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RasClock - Raspberry Pi Real Time Clock Module

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Product Description

RasClock v5.0 is a highly accurate real-time clock that plugs directly into the Raspberry Pi and includes a battery backup. The module is installed directly into the GPIO header pins with no wiring or soldering. It uses the 3.3V, GND SDA and SCL pins. The module stays within the outline of the Raspberry Pi and sits lower than video output so can be easilly installed within most cases.


RasClock uses one of the following chips:


RasClock features:

  • Very accurate timekeeping (typically ±3ppm or <2 minutes deviation per year).
  • Integrated crystal that compensates for temperature and age.
  • The supplied 36mA 3V battery will keep the time for around two years if the device isn't used and considerably longer if it is.
  • Coin Battery Included!
  • Fits inside most cases easilly inclusing the ModMyPi Raspberry Pi Case, PiBow's and Multicomp Cases.

Why do I need a Real Time Clock for my Raspberry Pi?

The Raspberry Pi is an ultra-small and ultra-low cost computer. In order to achieve this size and price, several non-essential items usually found on a computer had to be omitted. Laptops and computers keep time when the power is off by using a pre-installed, battery powered 'Real Time Clock' (RTC). However, this Real Time Clock module is not included with the Raspberry Pi. To keep time, the Raspberry Pi updates the date and time automatically over the internet via Ethernet or WiFi.
The Raspberry Pi will simply revert back to the standard date and time settings (usually 30 November 1999) when the network connection is removed. So, for projects which have no internet connection, you may want to add a low cost battery powered RTC to help your Pi keep time!
The RasClock has been specifically designed for use with the Raspberry Pi and, unlike other RTC's available, can plug directly in to the Raspberry Pi's GPIO Ports.

Complete installation and set up instructions can be found at http://afterthoughtsoftware.com/products/rasclock

Additional tutorial can be found at https://www.modmypi.com/blog/installing-the-rasclock-raspberry-pi-real-time-clock

Set up is easy, simply follow the guide Outlined in the ModMyPi Raspberry Pi Tutorial Here!


How to check what version of hardware and software running on your Raspberry Pi:


pi@raspberrypi:/$ sudo bash
pi@raspberrypi:/# cat /proc/cpuinfo
Processor       : ARMv6-compatible processor rev 7 (v6l)
BogoMIPS        : 697.95
Features        : swp half thumb fastmult vfp edsp java tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x0
CPU part        : 0xb76
CPU revision    : 7

Hardware        : BCM2708
Revision        : 0003
Serial          : 00000000c0ae10d9

root@raspberrypi:/# cat /etc/debian_version

root@raspberrypi:/# uname -or
3.6.11+ GNU/Linux


How to configure RTC module and remove fake-hwclock (when using wheezy 3.6.11):

sudo bash

apt-get remove fake-hwclock
rm /etc/cron.hourly/fake-hwclock
update-rc.d -f fake-hwclock remove
rm /etc/init.d/fake-hwclock


wget http://afterthoughtsoftware.com/files/linux-image-3.6.11-atsw-rtc_1.0_armhf.de
dpkg -i linux-image-3.6.11-atsw-rtc_1.0_armhf.deb
cp /boot/vmlinuz-3.6.11-atsw-rtc+ /boot/kernel.img

echo "i2c-bcm2708" >> /etc/modules
echo "rtc-pcf2127a" >> /etc/modules

Add the following to /etc/rc.local before "exit 0":

# Referring to
# http://www.raspberrypi-spy.co....
# to get the RasPi board version:
REV=$(cat /proc/cpuinfo|grep Revision)
# Extract revision
REV=${REV##*: }
# Make it readable as hex
# Convert hex to dec
if [ $REV -lt 4 ]
  echo pcf2127a 0x51 > /sys/class/i2c-adapter/i2c-0/new_device
  echo setting rtc to i2c-0
  echo pcf2127a 0x51 > /sys/class/i2c-adapter/i2c-1/new_device
  echo setting rtc to i2c-1
( sleep 2; hwclock -s ) &


Then reboot:


Console should show similar to:

[   32.897890] rtc-pcf2127a 0-0051: chip found
[   32.899695] rtc-pcf2127a 0-0051: rtc core: registered rtc-pcf2127a as rtc0
[   32.899757] i2c i2c-0: new_device: Instantiated device pcf2127a at 0x51

Then later nearer the login prompt:

Board revision: X
setting rtc to i2c-X


Log back in and verify date is correct:



Set hardware clock to current system time:

sudo hwclock -w



Verify hardware clock is set correctly:


sudo hwclock -r




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Product Reviews

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  1. What's not to like??? 5 Star Review

    Posted by on 6th Oct 2015

    These are great RTC modules. More accurate than the competition that I looked at. Includes a battery that should last years. Battery is replaceable by common, available, inexpensive CR1220. I've received good support on their web forum, although the newer Raspbians have support built-in to OS so there's not much left to even need "support" for. I'm running about 5 of these and haven't hit a dud yet.

  2. RasClock works well with our data logging applications 5 Star Review

    Posted by on 5th Dec 2014

    RasClock integrated very well with our use of the Raspberry pi as data loggers in the field. Very easy to use and, so far, trouble free.

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