A Battery-backed Real Time Clock

A Battery-backed Real Time ClockEdit

Hardware

We chose the Gravitech I2C-RTC board (available here), which provides a battery-powered RTC based on the Maxim DS1340-33 RTC chip.  There are drivers for this and many other RTC chips in the standard Linux kernel distribution.  A 3V lithium battery must be installed in the I2C-RTC board if you want it to keep time when the beaglebone is off.

Connection

The I2C-RTC is connected directly to the beaglebone's P9 expansion header using short lengths of 22 gauge wire as so:

I2C-RTC       Beaglebone
 Pin           P9 Pin #

VCC (+3.3V)       3
GND               1
SCL (clock)      19
SDA (data)       20

We soldered header pins into the holes on the I2C-RTC.  On that end of the hook-up wires, we crimped and soldered header plug contacts.  At the Beaglebone end of the hook-up wires, we soldered a male DB9-style pin to the other end of the wire.  The latter can be lodged gently but firmly into the appropriate P9 header pin-slots.  For use in the field, we will solder one end of wire directly into the I2C-RTC board, then use DB9-style male pins on the other end, but with heat-shrink tubing covering any metal surface emerging from the board or header block at the ends.  The I2C-RTC board will be attached to the lid of the acrylic case.

When correctly wired, the board's LED will be lit when the I2C-RTC is operating from Beaglebone power. However, there is a solder-trace which can be removed to disable this for low-power field deployments.

Detecting and Enabling the Device

If you install the i2ctools package on your SensorGnome, and then open a shell window, you can detect the I2C-RTC using the i2cdetect command:

# i2cdetect -y -r 3
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- UU -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --                       

The UU in row 60, column 8 indicates a device was detected at address 0x68, which is correct for ds1307-compatible devices such as this one.  To use the I2C-RTC, issue the following command:

# echo ds1307 0x68 > /sys/class/i2c-dev/i2c-3/device/new_device 

which will generate these lines in your system log, visible via dmesg | tail

[  125.921337] i2c i2c-3: new_device: Instantiated device ds1307 at 0x68
[  125.944866] rtc-ds1307 3-0068: rtc core: registered ds1307 as rtc1
[  125.951495] rtc-ds1307 3-0068: 56 bytes nvram

Note: the i2ctools package is not required in order to use the device, but is helpful for debugging connection problems.

Getting and Setting the RTC date/time

The device name highlighted above (rtc1) indicates where in /dev the device is to be found.  You can see its date/time using hwclock, as so:

# hwclock --show -u -f /dev/rtc1         ## show the i2c-rtc's date/time in UTC
Tue May 14 17:34:38 2013  0.000000 seconds

You can copy the current system clock (presumably already correctly set via ntpdate, GPS, or by hand) to the I2C-RTC by doing:

# hwclock --systohc -u -f /dev/rtc1      ## set the i2c-rtc clock from the system date/time

And conversely, you can set the system clock from the I2C-RTC by doing:

# hwclock --hctosys -u -f /dev/rtc1      ## set the system date/time from the i2c-rtc

If your kernel generated a device name other than rtc1, you must use that one instead.

Accuracy

In an over-the-weekend lab test, we found the I2C-RTC drift to be on the order of 1 second per day relative to network time via ntpdate.