piclist 2002\09\07\150325a >
Thread: variable speed serial communication
www.piclist.com/techref/io/serials.htm?key=serial
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face BY : Brendan Moran email (remove spam text)



Hi, everyone,

I'm working on a project where I will need a variable speed serial link
from a PC to a chip.  I want to do this with discreets if possible, but if
I can't, I'll just use a micro to do it.

The communication will consist of command vectors, acknowledges and data bytes.

The specifications state that:
-Each command vector is 22 bits long, but any zero padding is ignored, as
are any number of 0s between vectors.
-There are two clock modes: Transfer and Acknowledge
-Transfer may be executed at any speed, but Acknowledge is fixed at 2,
3,3.578,4,5,6,6.6666,7,8, 9,10,11, or 12MHz, and the frequency must be very
accurate, as the device is informed what frequency it is running at.
-The data line must be tristateable
-Acknowledge consists of setting the clock frequency, and tristating the
data line, then wating for a high to low transition on the data line.


I'm not sure a PC parallel port will cut it for this job...  I've been told
that the maximum polling frequency on a parallel port is ~5kHz.

The best parallel port solution I've come up with is this:  use a 4MHz
clock generator outside the PC, and have it enabled by a
flipflop.  Configure the flipflop to clock on high-to-low transitions on
the data line, so that the 4MHz clock is disconnected when the flipflop
receives a high-to-low transition. When the 4MHz clock is disabled, enable
a 1kHz clock, and read/write to the device on that freq.  Also use the
flipflop to inform the PC when the Acknowledge has been received.

The target device has TTL compatible inputs.

Does anyone see a better solution than this?

--Brendan

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