NB2DSK01 - 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 DS1391U-33, from Maxim. 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.

Figure 1. RTC (U2), external crystal (Y1) and
related circuitry.

The device operates from the NB2DSK01's 3.3V power supply, but also has a battery backup, courtesy of a 3V lithium cell (CR2032). An associated test point (labeled 'VBAT' and designated TP15) allows the cell's voltage level to be verified.

Figure 2. Lithium backup power cell (3V).

The primary power supply (3.3V) is internally monitored. Should the supply fail and fall below a threshold level – typically 2.88V – the device will switch to using the battery backup. Failure of the primary power supply results in the (active Low) Reset output being asserted, to flag the condition to the NanoTalk Controller. This signal will remain asserted until the primary power is restored, becoming deasserted after a delay of typically 160ms.

The device uses an external 32.768kHz crystal to provide the input to its internal oscillator circuitry. A test point (TP2) is supplied for verification of the generated frequency.

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

Location on Board

The DS1391 device (designated U2) and the 32.768kHz crystal (designated Y1) are both located on the solder side of the board.

The 3V lithium battery is located in a cell holder (designated BT1), which is located on the component side of the board, directly below the 6-way mini-DIN connector used to provide the PS/2 Mouse port.

The test point for the 3V supply (TP15) is located as part of the array of test points covering power supply voltages on the board, directly below the power connectors.

The test point for the 32.768kHz crystal (TP2) is located to the left of the battery cell holder.

Schematic Reference

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

• Sheet 20 (CLK_DS1391U_RTC.SchDoc, entitled SPI Real Time Clock)

• Sheet 21 (CON_BATT_COIN.SchDoc, entitled Lithium Battery Holder – CR2032).

Further Device Information

For more information on the DS1391, refer to the datasheet (DS1390-DS1393.pdf) available at www.maxim-ic.com.