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45c1d967a7
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@dh-electronics.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221202104127.122761-1-cniedermaier@dh-electronics.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
63 lines
2.0 KiB
YAML
63 lines
2.0 KiB
YAML
# SPDX-License-Identifier: GPL-2.0
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%YAML 1.2
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---
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$id: http://devicetree.org/schemas/serial/rs485.yaml#
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$schema: http://devicetree.org/meta-schemas/core.yaml#
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title: RS485 serial communications
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description: The RTS signal is capable of automatically controlling line
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direction for the built-in half-duplex mode. The properties described
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hereafter shall be given to a half-duplex capable UART node.
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maintainers:
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- Rob Herring <robh@kernel.org>
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properties:
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rs485-rts-delay:
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description: prop-encoded-array <a b>
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$ref: /schemas/types.yaml#/definitions/uint32-array
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items:
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items:
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- description: Delay between rts signal and beginning of data sent in
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milliseconds. It corresponds to the delay before sending data.
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default: 0
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maximum: 100
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- description: Delay between end of data sent and rts signal in milliseconds.
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It corresponds to the delay after sending data and actual release
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of the line.
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default: 0
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maximum: 100
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rs485-rts-active-low:
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description: drive RTS low when sending (default is high).
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$ref: /schemas/types.yaml#/definitions/flag
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rs485-rx-active-high:
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description: Polarity of receiver enable signal (when separate from RTS).
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True indicates active high (default is low).
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$ref: /schemas/types.yaml#/definitions/flag
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linux,rs485-enabled-at-boot-time:
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description: enables the rs485 feature at boot time. It can be disabled
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later with proper ioctl.
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$ref: /schemas/types.yaml#/definitions/flag
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rs485-rx-during-tx:
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description: enables the receiving of data even while sending data.
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$ref: /schemas/types.yaml#/definitions/flag
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rs485-term-gpios:
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description: GPIO pin to enable RS485 bus termination.
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maxItems: 1
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rs485-rx-during-tx-gpios:
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description: Output GPIO pin that sets the state of rs485-rx-during-tx. This
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signal can be used to control the RX part of an RS485 transceiver. Thereby
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the active state enables RX during TX.
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maxItems: 1
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additionalProperties: true
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...
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