SpiConnectionSettings should have a property for chip select active state

[shaggygi]

This proposal is to add a chip select active state property to the SpiConnectionSettings class.

Rationale and Usage

While most SPI devices are selected with an active-low chip select pin, there are devices that are active-high. To overcome this today (to my current knowledge) is adding additional hardware logic to inverse the select line between master/slave devices.

Proposed Changes

The proposed change is to add a new property for a chip select active state.

• This could be named ChipSelectActiveHigh or similar. It could have a default value of false that represents active-high since this is the minority use.
• It also doesn't have to be included in the SpiConnectionSettings constructor as is similar to other properties like ClockFrequency, Mode, etc..

var spiConnectionSettings = new SpiConnectionSettings(0, 0)
{
    ChipSelectActiveHigh = true,
    ClockFrequency = 1000000,
    Mode = SpiMode.Mode0
};

References

• I commented on similar topic in another proposal Provide a new GpioSpiDevice.

Open Questions

1. Windows 10 IoT appears to only allow for active-low state. Is there a way to change this in a system setting or within Windows.Devices.Spi API?
2. It appears you can change this in Linux, but I have not tested. See https://01.org/linuxgraphics/gfx-docs/drm/driver-api/spi.html. Need to review if this can be performed programmatically or must be hardcoded in SPI settings.

[joshfree]

@krwq could you please investigate for 3.0?

[krwq]

@shaggygi got any specific devices in mind? (just curious)

[shaggygi]

@krwq I came across this when trying to use with bit-banging GPIO pins. You can use SPI with a 74595 logic chip. While I have a partial binding started for this chip, it could be used with something like I2C/SPI Character LCD Backpack using SPI (with active-high CS).

While reviewing this topic, I found that many processors allow CS enable to be configured to Low or High. I also came across various SPI tools where they can be configured and blogs/forums where users state they have a device that needs to be active-high enabled. However, there was never a specific device name mentioned where I could verify the CS interface.

I agree most cases will be active-low CS as that appears to be the preferred method. I'm just thinking this option might need to be available sooner than later. Hope this helps.

[joperezr]

Given that this doesn't really apply to every device, I think that I would prefer if the active state would be passed in (with a paramter on the constructor) on the binding itself. Most of the devices I've worked with only allow having one state for the CS, so I don't think this should go on the SpiConnection class IMHO.

[krwq]

@joperezr I think quite opposite - I'd leave the constructors to the absolute requirement you have to set and move everything unusual to settings which in this case would be something really similar to what @shaggygi has proposed (I'd perhaps change bool to the enum but the same idea).

[shaggygi]

@krwq I like your idea better with an enum. Not sure if you were thinking of a new enum or possibly use PinValue where it would accept a few ways. If so, we might want to use ChipSelectActiveState as the property name and still use Low/false/0 the default.

// By PinValue.Low or PinValue.High
var spiConnectionSettings = new SpiConnectionSettings(0, 0)
{
    ChipSelectActiveState = PinValue.High,
    ClockFrequency = 1000000,
    Mode = SpiMode.Mode0
};

// By false or true
var spiConnectionSettings = new SpiConnectionSettings(0, 0)
{
    ChipSelectActiveState = true,
    ClockFrequency = 1000000,
    Mode = SpiMode.Mode0
};

// By 0 or 1
var spiConnectionSettings = new SpiConnectionSettings(0, 0)
{
    ChipSelectActiveState = 1,
    ClockFrequency = 1000000,
    Mode = SpiMode.Mode0
};

[shaggygi]

But then again, if we added an enum for #202, we might want to group related enums under the SPI namespace like ChipSelectState and DataFlow or similar?

var spiConnectionSettings = new SpiConnectionSettings(0, 0)
{
    ChipSelectState = ChipSelectState.ActiveHigh,
    DataFlow = DataFlow.MsbFirst,
    ClockFrequency = 1000000,
    Mode = SpiMode.Mode0
};

[krwq]

Yes this looks more or less correct - once I (or someone else) start working on this we can take a closer look at the exact shape and what makes most sense (probably makes sense to compare to other implementations like python/C and see what they call these so that we call them similarly).