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Diffstat (limited to 'Documentation/spi/spi-summary.rst')
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diff --git a/Documentation/spi/spi-summary.rst b/Documentation/spi/spi-summary.rst index 7f8accfae6f9..6e21e6f86912 100644 --- a/Documentation/spi/spi-summary.rst +++ b/Documentation/spi/spi-summary.rst @@ -614,6 +614,89 @@ queue, and then start some asynchronous transfer engine (unless it's already running). +Extensions to the SPI protocol +------------------------------ +The fact that SPI doesn't have a formal specification or standard permits chip +manufacturers to implement the SPI protocol in slightly different ways. In most +cases, SPI protocol implementations from different vendors are compatible among +each other. For example, in SPI mode 0 (CPOL=0, CPHA=0) the bus lines may behave +like the following: + +:: + + nCSx ___ ___ + \_________________________________________________________________/ + • • + • • + SCLK ___ ___ ___ ___ ___ ___ ___ ___ + _______/ \___/ \___/ \___/ \___/ \___/ \___/ \___/ \_____ + • : ; : ; : ; : ; : ; : ; : ; : ; • + • : ; : ; : ; : ; : ; : ; : ; : ; • + MOSI XXX__________ _______ _______ ________XXX + 0xA5 XXX__/ 1 \_0_____/ 1 \_0_______0_____/ 1 \_0_____/ 1 \_XXX + • ; ; ; ; ; ; ; ; • + • ; ; ; ; ; ; ; ; • + MISO XXX__________ _______________________ _______ XXX + 0xBA XXX__/ 1 \_____0_/ 1 1 1 \_____0__/ 1 \____0__XXX + +Legend:: + + • marks the start/end of transmission; + : marks when data is clocked into the peripheral; + ; marks when data is clocked into the controller; + X marks when line states are not specified. + +In some few cases, chips extend the SPI protocol by specifying line behaviors +that other SPI protocols don't (e.g. data line state for when CS is not +asserted). Those distinct SPI protocols, modes, and configurations are supported +by different SPI mode flags. + +MOSI idle state configuration +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Common SPI protocol implementations don't specify any state or behavior for the +MOSI line when the controller is not clocking out data. However, there do exist +peripherals that require specific MOSI line state when data is not being clocked +out. For example, if the peripheral expects the MOSI line to be high when the +controller is not clocking out data (``SPI_MOSI_IDLE_HIGH``), then a transfer in +SPI mode 0 would look like the following: + +:: + + nCSx ___ ___ + \_________________________________________________________________/ + • • + • • + SCLK ___ ___ ___ ___ ___ ___ ___ ___ + _______/ \___/ \___/ \___/ \___/ \___/ \___/ \___/ \_____ + • : ; : ; : ; : ; : ; : ; : ; : ; • + • : ; : ; : ; : ; : ; : ; : ; : ; • + MOSI _____ _______ _______ _______________ ___ + 0x56 \_0_____/ 1 \_0_____/ 1 \_0_____/ 1 1 \_0_____/ + • ; ; ; ; ; ; ; ; • + • ; ; ; ; ; ; ; ; • + MISO XXX__________ _______________________ _______ XXX + 0xBA XXX__/ 1 \_____0_/ 1 1 1 \_____0__/ 1 \____0__XXX + +Legend:: + + • marks the start/end of transmission; + : marks when data is clocked into the peripheral; + ; marks when data is clocked into the controller; + X marks when line states are not specified. + +In this extension to the usual SPI protocol, the MOSI line state is specified to +be kept high when CS is asserted but the controller is not clocking out data to +the peripheral and also when CS is not asserted. + +Peripherals that require this extension must request it by setting the +``SPI_MOSI_IDLE_HIGH`` bit into the mode attribute of their ``struct +spi_device`` and call spi_setup(). Controllers that support this extension +should indicate it by setting ``SPI_MOSI_IDLE_HIGH`` in the mode_bits attribute +of their ``struct spi_controller``. The configuration to idle MOSI low is +analogous but uses the ``SPI_MOSI_IDLE_LOW`` mode bit. + + THANKS TO --------- Contributors to Linux-SPI discussions include (in alphabetical order, |