You can take 4 GPIO pins and feed them into a decoder, say a '138. Then use the CS pin as the enable for the decoder. Based on the settings of the 4 GPIO pins, the appropriate CS pin will be activated only when the base CS is active. This means there is no complication added to the SW. Just set your four bits, and then read from or write to the baseline address and the desired devcie will be activated and the CS will be timed to the main CS strobe…
Gerald
Excerpts from Gerald Coley's message of Sat Feb 20 18:13:47 -0500 2010:
You can take 4 GPIO pins and feed them into a decoder, say a '138. Then use
the CS pin as the enable for the decoder. Based on the settings of the 4
GPIO pins, the appropriate CS pin will be activated only when the base CS is
active. This means there is no complication added to the SW. Just set your
four bits, and then read from or write to the baseline address and the
desired devcie will be activated and the CS will be timed to the main CS
strobe..
That is a fantastic idea. I wish I had thought of this. Thanks a ton!
- Ben
Glad to help! I have been thinking on that and I went back to the “old” days and reapplied it to this situation.
Gerald
May I still add three comments:
When I, in my previous email, referred to I2C and USB I meant the
BeagleBoard expansion capabilities in general, not just A/D and D/A
converters. And remember that you can implement a complex, high
performance A/D and D/A subsystem that communicates with BeagleBoard
via HS USB 2.0 with the nominal maximum speed of 480 Mbit/s!
And, when I was referring to McBSP (sorry about previous typo), I had
the audio A/D and D/A I2S applications, such as those described in the
OMAP3530 manual chapter 20.6 (starting page 2988), in mind. Here again
the GPIO CS multiplexing that I proposed works well.
Thirdly, there are many serious converters that can work in the TDMCA
mode. Up to 30 of them can be daisy-chained for input (A/D) and
another 30 for output (D/A) without extra hardware support. The
BeagleBoard epansion connector McBSP1 and McBSP3 can both easily
implement the TDMCA protocol (even with the SDMA support if you wish)
and so you have 2 x (30 + 30) = 120 converters on-line with no chip
select signals needed. See TI PCM1792A data sheet pages 46 - 53 for
details. But logic level converting buffers are, unfortunately,
typically needed.
As there are so many flavors of converters available with half a dozen
of popular protocols, the optimal solution is usually highly
application dependent.
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