Drivers are subdivided in 3 levels: I/O level, includes basic I/O functions to control and read hardware data lines; bus level, over I/O level we can create a bus concept, implemented buses are for example SPI and I2C bus, SPI bus implementation will simplify device drivers that makes use of it. Finally we have device driver level: over bus implementation and I/O level are implemented all spacific device derivers.
There is a device driver for each device or for each family of devices: some device driver may be able to handle more than one device, for example I2C memories are very similar each other, a full family of memories can be driven by a single device driver.
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