[PATCH V2 1/4] dt_bindings: rs485: Add binding for GPIO that controls Rx enable during Tx
Rob Herring
robh at kernel.org
Wed Nov 30 13:15:37 PST 2022
On Wed, Nov 23, 2022 at 01:30:01PM +0100, Christoph Niedermaier wrote:
> This patch adds a binding for a generic definition of an output GPIO that
> indicates the state of rs485-rx-during-tx. The idea is that the hardware
> already controls the option receiving during sending before it gets to the
> signal receiving hardware. The standard RS485 is a half-duplex bus that in
> most cases is driven by an UART controller. The advantage of using this
> GPIO is that it is independent of the capabilities of the UART core and
> the UART driver. On the hardware side the interface to the bus is
> controlled by a transceiver, that has a pin called RE (RX enable) or
> similar, which connects the bus to the RX signal of the UART controller.
> The GPIO can switch between two states to control the RE pin via an
> electrical circuit:
> - Active:
> The RE pin is always active. The UART RX see everything on the bus and
> therefore also what happens with the TX signal on the bus.
> - Inactive:
> The RE pin is always active, but during sending on the bus the pin RE is
> inactive. So basically the receiving during sending is suppressed.
>
> A possible circuit diagram could look like this:
> ┌──────────────────┐
> │ RS485 │
> TX ───────────────┤D │
> │ Transceiver │
> RTS ────┬──────────┤DE │
> │ │ │
> │ ┌─────┐ │ │
> └─┤& │ │ │
> │ ├──┤!RE │
> !rx_during_tx_gpio ──────┤ │ │ │
> └─────┘ │ │
> │ │
> RX ───────────────┤R │
> │ │
> └──────────────────┘
>
> Here the RTS pin of the UART core is used to control TX via the transceiver
> pin DE (Drive Enable). RE and rx_during_tx_gpio are active low.
>
> Signed-off-by: Christoph Niedermaier <cniedermaier at dh-electronics.com>
> ---
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: Rob Herring <robh+dt at kernel.org>
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt at linaro.org>
> Cc: Alexander Dahl <ada at thorsis.com>
> Cc: Marek Vasut <marex at denx.de>
> Cc: kernel at dh-electronics.com
> Cc: devicetree at vger.kernel.org
> To: linux-serial at vger.kernel.org
> To: linux-arm-kernel at lists.infradead.org
> ---
> V2: - Rework of the commit message
> - Rework GPIO property comment
> ---
> Documentation/devicetree/bindings/serial/rs485.yaml | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/serial/rs485.yaml b/Documentation/devicetree/bindings/serial/rs485.yaml
> index 90a1bab40f05..6d780911e342 100644
> --- a/Documentation/devicetree/bindings/serial/rs485.yaml
> +++ b/Documentation/devicetree/bindings/serial/rs485.yaml
> @@ -51,6 +51,10 @@ properties:
> description: GPIO pin to enable RS485 bus termination.
> maxItems: 1
>
> + rs485-rx-during-tx-gpios:
> + description: Output GPIO pin that indicates the state of rs485-rx-during-tx.
An output sets the state. An input samples or indicates the state.
This should include something about the active state: The active state
enables RX during TX.
> + maxItems: 1
> +
> additionalProperties: true
>
> ...
> --
> 2.11.0
>
>
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