SatNav updated

A slight tweak to the software to improve the OLED display while downloading a log to USB stick takes the software from version 2.30 to 2.50 but the change is minimal and the latest version will be uploaded after tonight’s Bristol group meeting.

The final version of the hardware has been upgraded from a breadboard approach to use a custom built PCB and will be on display at the Wakefield show. Fitted with a 4400mAh LiPo battery, the unit consumes about 175mA and thus gives a battery life of about 15 hours. Current consumption whilst shutdown is about 25µA. Bare PCBs will be available for sale:

The information for the forthcoming Wakefield show has been updated.

The final version now uses four plugin modules: a GPS module which deals with the satellite GPS signals; an OLED display module which provides a display in lieu of an HDMI monitor; an analogue to digital module which allows voltages to be measured and a power boost board which allows the unit to be run from a battery.

The problem with a breadboard approach is that wiring it up is tedious and error prone. Previous versions used intricate wiring on a breadboard that required enormous patience (not my strong point) and much correction of minor wiring errors. It is a good approach while each unit is breaking new ground but once a design is finalised there is a huge
advantage in moving to a custom PCB.

I worked out that there was (just) enough space on a 68mm x 60mm PCB to mount all four modules, each with a header socket and the few components with plenty of space around each for soldering. I designed this to be a ‘multi-HAT’ board, accommodating the four modules and plugging in to a Pi Zero.

I laid out an approximate design for the PCB on 28 April and with some considerable help from a colleague who knew how to specify PCBs we reached a consensus on the design (version 7) by 4 May and I ordered it from China the same day. It was delivered on 9 May. Pretty good service!

The ‘Mulit-HAT’ board can now be tested with no ancillary boards plugged in (supplying power directly to the micro
USB socket on the Pi Zero). Each module could then be added in turn and tested individually.

SatNav is one of my favourite apps since it works so well with RiscOSM on my Pi-Top. I would like to use SatNav with RiscOSM on my PineBook laptop (and hopefully sometime soon with the PineBook Pro). Is there any chance that SatNav will be adapted to work with a USB GPS receiver?

Is there any chance that SatNav will be adapted to work with a USB GPS receiver?

At present SatNav examines the serial port using the code below. It should be possible to adapt – how are the NMEA sentences obtained over USB? SatNav does not send any data to the GPS module, just expects it to stream serial data continuously at a default baud rate (its defalt mode).

      10 : REM >SatNav
      20 : :
      30 : REM This programme is SatNav and is version 2.50 (11-May-22)

    2310 : IF serial%=TRUE THEN
    2320 :   REM Set serialOK% to FALSE IF zero or top bit set char recd
    2330 :   serialOK%=TRUE
    2340 :   REM Try to obtain (only) up to twenty characters in each poll
    2350 :   if%=0
    2360 :   REPEAT
    2370 :     SYS "OS_SerialOp",4,-1 TO ,r1% ;r2%
    2380 :     if%+=1
    2390 :     IF (r2% AND 2)=0 THEN
    2400 :       REM Character in buffer (C clear)
                 ...
    2950 :     ENDIF
    2960 :   UNTIL (r2% AND 2) OR if%>100:REM Buffer empty or 100 attempts
    2970 :   REM If there are serial errors in line$(0)
    2980 :   REM Then you need to shut down
    2990 :   IF serialOK%=0 THEN
    3000 :     serstat%=serstat% OR 16
    3010 :     flag2$="NoSer"
    3020 :     *FX 21,1
    3030 :     SYS "OS_SerialOp",5,3,0:REM Baud rate to slow
    3040 :     SYS "OS_SerialOp",1,0:REM Set 8bit, 1 stop bit, no parity
    3050 :     SYS "OS_SerialOp",0,&3E:REM Set no handshaking
    3060 :     SYS "OS_Byte",2,2:REM Enable serial input buffer/kbd input
    3070 :     *FX 21,1
    3080 :     SYS "OS_SerialOp",5,7,0:REM Baud rate to 9600
    3700 : ELSE
    3710 :   serstat%=(serstat% AND &FB) OR 16
    3720 : ENDIF

I am not sure how to include code fragments so forgive me for a minimal code fragment. My USB GPS receiver is a PL2303 device so I use Colin’s SerialUSB module.
REM 8 bit word, 1 stop bit, no parity
input is as follows, the next two lines should be a single one but Textile is not happy with it all in one line.
in%= OPENIN
(“SerialUSB#baud=4800;stop=1;noparity;nohandshake;noblock:”)
REPEAT
G% = 0
STRING$=""
WHILE G% <> 10
G%=BGET#in%
IF G% <> 10
STRING$ += CHR$(G%)
ENDWHILE
REM Now need to decode string to check for $GPRMC etc.
Hope that makes sense.

I am considering splitting off the various bits of SatNav which perform specific tasks with hardware.

driving a PaPiRus via SPI;
receiving serial data for NMEA sentences;
driving an OLED display;
reading voltages via an ADS1015;
software control of power switching.

For the moment I’ll leave it as a single app. Do you have to issue a ‘CLOSE in%’ command at all? I’ll have a look at incorporating it next month sometime …

Great news that you are interested in adapting your software to include USB GPS receivers. Yes you need to ‘CLOSE#in%’

G% = 0
STRING$=""
WHILE G% <> 10
G%=BGET#in%
IF G% <> 10
STRING$ += CHR$(G%)
ENDWHILE

This looks like a substitute for GET$#in%. Does this keyword work with a serial stream?

@Steve great question, yes GET$#in% works with a serial stream from this type of device and so saves some lines of code.

GPS modules that use multiple satellite sources, for example GPS and GLONASS, transmit extra NMEA sentences that SatNav 2.50 does not understand. I am working on the next version that will work with such sources.

My testing at the moment uses an Adafruit GPS ultimate module (5440) in place of the previous (740) on a machine with a 800×480 touch screen, OLED display, power control and Lithium Polymer battery. It consumes 700mA so the battery life is down to about 2½ hours. The updated GPS module now uses both American (GPS) and Russian (GLONASS) satellites. The NMEA sentences are different and so it can ‘see’ the GPS satellites but doesn’t get a fix as the GPRMC messages are sent as GNRMC. So I have been chasing three problems – the device would not turn off as I had forgotten to solder one end of R4; the OLED display would not work as I had not soldered the GND pin; the device would get a fix but be unable to communicate this to SatNav.

Chasing three problems at once seems to take a very long time ….

SatNav is now at version 2.52 and available from !Store.

SatNav has been updated to version 2.54 and is available from !Store. This now allows USB-connected GPS modules (default speed 9600 baud) so will work, for example, on the Pinebook and Pinebook Pro.