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How to turnoff UART to free GPIO (only on ath79 processors)

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You may need to disable the UART, to be able to use the TX and RX as GPIO-pin. The fact that the processors AR71xx/AR913x/AR724X/AR933X - UART default is locked and uses internal resources (buffer interrupt) for interaction with the driver.

So, it is possible to turn off the built-in UART by simple script, without the NO need to patch/recompile firmware.

Method (NO need re-build the Kernel):

Install io:

opkg update && opkg install io

or use this installation method:

cd /tmp
opkg install io_1_ar71xx.ipk

Create an empty file (if it is from the console):

touch /usr/sbin/uart_gpio

Paste the script in empty file:

# Bitwise operations: & = And, | = Or, ^ = xOr, << = Left Shift
bit1="1<<1"            # using bit1 for AR724x/AR933x
bit8="1<<8"            # using bit8 for AR71xx/AR913x
detect_value=0x`io -4 $detect_addr | cut -f3 -d' '`
detected_result=$(printf "0x%4.4x" $(($detect_value & $rev_id_maj_msk)))
func_value=0x`io -4 $func_addr | cut -f3 -d' '`
# depending on the detected rev_id of CPU -
# it will be use specific bit# as case_bit variable, or exit
case "$detected_result" in
# AR7240/AR7241/AR7242/AR9330/AR9331
0x00c0 | \
0x0100 | \
0x1100 | \
0x0110 | \
0x1110 )
# AR71xx/AR913x
0x00a0 | \
0x00b0 )
# AR9341/AR9342/AR9344
0x0120 | \
0x1120 | \
0x2120 )
    echo -e "No need to disable UART on AR934x processors,\n \
    just use sysfs to reprogram GPIOs."
    exit 0
* )
    echo "Can't detect your CPU, must be Atheros!"
    exit 1
# we using Bitwise xOr operation to switching bit# state (0 or 1)
io -4 $func_addr $(printf "0x%8.8x" $(($func_value ^ $case_bit)))
# read bit# state and depending on the state - print some info
if [ $(($func_value & $case_bit)) = $(($case_bit)) ]; then
    echo "Hardware UART is turned OFF"
    # You can use this line for automatic configuring GPIOs via sysfs
    # or you can load other modules that use these GPIOs
    echo "Hardware UART is turned ON"

Assign the right execution:

chmod +x /usr/sbin/uart_gpio

And actually the launch itself uart_gpio (if launch again it turn on UART back) …

Of course, it turn off UART until the next reboot the device, ie, you need to automate the process, for example by writing command in /etc/rc.local. After that you need and you can configure for yourself GPIO via /sys/class/gpio/export… well, I think it is not necessary to go into details, because who need to turn off the UART for some purposes - he will understand.

Let me remind you, for AR724x/AR933x:

 GPIO9  |  GPIO10

Forum discussion:

docs/guide-user/hardware/ · Last modified: 2018/03/04 16:28 by bobafetthotmail