NanoBoard 3000 - SPI Real-Time Clock

The current date and time are made available to the NanoTalk Controller through the provision of a low-voltage, SPI Real-Time Clock (RTC) device – a PCF2123, from NXP. The device provides information in terms of: Year, month, day, hours, minutes, and seconds. The calendar automatically adjusts for months with fewer than 31 days and includes leap-year correction.

The device operates from the motherboard's 3.3V power supply, but also has a battery backup, courtesy of a 3V lithium cell (CR2032). An external 32.768kHz crystal is used to provide the input to the device's internal oscillator circuitry.

RTC (U27), external crystal (Y3) and cell holder
(BT1) for the lithium backup power cell.

The NanoTalk Controller communicates with the RTC device over the SPI bus, with the RTC being a slave SPI device.

Location on Board

The PCF2123 device (designated U27) and the 32.768kHz crystal (designated Y3) are both located on the component side of the board, next to the cell holder for the lithium battery, and to the right of 'User Header B'.

The 3V lithium battery is located in a cell holder (designated BT1), which is located on the component side of the board, to the top-left of the User FPGA (U8).

3V lithium cell inserted.

Test points for the 3V battery supply (labeled BAT, designated TP12), the 32.768kHz crystal (labeled 32K, designated TP11) and ground (labeled GND, designated TP8) are located to the right of the User Prototyping Area.

RTC-related test points.

Schematic Reference

The RTC circuitry can be found on the following sheets of the motherboard schematics:

• Sheet 18 (CLK_PCF2123_RTC.SchDoc, entitled SPI Real Time Clock)

• Sheet 19 (CON_BATT_COIN.SchDoc, entitled Lithium Battery Holder – CR2032).

Further Device Information

For more information on the PCF2123, refer to the datasheet (PCF2123_1.pdf) available at www.nxp.com.