Interfacing via SPI

We have an example, minimal SPI-based component here.

Let's take a closer look at this example.

Python

In addition to the usual requisite imports, we have:

from esphome.components import spi

This allows us to use some functions we'll need for validation and code generation later on.

DEPENDENCIES = ["spi"]

Given that this component will use--and consequently depend on--SPI, we must add it as a dependency. This allows ESPHome to understand this requirement and generate an error if it is not met.

empty_spi_component_ns = cg.esphome_ns.namespace("empty_spi_component")
EmptySPIComponent = empty_spi_component_ns.class_(
    "EmptySPIComponent", cg.Component, spi.SPIDevice
)

This is some boilerplate code which allows ESPHome code generation to understand the namespace and class names that the component will use. Note that EmptySPIComponent is inheriting from both Component and SPIDevice.

CONFIG_SCHEMA = ...

This defines the configuration schema for the component as discussed here. In particular, note that the schema is extended with .extend(spi.spi_device_schema(cs_pin_required=True)) since this is an SPI component/platform. Also note that we can configure whether or not a CS pin is required for this component.

Finally, in the to_code function, we have:

await spi.register_spi_device(var, config)

Since this is an SPI device, we must register it as such so it is handled appropriately by ESPHome.

C++

The C++ class for this example component is quite simple.

class EmptySPIComponent : public Component, 
                    public spi::SPIDevice<spi::BIT_ORDER_MSB_FIRST,spi::CLOCK_POLARITY_LOW, 
                            spi::CLOCK_PHASE_LEADING,spi::DATA_RATE_1KHZ> { ... };

As mentioned here, all components/platforms must inherit from either Component or PollingComponent; our example here is no different. Note that, since it's an SPI device, it also inherits from SPIDevice which uses a C++ template to define the SPI bus parameters such as bit order, clock polarity and speed.

Finally, the component implements the usual set of methods as described here. This is all that's required for our minimal SPI component!