Real time clock and extra storage

With the RTCMem Nanoshield you add a real time clock to your project, and 32KB more of non-volatile memory from an EEPROM chip embedded in the board. Interfacing with the RTC and the EEPROM memory is done through an I2C bus, using only 2 pins of your microcontroller. We provide open source Arduino libraries to access these resources.

The RTC chip on the board tracks day of month, day of week, month, year, hours, minutes and seconds. In the absence of a power supply, a lithium battery allows keeping track of the time for up to 10 years¹. Unlike the majority of the RTC boards in the market, where the clock can show a deviation of up to ±2 seconds per day (±20ppm), the RTCMem Nanoshield has a trimmer that is factory calibrated, such that the maximum deviation is ±0.5 seconds per day (±5ppm)¹.

The RTC chip also contains alarm configuration based on day of week, hours, minutes and seconds, possibly generating an event at a predetermined time. This event can cause an interrupt, that can be verified either via the INT pin or via an I2C command. Similar to the alarm feature, there is also a timer feature that can generate an event after a preconfigured period of time.


  • PCF8563 IC manufactured by NXP Semiconductors.
  • 256Kbit (32KB) CAT24C256 EEPROM manufactured by ON Semiconductor.
  • Three configurable bits for I2C address selection for the EEPROM chip, allowing simultaneous use of up to 8 chips on the same I2C bus (more details on the schematics).
  • Battery holder for CR/BR 1216, 1220 or 1225 batteries.
  • Trimmer for crystal calibration
  • Alarm feature
  • Timer feature
  • PCB pad to measure the RTC output clock.
  • CR1220 battery included.
  • Power supply input for 3.3V or 5V (configurable via solder jumpers).
  • Access to the interrupt signal from the RTC chip (configurable via solder jumper).


RTC diagram


RTC block diagram

RTCMem Arduino Function
INT D3# RTC interrupt (optional)
3V3 3V3 3.3V power supply input (optional)
SDA A4 I2C data
SCL A5 I2C clock
VCC VCC 5V power supply input
GND GND Reference voltage (ground)
Pin description

Electrical specifications

Power supply: must be provided on VCC pin, with a range from 4.5V to 5.5V (5V typical). Optionally, it can also be done through the 3V3 pin with a range from 3V to 3.6V (3.3V typical); in this case, change the SUP jumper configuration according to the instructions provided in the schematics.

Current consumption: the maximum current consumption is approximately 5mA, while the board is active and supllied via the VCC or 3V3 pins. When no supply is provided by these pins, and the RTC is only battery powered, the current consumption is of 250nA¹.

Logic levels: the I2C interface and the INT ouput pin (optional) are factory configured to operate with a logic level of 5V, but can be reconfigured to operated with 3.3V (see instructions in the schematics).

¹ The duration of the battery and clock deviation are estimated considering an ambient temperature of 25ºC.