Raspberry pi and shift register

Question

I have a project using a Raspberry pi and an 8 bit shift register to pull one of the 8 pins to ground if an event occurs, but leave it at 5 v 99% of the time. Ideally this will be done with Python, but I am open to other languages if necessary.

I have basically 3 parts to this question.

1. Any recommendation on which shift register will perform this task with ease?
2. Is there a source for an easy to understand sample code to work with the above recommendation?

3. Am I going about this correctly or is there a better (easier, more reliable) way?

    PI-|----| S |---------|Arduino_1_Reset
            | H |---------|Arduino_2_Reset
            | I |---------|Arduino_3_Reset
            | F |---------|Arduino_4_Reset
            | T |---------|Arduino_5_Reset
            | R |---------|Arduino_n_Reset
   

Answer 1

As @lenik said, a shift register is inappropriate for this task. You could try using a 3 to 8 decoder. This takes 3 I/O lines in (from your RPi) and turns on one of the 8 outputs based the numeric value of the input signals.

Here is an online applet that demonstrates the functionality of a 3 to 8 decoder: http://teahlab.com/3_to_8_decoder/

Answer 2

You need to find a shift register where you can enable or disable the outputs while shifting, possibly a 74HC595 might do. Have the normal pull-resistors on the Arduino reset inputs, and when shifting put the shift register outputs into high impedance mode. You'll need three of the gpio pins to drive it - one for clock, one for serial in, one for enable.

74HC4094 should also work - they are what's used when driving parallel input 3.2" TFT displays.

Answer 3

The shift register is such a funny thing, that will push "0" or "1" down the chain until the end, so once you reset Arduino_1, there's nothing you can do to avoid resetting Arduino_2 and the others on the next shift register clock.

You definitely need a better HW design.

Answer 4

I would suggest to consider using i2c port expanders, like PCF8574 (8 bit) or MCP23017 (16bit).

You will need 2 wires (I2C clock and data) to connect to your IC and a common ground. You may also want to take power from RPi, that makes 4 wires total to your expander.

Unfortunately, I have no experience with I2C on RPi (only on Arduino), so can't provide more detailed info.