Configuring ext signals

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Pin functions (mux)

By default the UART2 pins are set up as GPIOs, and UART3 is set up as an UART. The following describes how to change pin functions under a 3.2 kernel.


As root,

mkdir /debug
mount -t debugfs none /debug

In the /debug/omap_mux directory which appears, the files relevant to the EXT connector are uart2_cts, uart2_rts, uart2_rx, uart2_tx, uart3_rx_irrx, uart3_tx_irtx. They are mapped to the pad configuration registers. Reading one of these files reports the current configuration of the pin. Writing the appropriate pattern to the file changes the configuration of the pin.

Pad Configuration Register

As described in OMAP35x Technical Reference Manual (Rev. Y), section 7.4.4 [1]

Bits Name Comments
8 INPUTENABLE 0: output only. 1: bidirectional.
4 PULLTYPESELECT 0: pull-down. 1: pull-up.
3 PULLUDENABLE 0: pull not active. 1: pull active when pin direction is configured as input.
2,1,0 MUXMODE See table below.

Mux modes

Mode 0 Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode 6 Mode 7
uart2_cts mcbsp3_dx gpt9_pwm_evt NA gpio_144 NA NA safe_mode
uart2_rts mcbsp3_dr gpt10_pwm_evt NA gpio_145 NA NA safe_mode
uart2_tx mcbsp3_clkx gpt11_pwm_evt NA gpio_146 NA NA safe_mode
uart2_rx mcbsp3_fsx gpt8_pwm_evt NA gpio_147 NA NA safe_mode
uart3_rx_irrx NA NA NA gpio_165 NA NA safe_mode
uart3_tx_irtx NA NA NA gpio_166 NA NA safe_mode


Example of changing UART2_RTS from GPIO to UART mode [2]:

# cat /debug/omap_mux/uart2_rts
name: uart2_rts.gpio_145 (0x48002176/0x146 = 0x010c), b ab25, t NA
signals: uart2_rts | mcbsp3_dr | gpt10_pwm_evt | NA | gpio_145 | NA | NA | safe_mode

# echo 0x0108 > /debug/omap_mux/uart2_rts
# cat /debug/omap_mux/uart2_rts
name: uart2_rts.uart2_rts (0x48002176/0x146 = 0x0108), b ab25, t NA
signals: uart2_rts | mcbsp3_dr | gpt10_pwm_evt | NA | gpio_145 | NA | NA | safe_mode

The value was changed from 0x010c to 0x0108. In both numbers, bit8 is 1 for input enable, bit4 is 0 for pull-down, and bit3 is 1 for pull enable. Bits 2,1,0 changed from 4 to 0, which changed the mux mode from 4 to 0.

Power supply

Currently always set to supply 2.8V, changing this requires patching bootloaders (xload and u-boot).

This is connected to VAUX3 supply on PMIC, with these programmable voltages (200mA max): 1.5V, 1.8V, 2.5V, 2.8V and 3.0V, with 2.8V as default.

Warning: at the time of this writing, both bootloaders (xload and u-boot) set this to 2.8V unconditionally, so don't rely on this providing other voltages during reboot until you patch both bootloaders. The digital I/O pins use 1.8V logic and must not be connected to the 2.8V from the power pin!


Can be controlled using GPIO sysfs class device, as described in beagleboard tutorials here.

The pins must be set as GPIO using the pin mux.

In the following commands, NNN is the GPIO number, e.g. 145.

#enable gpioNNN folder, to prepare for the other commands
echo NNN > /sys/class/gpio/export

#reads "in" or "out" depending on what was written to it
cat /sys/class/gpio/gpioNNN/direction

#when direction is "out", reads 0 or 1 depending on what was written to it
#when pinmux specifies bidirectional and direction is "in", reads 0 or 1 depending on voltage at pin
#when pinmux specifies output-only and direction is "in", always reads 0
cat /sys/class/gpio/gpioNNN/value

#set value to 0, i.e. output low (only works when direction is "out")
echo 0 > /sys/class/gpio/gpioNNN/value

#set value to 1, i.e. output high (only works when direction is "out")
echo 1 > /sys/class/gpio/gpioNNN/value

#set direction to "in"
echo in > /sys/class/gpio/gpioNNN/direction

#set direction to "out" and value to 0, i.e. output low 
echo out > /sys/class/gpio/gpioNNN/direction

#set direction to "out" and value to 1, i.e. output high
echo high > /sys/class/gpio/gpioNNN/direction

#set direction to "out" and value to 0, i.e. output low 
echo low > /sys/class/gpio/gpioNNN/direction

#disable gpioNNN folder
echo NNN > /sys/class/gpio/unexport

Controlling the GPIOs with shell script like this is very slow; if speed is required, choose another language to write to the files more efficiently.


UART2 is mapped to /dev/ttyO1 and UART3 is mapped to /dev/ttyO2.

The line is high when idle. Each item transmitted has a start bit (low); 5, 6, 7 or 8 data bits; an optional odd or even parity bit; and 1 or 2 stop bits (high).

These UARTs support a wide range of baud rates. UART2 has the RTS and CTS pins brought out to the EXT connector, while UART3 does not.

See the Serial Programming Guide for POSIX Operating Systems.


On older firmwares, kernel messages are directed there. SuperZaxxon or later have that disabled.

On all firmwares released so far terminal with a shell is attached to UART3. The port runs at 115200 8N1 baud rate.

Disabling kernel messages

To disable kernel mesages, you need to edit kernel boot arguments. Probably easiest way to do it is to create autoboot.txt and place on root directory of a card in slot1 with this content:

setenv bootargs ubi.mtd=4 ubi.mtd=3 root=ubi0:rootfs rootfstype=ubifs rw rootflags=bulk_read

Alternatives are using u-boot environment (configure through USB or UART3 serial before system boots up) or patching and reflashing u-boot itself.

Disabling attached terminal

For this you need to edit /etc/inittab on pandora rootfs and comment out these lines:

#S0:2345:once:/sbin/getty 115200 ttyS0
#O2:2345:once:/sbin/getty 115200 ttyO2


The UART2 pins on the OMAP could be configured for PWM, but there does not seem to be an interface in the Pandora's OS to do this.

A wide variety of PWM output can be improvised with a UART. The start bit is always low and the stop bit(s) are always high, but the other bits depend on the character sent and the parity selection. The number of bits per character can be changed between 7 and 12, giving several options for frequency division within each available baud rate.

_^_^_^_^_^    8 data bits; frequency=baudrate/2; duty cycle 50%

_^^^^^^       5 data bits; frequency=baudrate/7; duty cycle 6/7

__________^^  8 data bits, even parity and 2 stop bits; frequency=baudrate/12; duty cycle 1/6