4

Analog Devices

Timekeeping

A 32.768kHz crystal oscillator is used as the time base for the real-time clock counter. The oscillator can

be turned on or off under software control. The oscillator must be on for the real-time clock to function.

The real-time clock counter is double buffered. This allows the controller to read time without the data

changing while it is being read. To accomplish this, a snapshot of the counter data is transferred to a

read/write buffer, which the user accesses.

Device Control Byte

The on/off control of the 32.768kHz crystal oscillator is done through the device control byte. This byte

can be read and written through the Clock Function commands.

Device Control Byte

7

6

5

4

3

2

1

0

U4

U3

U2

U1

OSC

OSC

0

0

Bit 0 - 1

0

No function

Bits 0 and 1 are hard-wired to read all 0’s.

Bit 2 - 3

OSC

Oscillator Enable/Disable

These bits control/report whether the 32.768 kHz crystal oscillator is running. If the oscillator is running,

both OSC bits will read 1. If the oscillator is turned off these bits will all read 0. When writing the device

control byte both occurrences of the OSC bit should have identical data. Otherwise, the value in bit

address 3 (bold) takes precedence.

Bit 4 - 7

Un

General-purpose user flags

These nonvolatile bits have no particular function within the chip. They can be read and written under the

control of the application software.

Real-Time Clock

The real-time clock is a 32-bit binary counter. It is incremented once per second. The real-time clock can

accumulate 136 years of seconds before rolling over. Time/date is represented by the number of seconds

since a reference point, which is determined by the user. For example, 12:00 a.m., January 1, 1970 could

be a reference point.

Clock Function Commands

The “Clock Function Flow Chart” (Figure 5) describes the protocols necessary for accessing the real-time

clock. With only four bytes of real-time clock and one control byte the DS1904 does not provide random

access. Reading and writing always starts with the device control byte followed by the least significant

byte of the time data.