RTC Cape

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RTC Cape

The RTC Cape provides time tracking capability for the BeagleBone Black even when the power is lost. This cape features the DS1338 I2C-based real-time clock/calendar and a holder for a coin battery type CR2032. Setup, reading, and setting up time is easy with the provided device tree overlay.

Revision Changes

Revision A1

Initial release of RTC Cape.

Software Support & Compatibility

  • All revisions are compatible with BeagleBone and BeagleBone Black using 3.8 kernel.
  • Latest software for BeagleBone Black can be downloaded here.

Getting Started

The instructions below are performed on Debian release 2014-05-14 (Linux beaglebone 3.8.13-bone50 #1 SMP Tue May 13 13:24:52 UTC 2014 armv7l GNU/Linux).

  • Connect your BeagleBone Black to the internet, log in as root, download the device tree files and extract them to /lib/firmware of your BBB:

   cd /lib/firmware
   wget https://github.com/CircuitCo/RTC-Cape/raw/docs/BBB-RTC-01-00A1.tar.gz
   tar xvzf BBB-RTC-01-00A1.tar.gz

  • Load the device tree overlay for your RTC Cape:

   echo BBB-RTC-01:00A1 > /sys/devices/bone_capemgr*/slots

  • Check to ensure the overlay is loaded. In the messages below, the DS1338 is registered as rtc1:

   dmesg | tail
       [ 5721.844355] bone-capemgr bone_capemgr.9: slot #7: 'Override Board Name,00A1,Override Manuf,BBB-RTC-01'
       [ 5721.844607] bone-capemgr bone_capemgr.9: slot #7: Requesting part number/version based 'BBB-RTC-01-00A1.dtbo
       [ 5721.844656] bone-capemgr bone_capemgr.9: slot #7: Requesting firmware 'BBB-RTC-01-00A1.dtbo' for board-name 'Override Board Name', version '00A1'
       [ 5721.847380] bone-capemgr bone_capemgr.9: slot #7: dtbo 'BBB-RTC-01-00A1.dtbo' loaded; converting to live tree
       [ 5721.848460] bone-capemgr bone_capemgr.9: slot #7: #3 overlays
       [ 5721.860065] omap_i2c 4802a000.i2c: bus 2 rev0.11 at 100 kHz
       [ 5721.902076] rtc-ds1307 2-0068: rtc core: registered ds1338 as rtc1
       [ 5721.902147] rtc-ds1307 2-0068: 56 bytes nvram
       [ 5721.902274] omap_i2c 4802a000.i2c: unable to select pin group
       [ 5721.902468] bone-capemgr bone_capemgr.9: slot #7: Applied #3 overlays.

  • Read the time from the RTC chip. It should look like something below since the time has not been set:

   hwclock -r -f /dev/rtc1
   Sat 01 Jan 2000 12:06:17 AM UTC  -1.004531 seconds

  • Update the system time of the BeagleBone Black (internet connection required) and write that to the RTC chip using hwclock command:

   ntpdate -b -s -u pool.ntp.org
   hwclock -w -f /dev/rtc1

  • Read the time again. This time it should have been updated.

   hwclock -r -f /dev/rtc1
   Fri 16 May 2014 10:57:12 PM UTC  -0.755959 seconds

Helpful Links


Followings are some specifications of the RTC Cape:

Mechanical Specifications

Size 2.15" x 2.825"
Layers 2
PCB Thickness .062"
RoHS Compliant Yes

Signal Usage

RTC Cape
Power VDD_3V3EXP
Squarewave Output Pin GPIO1_29


Please visit Where To Buy for a list of distributors who carry BeagleBoardToys' capes.

Product Images


RTC Cape Revision A1

To download all files at once please click here

To go back to the cape list, please click here