I2C RS485 Chip

[Dave Boland]

A design I was working on was going to use the MAX3140 SPI
<--> RS-485 chip. This is an integrated UART and set of
drivers. The plan has changed and the RS-485 portion moved
to a daughter card. Since all cards will be connected via
I2C, I was looking for a similar chip similar to the 3140,
but with an I2C interface. Can't find any, but perhaps
someone knows of one.

The alternative (and actually a better one in some ways) is
to use a processor with I2C port and a UART port, and
drivers. Problem with this idea is more software needs to
be done (no time) and more board space (not much of that
either).

Help appreciated.

Dave,

 

[PeteS]

You can actually use SPI pins (usually) for I2C. See this page (for
example)

http://www.i2cchip.com/mix_spi_i2c.html

I would also check the datasheet on whatever you are sourcing the SPI
from, as it may support I2C directly (with a little register tweaking).

Cheers

PeteS

 

[Tony]

The cheapest way will probably be to use a microcontroller that has
UART and I2C on-chip - there are plenty around (although there are
more with UART and SPI, which is what we're using in the same
scenario).
Tony

You can actually use SPI pins (usually) for I2C. See this page (for
example)

http://www.i2cchip.com/mix_spi_i2c.html

I would also check the datasheet on whatever you are sourcing the SPI
from, as it may support I2C directly (with a little register tweaking).

Cheers

PeteS

Tony (remove the "_" to reply by email)

 

[Dave Boland]

Tony,

I agree that using a microcontroller has a number of
advantages. There are some disadvantages though. One is
that is that code will need to be developed and maintained.
This translates into development time and cost.

The other problem is that few, if any, microcontrollers can
be used as an I2C slave directly. To do this with the PIC,
Atmel, Zilog, and most flavors or 8051/52 requires doing it
in software with IRQ's. This solution may be a performance
problem.

I'm arguing to keep the RS-485 port on the processor card so
that SPI can be used (no -CS's are passed to the daughter
cards). SPI has a much faster data transfer rate, so it
will work much better than I2C (even the high speed I2C).


Dave,