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'[EE]: variable speed serial communication'
2002\09\07@150325 by Brendan Moran

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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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2002\09\09@030109 by Vasile Surducan

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Brendan, if I'll be in your skin, I'll use a fixed speed serial link to
PC, the faster I can afford (  up to 500kB on serial ? or much more on USB
) and a local PIC for the rest of the job. However I have no ideea how you'll send
an acknowledge at every 12MHz clock from a PIC...


regards,
Vasile
http://www.geocities.com/vsurducan


On Sat, 7 Sep 2002, Brendan Moran wrote:

{Quote hidden}

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2002\09\09@031602 by Brendan Moran

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At 10:12 AM 09/09/2002 +0300, you wrote:
>Brendan, if I'll be in your skin, I'll use a fixed speed serial link to
>PC, the faster I can afford (  up to 500kB on serial ? or much more on USB
>) and a local PIC for the rest of the job. However I have no ideea how
>you'll send
>an acknowledge at every 12MHz clock from a PIC...

Thanks for the thought, Vasile.

A mid-range PIC can't handle it.  I'd have to use an 18 series at 48MHz,
and even that's close...  If I were going to use a programmable part, and I
were to use a PIC, I think I'd go with an 18 series PIC.  I'd operate on an
SRAM, and send up to 16k at a time.

I think I'd use a constant 3MHz clock...  I'd prefer, though, to use a
system built on discreets.  I'm trying to make this so that it doesn't use
programmable parts, though I may not be able to avoid it...


Regards,
--Brendan

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