6

Analog Devices

Write Clock [99h]

The write clock command is used to set the real-time clock counter and to write the device control byte.

After issuing the command, the bus controller writes first the device control byte, which becomes

immediately effective. After this the bus controller sends the least significant byte through the most

significant byte to be written to the real-time clock counter. The new time data is copied from the

read/write buffer to the real-time clock counter and becomes effective as the bus controller generates a

reset pulse. If the oscillator is intentionally stopped, the real-time clock counter behaves as a four-byte

nonvolatile memory.

1-Wire Bus System

The 1-Wire bus is a system, which has a single bus controller and one or more peripherals. In all instances

the DS1904 is a peripheral device. The bus controller is typically a microcontroller. The discussion of this

bus system is broken down into three topics: hardware configuration, transaction sequence, and 1-Wire

signaling (signal types and timing). A 1-Wire protocol defines bus transactions in terms of the bus state

during specified time slots that are initiated on the falling edge of sync pulses from the bus controller. For

a more detailed protocol description, refer to Chapter 4 of the Book of iButton® Standards.

Hardware Configuration Figure 6

Hardware Configuration

The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to

drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have open-

drain or three-state outputs. The 1-Wire input of the DS1904 is open drain with an internal circuit

equivalent to that shown in Figure 6. A multidrop bus consists of a 1-Wire bus with multiple peripherals

attached. The 1-Wire bus has a maximum data rate of 16.3kbits per second and requires a pull-up resistor

of approximately 5kΩ.

The idle state for the 1-Wire bus is high. If for any reason a transaction needs to be suspended, the bus

must be left in the idle state if the transaction is to resume. If this does not occur and the bus is left low

for more than 120μs, one or more of the devices on the bus may be reset. Since the DS1904 gets all its

energy for operation through its VDD pin it will not perform a power-on reset if the 1-Wire bus is low for

an extended time period.

Transaction Sequence

The protocol for accessing the DS1904 via the 1-Wire port is as follows:

•

Initialization

•

ROM Function Command

•

Clock Function Command

BUS CONTROLLER

DS1904 1-Wire PORT

OPEN-DRAIN

PORT PIN

Rx = RECEIVE

Tx = TRANSMIT

Rx

Tx

VPUP

Rx

Tx

5µA,

TYP

100Ω

MOSFET

DATA