Setting the system clock in Psion SIBO computers via an attached GPS

Psion MC/Series 3/3a time setting via serially attached GPS receiver that outputs NMEA

Uses the CALL $058B to retrieve the "country data" structure - this includes UTC offset (in minutes) and "Summer Time" flag

NOTES: DATE IS NOT SET - ONLY TIME! FIX STATUS IS NOT CHECKED!

After opening the serial port we

• get 255 bytes of NMEA
• LOCate GPRMC, get GPS traceable UTC time from next 7-12 bytes
• calc time (in seconds from 1/1/1970) from that and UTC offset and the Psion's "Summer Time" setting
• Add a "delay processing time" fudge factor of ... 1s?
• call OS function $038B to set the time

Time is expressed as:

1690166270 or $64BDE3F3 (32-bit number) - i.e. can be split into $64BD & $E3F3 as below

(as Hex $nnnnNNNN - split into hi & low part $nnnn & $NNNN or just by simple maths ;-) )

Tested working on the MC/Series 3/Series 3a

Assumes a GPS receiver attached to the serial port, running at 9600 8N1 Assumes the GPS has a valid fix

Assumes GPRMC or GPGGA sentences which contain UTC time after "GPRMC," or "GPGGA,"

$GPRMC,170805.000,A,5337.5945,N,00300.7381,W,0.00,281.17,250523,,,D7D $GPVTG,281.17,T,,M,0.00,N,0.01,K,D34 $PGTOP,11,36F $GPGGA,170806.000,5337.5945,N,00300.7381,W,2,9,1.13,4.2,M,49.5,M,0000,000040

The "GenGetCountryData" OS call may or may not be meaningless on the MC - certainly its clock has no concept of "Summer Time" like the 3/3a does. So stime% is set to 1 in the MC's OPL to get UTC to BST working (in summer!). Also (obviously!) all the graphics commands such as gAT, gPRINT, etc. are meaningless on the MC (I also removed them from the Series 3 code too).

I also had to change the sentence to search for to GPGGA rather than GPRMC on the S3 & MC, possibly something to do with serial overruns or maybe something else entirely...?

Shared in the hope it might prove useful to someone. somewhere ;-)

:: Chris Farrow @zedstarr July 2023 ::