NMEA is an acronym for the National Marine Electronics Association.
STC15W408ASMCU with > 4K flash.
- The firmware must be compiled with the
WITH_NMEAflag. The makefile settings should be updated for large firmware:
STCCODESIZE ?= 8184
SDCCOPTS ?= --code-size $(STCCODESIZE) --stack-auto --iram-size 256 --xram-size 256 --data-loc 0x30NOTE: The above settings are applied automatically when calling make with the
NMEA=1parameter:make NMEA=1
- Serial port speed can be changed in nmea.h according to your device:
#define BAUDRATE 9600- The user can change the auto update interval or disable it. Available options are:
Off,3 hours,6 hours,12 hoursand24 hours(see theNMEA_AUTOSYNCenum).
Synchronization is forced when one of the NMEA settings is changed and can also be forced by long pressing the S1 and S2 buttons from the main screen (HH:MM).
I wanted an NMEA device to be ON only during the synchronization process. To achieve this, I added a transistor switch connected to pin 6 (P1.3). There is also an LED indicating that the NMEA device is on.
The firmware turns on the NMEA device according to the user-defined update interval, waits for data from the device, and turns it off. The maximum duration of the synchronization process is 30 minutes, it is hardcoded (see the NMEA_MAX_SYNC_DURATION constant).
The NMEA data is received on pin 21.
Note: In my version of this DIY kit, both pins 6 and 21 are not connected to anything else on the board (just "hanging in the air"). This means that the wires to these pins musted be soldered in advance, before soldering the 7-segments indicators.
I experimented with two types of GPS receivers: NEO-6MV2 and NEO-M8N. Both work great, but the second one receives more satellites, supports multiple GNSS systems (Beidou, Galileo, GLONASS, GPS/QZSS), returns the result quicker, consumes less power.
Note: Strings returned by the
NEO-M8Nreceiver have the ($GNRMC) prefix instead of the$GPRMCreturned by theNEO-6MV2.The first two characters indicate the signal source. The prefix
GPmeans GPS, the prefixGNis used in case of mixed signals.The firmware supports both types of receivers out of the box.
It is not practical to use GPS synchronization indoors because the receiver does not receive satellite signals well. But the situation improves when the clock is placed at a window.
For the NTP syncronisation I decided to use a WeMos D1 Mini board. It has a build-in voltage regulator (5V -> 3.3V), so it can be powered directly from 5V and it also has a USB-C connector so can be directly connected to a laptop to upload code.
The transistor switch together with the LED and connector are mounted on a piece of prototyping board soldered to the WeMos D1 mini board.
The code is pretty simple, can be found here and uploaded to the board using the Arduino studio.
On the first run (or when the saved WiFi network is not available), it creates a WiFi access point with the name Hodiki and password 24diyclock24. Once connected to this access point, you can specify the WiFi network (name and password) that should be used for sending NTP requests.
The original case has enough space for the board:
The LED is indicating that the synchronization is in progress:
Note: This particular kit had been built before I decided to add the NTP support, so I had to solder the wires directly to the MCU (which is not very good).
