn't believe it before I encountered that
problem myself.


I believe the OP wasn't talking about SPI. But with SPI you are right.
Many devices, even simple DACs and ADCs have time-out circuits in there
and when SPICLK hasn't come for a certain period of time during a
transmission they will abort for the affected data set.


Actually I'm trying to do it any synch communications. SPI, I2C, and ICSP
will be some of the protocols I'll try and implement. Kinda sucks that there
is at time out and since SPI doesn't have an acknowledgement it makes it
even works ;/



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