From: Alexander Dahl [mailto:ada@xxxxxxxxxxx] Sent: Friday, October 28, 2022 8:21 AM > > Hello Christoph, > Hello Alex, > let me first say, for an upcoming new product we are currently > developing, we tried different wirings, all related to that !RE pin of > the transceiver. > > In a first hardware revision the engineer doing the schematic > connected !RE to GND (always on) and it took me a while to recognize > what was going on: everything send through TX was mirrored on RX, > which makes using lots of standard applications on that port just > fail. (To be fair, we need exactly that behaviour on a different > board.) > > In a second iteration !RE is directly connected to DE now, so RE is > now always the opposite of DE. DE itself is always connected to the > RTS line of the UART, which allows the driver to switch direction. > > More below. > > Am Thu, Oct 27, 2022 at 12:06:56PM +0000 schrieb Christoph Niedermaier: >> From: Rob Herring [mailto:robh@xxxxxxxxxx] >> Sent: Wednesday, October 26, 2022 10:59 PM >>> On Wed, Oct 26, 2022 at 06:50:46PM +0200, Christoph Niedermaier wrote: >>>> Add the binding for a generic definition of a GPIO, that controls whether Rx >>>> is connected or disconnected by an electrical circuit to have the ability >>>> to receive the signals on the bus during sending or disable receiving during >>>> sending. >>>> >>>> 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: Marek Vasut <marex@xxxxxxx> >>>> Cc: devicetree@xxxxxxxxxxxxxxx >>>> To: linux-serial@xxxxxxxxxxxxxxx >>>> To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx >>>> --- >>>> 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..0ebd7690f85d 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: GPIO pin to control RS485 Rx enable during Tx. >>> >>> Active state means do what? And inactive? This is an output gating the >>> RX signal or an input telling the receiver what to do during tx? The >>> description is not adequate. >>> >>> How does this property relate to 'rs485-rx-during-tx' Any combination of >>> the 2 being present or not is okay? If not, you need some constraints. >>> >>> Rob >> >> >> Hi Rob, >> >> I have improved the message: >> >> The standard RS485 is a half-duplex bus that in most cased is driven by an >> UART controller. 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 Rx signal >> of the UART controller. This patch adds a binding for a generic definition of a >> GPIO that 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. >> >> Is it now more understandable, or have I still not considered an aspect? > > Better. But what about the questions Rob asked? There's already a > property 'rs485-rx-during-tx' which leads to setting the flag > SER_RS485_RX_DURING_TX in serial core. As far as I understood from a > quick glance at core and drivers, this deals with half or full duplex > from the UART point of view (not the transceiver). > > You need to explain, what's the difference to the new property. > I suspect you want to somehow switch behaviour with that GPIO line? > Which driver should switch that line in the end and when? > > (For third iteration of our hardware we thought about a GPIO > controlling whether !RE is always on or connected to DE, but that's > probably not what you have in mind?) > > Maybe a simple generic schematic could help explain or maybe you find > a better name for the property? > > Greets > Alex Basically the idea is that this GPIO is an indicator for the state of the flag 'rs485-rx-during-tx', so that the hardware knows which mode for the receiving is wanted. My schematics: ┌──────────────────┐ │ RS484 │ Tx ───────────────┤D │ │ Transceiver │ RTS ────┬──────────┤DE │ │ │ │ │ ┌─────┐ │ │ └─┤& │ │ │ │ ├──┤!RE │ !rx_during_tx_gpio ─────┤ │ │ │ └─────┘ │ │ │ │ Rx ───────────────┤R │ │ │ └──────────────────┘ I muxed the UART CTS pin as GPIO and use it as rx_during_tx_gpio, because in my case this pin is unused anyway. It should be noted that I then use this pin LOW ACTIVE (in DT by GPIO_ACTIVE_LOW). I hope it has become clearer what I am doing. Do you still think I need another name? Regards Christoph
