I am working on a Blackfin 537 project running uClinux. The BF537 has a
built-in hardware SPI port with up to seven slave select outputs, of which I
can use four or five (the others aren't available because they share I/O pins
with something else I need more). I have 10 SPI devices I need to control.
I am looking for advice about the best way to access more SPI devices (the
way that will fit the best with how the Linux SPI driver works). I can think
of
four options:
#1: Chain multiple SPI devices together like shift registers using the
same slave select.
#2: Use GPIO lines as additional slave select outputs.
#3: Use software and GPIO lines to make an additional bit-banged
SPI interface for the slow devices.
#4: Demultiplex the existing slave select outputs.
Option #1 is straightforward enough, but makes it difficult to write
independent
drivers if the chained devices serve different logical functions. I suppose it
would be possible to make separate drivers for each device, then make them
connect to a virtual SPI port that would combine the data for the different
devices into a longer data string and send it out to the chain of devices.
Does
Linux already have such a virtual SPI port?
Option #2: It is easy enough to turn on a GPIO-based slave select before
causing an SPI transfer, but how can I tell when the transfer has finished and
it is safe to turn it off? Is there a hook in the SPI driver for that?
Option #3: If I bit-bang GPIO lines to control not only the slave selects (as
with option #2) but also the SPI data and clock signals, it will be easy to
control the sequence of events, but will be a lot slower than using the
hardware-assisted SPI interface. Does Linux already have code for a
bit-banged SPI interface? Where would I find docs for it?
Option #4: I don't know if it is practical on the Blackfin or not. What I
have
in mind is something like the way the 68HC16's queued SPI provides four
programmable slave selects that can be used individually to select one of
four devices, or demultiplexed to select one of up to 16 peripheral devices.
Does the Linux SPI driver provide a way to specify that a combination of
slave selects needs to be asserted to select a particular slave?
Thanks for any suggestions or advice.
Steve
---
Steve Strobel
Link Communications, Inc.
1035 Cerise Rd
Billings, MT 59101-7378
(406) 245-5002 ext 102
(406) 245-4889 (fax)
WWW: http://www.link-comm.com
MailTo:[EMAIL PROTECTED]
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