Serial I/O chip

ljbeng · Joined: 10 Feb 2004 Posts: 205

I have a 40 pin PIC with almost all I/O used up and we have a new spec to add 8 inputs/8 outputs so I was looking for a serial shift register that would gain me some IO using SPI or IIC. I saw a chip on Maxim website (MAX4820) and requested samples. It gets me 8 relay driver outputs. I would also need 8 dc (+12V) inputs..... Thanks.

Guest · Posted: Tue Mar 29, 2005 12:45 pm

della · Guest

I think you can better use the standard chips 74595 (for 8 outputs) and 74165 (for 8 inputs). There are also drivers in the CCS example directory. These chips run with SPI. And for the relay control add extra transistors or other cheap drivers (maybe also from the standard TTL series).
The special chips from MAXIM are really nice but I think they are more expensive an not easy to get from a distributor. Such special chips are always out of stock...

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

The standard 74xxx chips are cheap and good available but have one major drawback, there is no way you can check the input and/or output is correct. Just imagine there is some noise on the clock line, then the shift registers might get a 1-bit offset. Depending on your application you will have to decide whether or not to take this risk. The dedicated I/O-expander chips have more advanced features, for example allowing you to read back the settings.

Maxim hace some nice chips, but long lead times. A single sample is often quick available, but ordering 100 pieces might take 10 weeks.

Philips has several I2C expander chips like the PCF8574 (8 bit) and PCF8575 (16 bit). The PCF8574 is in stock with Farnell.com, PCF8575 seems more difficult to get.

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Don't like any of those suggestions, you can always make your own I2C device out of a really cheap micro.

jds-pic · Joined: 17 Sep 2003 Posts: 205

fuzzy · Joined: 26 Feb 2005 Posts: 64

I tryed to use PCF8575 to drive some relays by using uln2003. but i had a ot of difficulties.they seems not driving much current to activete uln2003 inputs. i needed 18 external I/O and I had to use 2 PCF.So I decided to use a PIC 16F876 as external I/O expander thata has 20 I/O and 2 pins for I2C or RS232.

It will cost as 2 PCF8575 and it is more versatile and strong.

I only have Problem connecting this PIC as slave on i2c.

in rs232 communication is perfect.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA