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These ADCs are marketed as single-channel 22 bit delta-sigma ADCs, but in reality their resolution is 21 bit with an overrange or underrange of 12% beyond Vref. In other words, "full scale" means +/- 2^20. This driver does not explicitly signal back to the user when an overrange or underrange occurs, but the user can detect it by comparing the raw value to +/- 2^20 (or the scaled value to Vref). The chips feature an extended temperature range and high accuracy, low noise characteristics, but their conversion times are slow with up to 80 ms +/- 2% (on the MCP3550-50). Hence, unlike the other ADCs supported by the driver, conversion does not take place in realtime upon lowering CS. Instead, CS is asserted for 8 usec to start the conversion. After waiting for the duration of the conversion, the result can be fetched. While waiting, control of the bus is ceased so it may be used by a different device. After the result has been fetched and 10 us have passed, the chip goes into shutdown and an additional power-up delay of 144 clock periods is then required to wake the analog circuitry upon the next conversion (footnote below table 4-1, page 16 in the spec). Optionally, the chips can be used in so-called "continuous conversion mode": Conversions then take place continuously and the last result may be fetched at any time without observing a delay. The mode is enabled by permanently driving CS low, e.g. by wiring it to ground. The driver only supports "single conversion mode" for now but should be adaptable to "continuous conversion mode" with moderate effort. The chips clock out a 3 byte word, unlike the other ADCs supported by the driver which all have a lower resolution than 16 bit and thus make do with 2 bytes. Calculate the word length on probe by rounding up the resolution to full bytes. Crucially, if the clock idles low, the transfer is preceded by a useless Data Ready bit which increases its length from 24 bit to 25 bit = 4 bytes (section 5.5 in the spec). Autosense this based on the SPI slave's configuration. Cc: Mathias Duckeck <m.duckeck@kunbus.de> Signed-off-by: Lukas Wunner <lukas@wunner.de> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.