Several constrained IoT devices are too simple to be connected to the Internet to get the current date/time via NTP.

This Python program is a daemon that allows (for example) a Raspberry Pi, which can easily be connected to the Internet and synchronize date and time using NTP, to publish the current date/time information via a NRF24L01 module.

IoT devices with a NRF24L01 transceiver, that is within range of the Raspberry PI can then listen on the fixed address "DTCLI" and receive the current year, month, day, hour, minute, second, and weekday.

Such client may subscribe to the current date/time information on each "reboot", or they may store the date/time information in a RTC and then synchronize periodically to maintain a relatively precise notion of the current time.

The date/time daemon publishes the current date/time information every 5 seconds per default.

The NRF24 Time Server is written in Python and uses the pigpiod deamon that allows easy access to the GPIO pins of the Raspberry PI. Please refer to http://abyz.me.uk/rpi/pigpio/pigpiod.html for more information on the pigpiod daemon.

The NRF24 Time Server also uses the py-nrf24 module that implements NRF24 communication using the pigpio daemon. Please visit the following link for further information: https://github.com/bjarne-hansen/py-nrf24

The pigpiod daemon, which is a prerequisite, can be downloaded and installed using instructions found here: http://abyz.me.uk/rpi/pigpio/download.html

The fastest approach on a Raspberry Pi running raspbian is to install using apt:

sudo apt-get update
sudo apt-get install pigpio python-pigpio python3-pigpio 

The nrf24_timed is a systemd service written in Python and depends on the pigpio, nrf24, and systemd modules.

The service code is in src/nrf24_timed.py file, the systemd service file is in nrf24_timed.service, and the configuration file can be found in etc/nrf24_timed.ini.

A small bash shell script installing the service on a Raspberry Pi is included in the install file.

Before executing the install script make sure you have Python >= 3.6, pip, and virtualenv installed. Make sure you are connected to the internet, as the install script will download and install ependencies using pip.

The install file will install the service in the following directories:

/usr/local/bin/nrf24_timed
/usr/local/etc
/usr/local/lib/systemd/system

Inside the /usr/local/bin/nrf24_timed folder a virtual environment venv will be created.

The install script will not start the service automatically, but will show the status of the service.

After installation the service can be started, stopped, and motitored using the standard systemctl and journalctl commands.

$ systemctl status nrf24_timed
$ systemctl start nrf24_timed
$ systemctl reload nrf24_timed
$ systemctl restart nrf24_timed
$ systemctl stop nrf24_timed

$ journalctl --unit nrf24*

The 'reload' command will send a SIGHUP signal asking the service to reload its configuration from /usr/local/etc/nrf24_timed.ini (default).

Configuration for pigpio, nrf24, and the nrf24_timed service can be found in the etc/nrf24_timed.ini file.

The data broadcast via NRF24 will be a binary payload pack'ed using the protocol format "\<B4sHBBBBBB".

B:   Fixed protocol marker 0xfe
4s:  Fixed string signature "TIME"
H:   year    
B:   month  
B:   day     
B:   hour   
B:   minute 
B:   second
B:   weekday (Monday is 1 and Sunday is 7)

Constrained IoT devices will be able to listen for the date/time being broadcastet and update a local RTC based on that information.

The default wiring for the Raspberry PI and the NRF24L01 module for a Raspberry PI Zero Wireless is:

NRF24L01
PIN side
+---+----------------------
|       *    *
+---+
|7|8|   purple |   -
+-+-+
|5|6|   green  |  blue
+-+-+
|3|4|   yellow | orange
+-+-+   
|1|2|   black  |  red
+-+-+----------------------

    NRF24L01                PI ZW
-------------------------------------
PIN DESC  COLOR           PIN  GPIO
1   GND   black   <--->   6     -
2   3.3V  red     <--->   1     - 
3   CE    yellow  <--->   22    25
4   CSN   orange  <--->   24     8
5   SCKL  green   <--->   23    11   
6   MOSI  blue    <--->   19    10 
7   MISO  purple  <--->   21     9 
8   IRQ           <--->   N/C   - 

I tend to always use the same colouring of the connecting cables as this helps me to easily keep track of the wiring, and the above color coding seems to be popular on a number of sites having articles about NRF24L01 transceivers.

#to de described#