On Thu, 01 Dec 2022 12:02:34 +0100, Christoph Niedermaier wrote: > Add a binding for a generic definition of an output GPIO that sets 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@xxxxxxxxxxxxxxxxxx> > --- > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Cc: Rob Herring <robh+dt@xxxxxxxxxx> > Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@xxxxxxxxxx> > Cc: Alexander Dahl <ada@xxxxxxxxxxx> > Cc: Marek Vasut <marex@xxxxxxx> > Cc: kernel@xxxxxxxxxxxxxxxxxx > Cc: devicetree@xxxxxxxxxxxxxxx > To: linux-serial@xxxxxxxxxxxxxxx > To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > --- > V2: - Rework of the commit message > - Rework GPIO property comment > V3: - Rework the binding description > - Rework message title > - Rework of the commit message > --- > Documentation/devicetree/bindings/serial/rs485.yaml | 6 ++++++ > 1 file changed, 6 insertions(+) > Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
