SPI - Design help for Interfacing with a large number of SPI slaves

Using Arduino Networking, Protocols, and Devices
#1

Hi all, I am trying to develop a system in which I can communicate with 30+ SPI slaves. The slaves are BMP280 pressure sensors. I currently have a working system with 16 SPI slaves, in which I use SIPO shift registers to control the slave select lines of each sensor (reducing the I/O requirements of the master). One key feature is that I would like to send a 'command' from the master to all the slaves synchronously. To do this I 'disable' the shift register outputs, using the 'enable' pin, and have a pull-down resistor on each of the slave select lines, ensuring they go to Ground.

This system works in practice, however, in my mind seems quite clunky and not particularly scalable ( i.e. for 64 sensors would need 64 resistors....) So I was just wondering if anyone had any advice/suggestions on a different way to produce this system to reduce the amount of hardware required, whilst still keeping the same functionality??

Many Thanks,

Fintan

#2

Couldn't you just use the SIPO shift register itself to select all slaves by loading it with zeros to set all outputs low?
Do you have a link to the shift register?

#3

Can you use an IO expander like this one? http://au.mouser.com/new/NXP-Semiconductors/nxp-PCAL6416A-io-expander/ or TCA9555, they seems to be very flexible and you coukd easily control a single CS or all at the same time.

The only issue is that those expanders normally have a reset state as Input so you need to make sure you set all as Output high straight after boot. You may be able to find one that has a pull-up enabled as default.

#4

ehm… can you not simply send 0x00 to all your shift registers prior of sending your command? Than you dont need any resistors at all

#5