Electronic – SPI with chip select to ground

spi

If you have a SPI bus with only one device can you connect chip select to ground?

I'm new to electronics, tinkering with a custom split keyboard design. I have a micro controller in the one half and a MCP23S17 IO expanded in the other. I have a five strand cable to connect the two. I think I need MOSI, MISO, SCK, Ground and 5v+. Do I need a dedicated chip select, or can I connect chip select to ground (pulled low?) if the only spi device on the spi bus in the IO expander?

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Most SPI devices use the CS as a framing signal as well as for bus sharing, so you can't get away with just tying it to GND. From a quick look at the datasheet this appears to be the case for the MCP23S17.

If you need to save pins, consider using the IC's I2C sibling MCP23017. If you need to save pins and absolutely must use the MCP23S17, you could probably contrive some circuit to set CS high (i.e. deasserted) when SCLK has been idle for a few microseconds at the end of a transaction. That would be a pretty ugly hack though, and it would be difficult not to violate timing requirements on the other end when it needs to assert CS at the start of the next transaction.

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Electronic – Interfacing a MCP23S17 (SPI) with a FPGA

The MCP23S17 is really meant to be connected to a microcontroller. I have used it successfully in a Blackfin-based project. It has a number of internal registers, just like the GPIO ports on a typical microcontroller. Each 8-bit port has a direction register, an input register and an output register, plus registers for input polarity and interrupt-on-change. There's also a global configuration register.

It does default to all-inputs at power up, so if that's all you need, then you just need to create a state machine that reads the two input registers. Note that you need to supply both a chip address byte and then a register address byte for each read cycle.

Also, you need to be aware that this chip has the funky feature of having two different address maps for the registers, depending on the setting of the "BANK" bit. Study this part carefully; it's pretty confusing.

The BANK bit is zero on power-up, so the two registers you want, GPIOA and GPIOB are found at addresses 12 and 13, respectively. Therefore, to read them both, you need to do two 24-clock SPI cycles:

CS:   1111000000000000000000000000111111110000000000000000000000001111
MOSI: xxxx0100aaa10000110000000000xxxxxxxx0100aaa10000110100000000xxxx
MISO: xxxx0000000000000000AAAAAAAAxxxxxxxx0000000000000000BBBBBBBBxxxx

"aaa" represents the chip address.
"AAAAAAAA" represents the data from port A
"BBBBBBBB" represents the data from port B

Note that everything is MSB-first.

Electronic – chip select on one to one spi connection

In general, it depends on the part. Some parts use the CS going high to process the data in the receive buffer.

Specifically for the Atmega168a in slave mode, yes, you can just ground the CS. However, you'll have no way to reset the SPI logic, so if you are off by one bit on one transaction that error will continue to propagate.

So, it's possible, but not recommended. Especially if you're still attempting to debug the connection.

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Electronic – Interfacing a MCP23S17 (SPI) with a FPGA

Electronic – chip select on one to one spi connection

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