Hi,
I'm preparing Linux for a device based on i.MX6UL by Freescale, now NXP.
The device uses RS485 interface, and I have noticed, that the current
driver SERIAL_IMX (driver/tty/serial/imx.c) doesn't do that correctly.
Thus, I have a question to you (four paragraphs below).
According to my understanding, at least for the i.MX family, the general
agreement is that CTS_B signal of the processor, which is an output,
should be used as "driver enable" (DE) signal of transmitter in the
RS485 transceiver chip. Typically this DE signal is "normal logic", ie.
high voltage == '1', while CTS_B is inverted logic signal, ie. low
voltage == '1'. UART peripheral of i.MX processor has bit field CTS in
register UCR2, and this bit field can be used for "manual" control of
the CTS_B signal.
That is what the driver SERIAL_IMX does for ports defined in device tree
as dedicated for RS485.
However, Linux can, and does, disable serial port, here, by putting '0'
into UCR1 register of the UART peripheral. But then, CTS bit field is
ignored and CTS_B a.k.a. DE signal line is high. For RS485 this ends up
in having wrestling match of connected transmitters.
The correct state should be that for unused or disabled UART assigned
for RS485 the DE signal line is low. That is something I want to
implement. I can see two ways of doing that in software (described
below), and my question is: which is preferred?
1) Prevent disabling UART peripheral. For that function imx_shutdown or
imx_uart_shutdown could check if RS485 is enabled (in rs485.flags) and
if yes, keep the enable bit in UCR1 asserted. This will not require
engaging other things, but will not allow to fully disable the port. I
don't know why Linux tries to do so, and I can't tell whether or not
that is a potential problem.
2) Use GPIO to define the state of the signal when UART peripheral is
disabled. That might be done by defining the GPIO muxed with CTS_B as
output in low state, and muxing the processor pin to CTS_B when enabling
the UART, and to GPIO when disabling it. This would allow to fully
disable UART, but will require undisclosed control of an additional
feature (GPIO channel) which somebody might want to use for another purpose.
I would appreciate letting me know your opinion on that.
Best regards,
Adam
---------------------
Adam Rudziński
A.R.f.
http://arf.net.pl
