6abe9ea8a5
rs485 allows for robust half-duplex serial communication. It is often implemented by attaching an rs485 transceiver to a UART. The UART's RTS line is wired to the transceiver's Transmit Enable pin and determines whether the transceiver is sending or receiving. Examples for such transceivers are Maxim MAX13451E and TI SN65HVD1781A: https://datasheets.maximintegrated.com/en/ds/MAX13450E-MAX13451E.pdf http://www.ti.com/lit/ds/symlink/sn65hvd1781a-q1.pdf In the devicetree, the transceiver itself is not represented, only the UART is. A few rs485-specific dt-bindings already exist and these go into the UART's device node. This commit adds a binding to set the RTS polarity. Most (if not all) transceivers require the Transmit Enable pin be driven high for sending, but in some cases boards may negate the pin and RTS must then be driven low. Consequently the polarity defaults to active high but can be inverted with the newly added "rs485-rts-active-low" binding. Document this binding in rs485.txt and in the two drivers fsl-imx-uart and fsl-lpuart that are about to be amended with support for it. Curiously, the omap_serial driver defaults to active low and already supports an "rs485-rts-active-high" binding to invert the polarity. This is left unchanged to retain compatibility, but the binding is herewith documented. Cc: Mark Jackson <mpfj@newflow.co.uk> Cc: Michał Oleszczyk <oleszczyk.m@gmail.com> Cc: Rafael Gago Castano <rgc@hms.se> Cc: Sascha Hauer <s.hauer@pengutronix.de> Acked-by: Rob Herring <robh@kernel.org> Signed-off-by: Lukas Wunner <lukas@wunner.de> Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
32 lines
1.2 KiB
Plaintext
32 lines
1.2 KiB
Plaintext
* RS485 serial communications
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The RTS signal is capable of automatically controlling line direction for
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the built-in half-duplex mode.
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The properties described hereafter shall be given to a half-duplex capable
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UART node.
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Optional properties:
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- rs485-rts-delay: prop-encoded-array <a b> where:
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* a is the delay between rts signal and beginning of data sent in milliseconds.
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it corresponds to the delay before sending data.
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* b is the delay between end of data sent and rts signal in milliseconds
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it corresponds to the delay after sending data and actual release of the line.
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If this property is not specified, <0 0> is assumed.
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- rs485-rts-active-low: drive RTS low when sending (default is high).
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- linux,rs485-enabled-at-boot-time: empty property telling to enable the rs485
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feature at boot time. It can be disabled later with proper ioctl.
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- rs485-rx-during-tx: empty property that enables the receiving of data even
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whilst sending data.
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RS485 example for Atmel USART:
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usart0: serial@fff8c000 {
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compatible = "atmel,at91sam9260-usart";
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reg = <0xfff8c000 0x4000>;
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interrupts = <7>;
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atmel,use-dma-rx;
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atmel,use-dma-tx;
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linux,rs485-enabled-at-boot-time;
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rs485-rts-delay = <0 200>; // in milliseconds
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};
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