+ This is a lua program intended to be run on an ESP8266 loaded with + nodemcu. It will parse pulses from a + pollin + DCF77 receiver module. It's + output on the serial port (usually available over USB) will be + NMEA GPZDA sentences + which can be understood by ntpd. +
++ First of all i wanted to learn more about DCF77 and gain experience + programming in lua. I also wanted to connect DCF77 to my (Raspberry Pi 2B + based) ntp server without compromising the accuracy of it's already available + PPS signal. When the receiver module looses it's signal it can easily generate + hunderds or thousands of interrupts a second which will definately harm the + PPS accuracy. By moving the processing to an ESP8266 the ESP will take the hit + and just output valid data when it has a decent signal. +
++ It receives data from a DCF77 module and outputs the same data as a GPS would + do. To do that, it has some (IMHO) neat tricks. Usually it will run in an + interrupt driven way, with a clean signal this will trigger twice a second + (one for the rising edge, one for the falling edge). When it detects way more + interrupts than expected, it switches to polling mode. It will use polling to + detect a clean signal, and only when it's clean enough it will change back to + interrupt mode again. +
++ To improve accuracy and reduce the time from the rising edge of the radio + signal to the delivery of the GPS sentence over the serial port it partially + calculates the required checksum for the sentences every minute. Doing so only + leaves the second characters to be added to the checksum. +
++ When a clean signal is received it will blink a led to let you know it's + working. Because LED's can be very bright when it gets dark you can set a + minimum ambient light which will be read from the adc. When the ambient light + is lower it will stop blinking. +
++ Of course you'll need an ESP8266. The ESP32 might actually be compatible, but + i don't know and haven't tested. And of course you'll need a DCF77 receiver + module. The pollin one i have is pretty cheap (around + EUR 10, + ignore the image on the site, it's the conrad module, but you'll get the + pollin one when you order), the popular conrad module might also work but + again: I haven't tested it. +
++ The nodemcu firmware can be build using + their build service. This program + requires the adc, bit, gpio and timer modules. If + you want to use + nodemcu-uploader to + upload the program, be aware that you will also need the file, + node and uart modules. +
++ To use the program upload it to your ESP8266 and if you want it to start + automatically when the ESP is started also upload the init.lua file. Restart + the ESP3288 and then wait :-) +
++ If you look at the code you'll see quite some print statements commented out, + those are there to help debugging. Unfortunately memory on the ESP8266 is too + constrained to be able to uncomment them all at the same time, so use with + caution. +
++ With this you can make ntpd think it's receiving from a GPS while it's + actually using your DCF77 receiver. To do that, first you'll have to instruct + udev to create a gps symlink. I use the following rules in + /etc/udev/rules.d/99-gps.rules: +
++KERNEL=="ttyUSB0", SYMLINK+="gps0" ++
+ After that configure ntpd to use it with a server and fudge line in + /etc/ntp.conf: +
++server 127.127.20.0 mode 0x58 minpoll 3 maxpoll 3 prefer +fudge 127.127.20.0 stratum 0 flag1 0 flag2 0 flag3 0 flag4 0 time1 0.0 time2 0.0292 refid DCF0 ++
+ Especially time2 should be tuned to your own setup. The command ntpq + -pn should now show something like this: +
++ remote refid st t when poll reach delay offset jitter +============================================================================== +*127.127.20.0 .DCF0. 0 l 4 8 377 0.000 -0.214 0.567 ++
+ Have fun! +
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