Knowledge Base

This demo shows how to write to and read from the internal battery-backed memory on the DS1307 RTC chip as used on the RTC Pi and Expander Pi with Python.

Step 1: With your Raspberry Pi switched off, install the RTC Pi on the Raspberry Pi GPIO port and insert a CR2032 coin battery into the battery holder. Using the RTC Pi without a battery installed may damage the RTC Pi and will stop it from appearing on the I2C bus.

Step 2: Follow the instructions on how to install and configure I2C on Raspbian Linux.

We will create a new Python program file for this tutorial called demo_memory_int.py. You can use your favourite text editor to write the program. You can find a complete example of demo_memory_int.py in the ABElectronics_Python_Libraries/RTCPi/demo folder.

At the top of your program, you must import the struct library and the RTC class from the RTCPi library.

from RTCPi import RTC

We need some helper functions to convert an integer into a four-byte array and a four-byte array into an integer.

def int_to_array(val):
    # convert an integer into a four byte array
    x = [0, 0, 0, 0]
    x[3] = val & 0xFF
    val >>= 8
    x[2] = val & 0xFF
    val >>= 8
    x[1] = val & 0xFF
    val >>= 8
    x[0] = val & 0xFF
    return x


def array_to_int(x):
    # convert a four byte array into an integer
    val = (x[0]<<24) + (x[1]<<16) + (x[2]<<8) + x[3]
    return val

Create a new instance of the RTC class

rtc = RTC()

We will define an integer to be written to the RTC memory

writeval = 176247

And now, convert the integer into an array of bytes

writearray = int_to_array(writeval)

Next, write the array to the RTC memory

rtc.write_memory(0x08, writearray)

We can now read four bytes from the RTC memory into an array to check that the data has been saved.

read_array = rtc.read_memory(0x08, 4)

Print the individual array values to the console.

print (read_array[0], read_array[1], read_array[2], read_array[3])

To convert it back to an integer, you must combine the array values into an integer and print it.

print (array_to_int(read_array))

To run the demo in a console window, run the following command:

python3 demo-memory-int.py

The complete code for this demo is as follows:

#!/usr/bin/env python

from RTCPi import RTC
import struct

"""
================================================
ABElectronics RTC Pi real-time clock | RTC memory demo

run with: python3 demo_memory_int.py
================================================

This demo shows how to write to and read from the internal battery 
backed memory on the DS1307 RTC chip
"""

def int_to_array(val):
    # convert an integer into a four byte array
    x = [0, 0, 0, 0]
    x[3] = val & 0xFF
    val >>= 8
    x[2] = val & 0xFF
    val >>= 8
    x[1] = val & 0xFF
    val >>= 8
    x[0] = val & 0xFF
    return x


def array_to_int(x):
    # convert a four byte array into an integer
    val = (x[0]<<24) + (x[1]<<16) + (x[2]<<8) + x[3]
    return val


# create a new instance of the RTC class
rtc = RTC()

# integer to be written to the RTC memory
writeval = 176247 

# convert the integer into an array of bytes
writearray = int_to_array(writeval)

# write the array to the RTC memory
rtc.write_memory(0x08, writearray)

# read four bytes from the RTC memory into an array
read_array = rtc.read_memory(0x08, 4)

# print the individual array values
print(read_array[0], read_array[1], read_array[2], read_array[3])

# combine the array values into an integer and print it
print(array_to_int(read_array))

Related Expansion Boards

RTC Pi

Real-time clock for the Raspberry Pi

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