I'm designing a controller, and for analog and digital I/O I want to use
serial I/F devices with some devices remote from the micro board in several
expansion boxes. I can't decide whether to use I2C or SPI/Microwire bus
systems. Ideally I wouldn't use both.

Each expansion box needs 4 or 8 latched outputs and 4 or 8 analog
(12bit) inputs. (I haven't yet decided exact no of channels required per box).

I don't really want to implement both buses, and would be happy with the
I2C bus which is very simple, hardware wise, but I can't seem to get all
the  chips in I2C with the functions I need.

I thought to use I2C PCF8574's for each set of 4/8x latched outputs, but
would
then have to use SPI devices for each set of 4/8 I/P multiplexed 12 bit A/D,
since I can't find a multichannel 12bit I2C A/D converter. I am considering
the LTC1298 with LTC1390 multiplexer or an LTC1290.
I don't need fast conversions. Any other suggestions?

(Interestingly the LTC1380 is an SMB/I2C bus 8 I/P MPX chip, but Linear
don't seem to do a matching 12 bit A/D converter with SMB/I2C bus. Does
anyone do one?)

Lastly, does anyone know what length of cables can I run between the micro
and each expansion box, and/or any information on max capacitance loading /
impedance levels for the two bus systems - I can't find this application
information anywhere in the datasheets?

Any help would be appreciated.

John

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 John Lawton Electronics: john@jle.co.uk   www.jle.co.uk
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