[gpsd-users] Problems processing pps from Maestro A2235 gps

[Winfried]

Dear all,

For a few days I have been struggling to sync the ntpd daemon to the 
Maestro A2235 gps module on an embedded device.

The A2235 sends out a 0.2 sec long pps pulse every second. However the 
designer of the board placed an inverter between the PPS signal and the 
GPIO pin that receives it. So in the device tree I added the 
'assert-falling-edge' property to trigger on the falling edge of the 
inverted signal.

The KPPS seems to work fine, a PPS event is generated every second.

However gpsd always reports very large PPS offsets, usually in the range 
-0.8 to -1.2 seconds. This offset stays more or less the same until I 
restart gpsd. I tried running ntpd -qg first, but that didn't help.

When I run gpsd with debug level 9 I see that the pulse is classified as 
an invisible pulse:

gpsd:PROG: KPPS:/dev/pps1 assert  1452268571.968440096, sequence: 17939, 
clear   0.000000000, sequence: 0 - using: assert
gpsd:PROG: KPPS:/dev/pps1 Assert cycle: 1000001, duration:       0 @  
1452268571.968440096
gpsd:RAW: PPS:/dev/pps1 Assert pps-detect invisible pulse
gpsd:PROG: PPS:/dev/pps1 Assert cycle: 1000001, duration:       0 @  
1452268571.968440096
gpsd:RAW: PPS:/dev/pps1 Assert categorized invisible pulse
gpsd:PROG: NTP: ntpshm_put(/dev/pps1 pps)  1452268571.000000000 @  
1452268571.968440096
gpsd:CLIENT: => client(0): 
{"class":"PPS","device":"/dev/pps1","real_sec":1452268571, 
"real_nsec":0,"clock_sec":1452268571,"clock_nsec":968440096,"precision":-20}\x0d\x0a
gpsd:INFO: PPS:/dev/pps1 Assert hooks called clock:  
1452268571.968440096 real:  1452268571.000000000: accepted
gpsd:PROG: PPS:/dev/pps1 Assert accepted @  1452268571.968440096 offset 
-0.968440096


When I start ntpd it very quickly classifies both the serial gps time as 
well as the pps shared memory segments as false tickers.

Can anybody tell me what is going on here ? Does the inverted pulse 
confuse gpsd ?


gpsd version used: 3.15
gpsd started with: /usr/sbin/gpsd -n -N -D9 /dev/ttymxc2 /dev/pps1


Thanks a lot in advance for any insight.

Best regards,

Winfried