How does the slave know when the master has read the data?  Is it=20
possible that the slave is sending data, and then sending the next data=20
before the master gets around to reading the first data?

I usually use 2 control lines, Master to Slave and Slave to Master. =20
Lets call them MS and SM.  When the slave wants to send data, it sets SM=20
high.  When the Master see SM high, it reads the data and sets MS high. =20
When the slave sees MS high, it sets SM low.  When the master sees SM=20
low, it lowers MS.  When the slave sees MS low, it knows it can send the=20
next set of data.  This works 100% of the time, with no chance of missed=20
data.

Kerry


Josh Koffman wrote:
> Hi all.
>
> I'm seeing some weird behaviour with the external interrupts, and I'm
> not sure what's going on. I'm using an 18F4321. This chip is
> exchanging data with another 18F4321 over a parallel bus (not the PSP,
> something I've worked up with a few control pins). I am using INT2 as
> an indicator that the slave chip has dropped the data ready (DR) line,
> put new data on the bus, and then reasserted DR. Because some other
> things are a bit more timing sensitive, I am polling for the INT2 flag
> in my mainline. I've basically moved as much as I can out of the ISR
> to help ensure a consistent response to the INT0 interrupt (more on
> that below). Everything works great...well, sort of.
>
> On INT0 I have a sensor that is producing and interrupt about every
> 1/12s (0.083s). Once I connect up the sensor, I find that INT2 doesn't
> consistently set the flag anymore. The sensor is dealt with in the ISR
> and triggers a burst of data out the UART. Instead of polling, I've
> tried dealing with it in the ISR. I've tried both edge settings
> (trigger on transition low and trigger on transition high).  I've
> checked the code and I can't see any unintentional clearings of the
> flag. Because it doesn't flag the change in the control signal, my
> data exchange stalls.
>
> I'm really confused by this one. I can't think of a reason why INT2
> isn't catching the transitions. Is there something I'm just missing
> here?
>
> Thanks!
>
> Josh
>  =20



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