
Doc. No: Unex-QSG-21-003
18/23
A printed version of this document is an uncontrolled copy
© 2021 Unex Technology Corporation – Company Confidential
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-301(v2)/SOM-
351 at least 20ms later than SOM-301(v2)/SOM-351 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-301(v2)/SOM-351 is 32KΩ-60KΩ Ohm)
2
Delay the boot-up of external GNSS module for 200ms until SOM-301(v2)/SOM-351
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-301(v2)/SOM-351 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
.