On úterý 25. září 2018 16:56:14 CEST, Marco Felsch wrote:
This patch fixes 'commit 9b3e4207661e ("pinctrl: mcp23s08: spi: Fix regmap
debugfs entries")'. For sure the MCP23S18 device has only 1 address pin
but this will get decoded into three internally ([1], section 1.4).
[1] http://ww1.microchip.com/downloads/en/DeviceDoc/22103a.pdf
Hi Marco, Section 1.4 only applies to I2C devices. Quoting the first
paragraph:
The ADDR pin is used to set the slave address of the MCP23018 (I2C only)
to allow up to eight devices on the bus using only a single pin.
Typically, this would require three pins.
Here's my summary of how it works among the whole family of these chips:
- MCP23O18 is an I2C device. Its address selection is a bit unusual, but it
can still provide 8 different addresses. In Linux, these are separate I2C
clients as usual.
- MCP23S18, on the other hand, is an SPI device. It does not support any
extra addressing apart from the SPI CS pin (see page 13 of the datasheet
you linked to). As far as I can tell, it's always just one device on a
given SPI CS signal.
- MCP23S17 is also a SPI device, but it supports a creative/hacky
"addressing" scheme where up to eight devices can share the same SPI CS
signal. On Linux, this opens an ugly can of warms with several "gpiochip"
instances within the same SPI client, but that's not relevant in the
context of this patch.
I know that the current code works with MCP23S17 -- we have a board with
two chips with a shared SPI CS, but a different "address" prefix. Looking
at the datasheet, MCP23S18 works in a different manner. I therefore don't
think that this patch which effectively unifies handling of both ...S18 and
...S17 is correct.
Can you please clarify what doesn't work for you, and how it should work?
Did my patch 9b3e4207661e broke something?
With kind regards,
Jan
