Doc. No: Unex-QSG-21-003
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© 2023 Unex Technology Corporation – Company Confidential
9.7.
Reset
The reset pin (PERST#, P22) is low active, and will reboot Linux when a rising edge of
input voltage (end of assertion) is detected. The reset pin is internally connected to 3.3V
with a 2KΩ pull-up resistor. Connecting this pin to an open drain or open collector driver is
recommended if the motherboard logic level is different from 3.3V TTL.
9.8.
1PPS
The 1PPS pin (P49) serves as the input of an external 1PPS (1 pulse per second)
signal. The start of a system time (UTC time) second will line up with the rising edge of this
time pulse signal. For timing and logic level of external 1PPS signal, please see
ECOMMENDED OPERATING CONDITIONS
. Leave this pin open if not used.
External 1PPS pins are exposed in both mPCIe interface pin 49 and I/O cable pin 5.
To avoid back powering and latch-up, external 1PPS should be fed to SOM-352 at least
20ms later than SOM-352 power up.
The 1PPS input pins (mPCIe pin 49 and I/O cable pin 5) will be pulled high by default
during boot-up (up to 200ms). If the 1PPS also serves as BOOT MODE pin in the external
GNSS module, for example like some Teseo III modules, it may cause boot-up failure of
the external GNSS module. If this happens, you may need choose one of the following
three solutions:
1
Add a 3KΩ PD resistor on the motherboard side. (The internal PU resistance of
SOM-352 is 32KΩ-60KΩ Ohm)
2
Delay the boot-up of external GNSS module for 200ms until SOM-352 boot-up
completed.
3
Pull low the PERST#/EX_RSTn signal during power up, and then release (or pull
high) the PERST#/EX_RSTn after the external GNSS module boot-up complete.
The 1PPS pin (P49) will remain LOW until the release of PERST#/EX_RSTn signal.
By default, the SOM-352 will use internal 1PPS signal. In order to change to external
1PPS and NMEA source, please issue the following commands in user space:
echo 100 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio100/direction
echo 0 > /sys/class/gpio/gpio100/value
# 0=Ext_GNSS, 1=Int_GNSS
To let the above setting take effect upon next boot-up, please append the above lines
in
/home/root/ext-fs/usr/bin/at_startup
.
