[chrony-users] Forcing chrony to use the clear (falling) edge of negative PPS

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Hi Everyone,

I have a Novatel FlexPak G2 that puts out a Negative PPS (because it's going through a TTL to RS232 converter, which flips the pulse). The pulse width is 1ms. I have gpsd (v3.22) and chrony (v4.1) working together on my Linux Mint 19.1 machine. 

I have configured chrony to be fed from gpsd as follows in the /etc/chrony.conf file:

refclock SHM 0 delay 0.5 refid NMEA 
refclock SOCK /var/run/chrony.ttyS0.sock precision 1e-7 refid PPS

Question:  how can I tell Chrony to use the falling edge (clear edge, which is the first leading edge of the negative PPS) ? 

In the manual it says that you can use the keyword "clear" if you are using the PPS reflcock i.e. reflclock PPS /dev/pps0:clear but since I am using gpsd, I am supposed to use the SOCK method, correct ? 

Putting refclock SOCK /var/run/chrony.ttyS0.sock:clear causes chronyd to error out. 

Any help appreciated, thanks!

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