DS1994

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WRITE PROTECT/PROGRAMMABLE EXPIRATION

The write protect bits (WPR, WPI, WPC) provide a means of write protecting the timekeeping data and

limiting access to the DS1994 when an alarm occurs (programmable expiration). The write protect bits

cannot be written by performing a single copy scratchpad command. Instead, to write these bits, the copy

scratchpad command must be performed three times. Please note that the AA bit is set, as expected, after

the first copy command is successfully executed. Therefore, the authorization pattern for the second and

third copy command should have this bit set. The read scratchpad command can be used to verify the

authorization pattern.

The write protect bits, once set, permanently write protect their corresponding counter and alarm

registers, all write protect bits, and certain control register bits as shown in Figure 7. The time/count

registers continue to count if the oscillator is enabled. If the user wishes to set more than one write protect

bit, the user must set them at the same time. Once a write protect bit is set it cannot be undone, and the

remaining write protect bits, if not set, cannot be set. The programmable expiration takes place when one

or more write protect bits have been set and a corresponding alarm occurs. If the RO (read only) bit is set,

only the read scratch and read memory function commands are available. If the RO bit is a logic 0, no

memory function commands are available. The ROM functions are always available.

Figure 7. WRITE PROTECT CHART

WRITE PROTECT BIT SET:

WPR

WPI

WPC

Data Protected from

User Modification:

Real-Time Clock

Real-Time Alarm

WPR

WPI

WPC

RO

OSC*

Interval Timer

Interval Time Alarm

WPR

WPI

WPC

RO

OSC*

STOP/START **

AUTO/MAN

Cycle Counter

Cycle Counter Alarm

WPR

WPI

WPC

RO

OSC*

DSEL

* Becomes write 1 only, i.e., once written to a logic 1, cannot be written back to a logic 0.

** Forced to a logic 0.

1-WIRE BUS SYSTEM

The 1-Wire bus is a system that has a single bus master and one or more slaves. In most instances, the

DS1994 behaves as a slave. The exception is when the DS1994 generates an interrupt due to a

timekeeping alarm. The discussion of this bus system is broken down into three topics: hardware

configuration, transaction sequence, and 1-Wire signaling (signal types and timing).

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 port of the DS1994 is open drain with an internal circuit

equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple slaves

attached. The 1-Wire bus has a maximum data rate of 16.3kbps and requires a pullup resistor of

approximately 5k

W. 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 120

ms, one or more of the devices on the bus can be reset.