Subject:
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Re: Digital outputs.
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 1 Jan 1998 10:06:32 GMT
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Original-From:
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David Kott <dakott@alpha.delta.AVOIDSPAMedu>
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Viewed:
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1414 times
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On Wed, 31 Dec 1997, Tom Brusehaver wrote:
<snip>
>
> G> Wich are the options to have 3 digital outputs in the handyboard?
> G> No matter if i have to do little modifications to the Hardware. I
> G> already know how to conect the keypad if you can tell me how
> G> obtain 3 outputs.. :)
>
>
> The SPI port is sitting there. I think you can get at 3 outputs from
> those pins (SD/RD/CLK).
</snip>
yet ANOTHER idea, if you are suitably masochistic, is to use the SPI port
for a much more extensible I/O system. As you know, SPI uses 4 basic
control signals, MOSI, MISO, CLK and SS... Ok, what we used to do in
Embedded Controllers class is hook up a Serial In/Parallel Out
(hereforward referred to as a SIPO) shift register. You must have at
least ONE output available, so, this pretty much eliminates a L293 or, if
you are bold enough, obtain additional outputs using a '138 as outlined in
the HandyBoard FAQ. A SIPO you might use is the 8 bit 74164.
Hook the DATA IN on the SIPO to the MOSI pin on the SPI. Hook the
SPI's CLK output to the SIPO's clock pin. Use a transparent latch to
update and hold the data on the outputs of the SIPO and update it with the
one output pin, perhaps a 74373.
To update the new 8 bits of data appearing at your latch's output, you load
the SPI's data register with the byte value that you want to have across
your new outputs. This data will be shifted out during the next 8 E-clock
cycles. After the SPI's data register is empty indicating that the SIPO
has the output byte on it's parallel outputs, pulse the single control
output to update the latch's outputs.
With this arrangement, you could, in theory, have many, many SIPO shift
register/Latch pairs; the Serial Data In of the next stage connected to
the last Parallel Output on the previous adjacent stage.
One would just have to make sure that you coordinated the number of stages
with the number of bytes outshifted by the SPI data register (naturally).
The downside to this arrangement is the time it takes to update a digital
output. The entire train (8 bits, 16 bits, 24 bits... more?) Need to be
loaded and shifted out to change just ONE output. The upside is, the data
will shift at 2 Mhz, which makes for a (250 ns * [8+2] ) 2.5 ms update
time. Not suitable for time critical applications, (PWM, Communication)
but not bad for bulk outputs.
I don't think I have explained my little circuit here very well.. perhaps
an ASCII graphic?
Output originally going to an L293D
+---------------------------------------+
|Or added via a '138 from the |
|expansion buss. |
| +--+----+
+----+---------+ +---------+ | LE |
| | SPI CLK |'164 PO0|----| '373 |----
| +-----------+ CP PO1|----| |----
| 68HC11 | SPI MOSI | PO2|----| |----
| +-----------+ Data PO3|----| |---- New
| | | PO4|----| |---- Digital
+--------------+ | PO5|----| |---- Outputs
| PO6|----| |----
| PO7|----| |----
+---------+ +-------+
Where:
PO# is a "Parallel Output" on a SIPO
Data is the "Serial Data Input" on a SIPO
CP is the SIPO's clock
-d
Win95/NT - 32 bit extensions and a graphical shell for a 16 bit patch to
an 8 bit operating system originally coded for a 4 bit microprocessor,
written by a 2 bit company that can't stand 1 bit of competition. -UGU
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Message is in Reply To:
 | | Re: Digital outputs.
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| (...) G> Wich are the options to have 3 digital outputs in the handyboard? G> No matter if i have to do little modifications to the Hardware. I G> already know how to conect the keypad if you can tell me how G> obtain 3 outputs.. :) Handyboards are (...) (27 years ago, 31-Dec-97, to lugnet.robotics.handyboard)
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