DS1207 datasheet(2/17 Pages) DALLAS

2 / 17 page

DS1207

021798 2/17

OPERATION – NORMAL MODE

The TimeKey has two modes of operation: normal and

program. The normal mode of operation provides the

functions of reading and writing the 384-bit secure

memory. The block diagram (Figure 1) illustrates the

main elements of the TimeKey when used in the normal

mode. To initiate data transfer with the TimeKey, RST is

taken high and 24 bits are loaded into the command reg-

ister on each low-to-high transition of the CLK input. The

command register must match the exact bit pattern

which defines normal operations with a function code of

read or write. If one of these patterns is not matched,

communications are ignored. If the command register is

loaded properly, communications are allowed to contin-

ue. Data is clocked out of the TimeKey on the

high-to-low transition of the clock. If the pattern matched

in the command register calls for a normal read or write,

the next 64 cycles following the command word are read

and data is clocked out of the identification memory. The

next 64 write cycles are written to the compare register

(Figure 2). These 64 bits must match the exact pattern

stored in the security match memory. If a match is not

found, access to additional information is denied. In-

stead, if a normal read mode is selected, random

garbled data is output for the next 384 cycles. If a normal

write cycle is selected and a match is not achieved, the

TimeKey will ignore any additional information. Howev-

er, when a security match is achieved, access is per-

mitted to write the 384-bit secure memory.

OPERATION - PROGRAM MODE

The program mode of operation provides the functions

of programming the identification and security match

memory, and setting and reading the amount of time the

TimeKey can be used. The block diagram in Figure 3 il-

lustrates the main elements of the TimeKey when used

in the program mode. To initiate the program mode, RST

is driven high and 24 bits are loaded into the command

The command register must match the exact bit pattern

that defines the program mode for the identification and

security match bits or the program mode for setting and

reading the amount of time for which the TimeKey can

be used. If an exact match for one of the seven function

codes of the program mode is not found, the remainder

of the program mode is ignored. When the command

cation and security match bits, the next 128 bits are writ-

ten to the identification and security match memory

(Figure 4). When this mode of operation is invoked, all

memory contents are erased.

SETTING AND READING TIME REMAINING

There are six functions of the program mode which are

used to set or read the amount of time for which the

TimeKey will allow full operation. To initiate any of the six

functions of the program mode used for setting and

reading time remaining, RST is driven high and 24 bits

are loaded into the command register on each

low-to-high transition of the CLK input. If the command

reading the 20-bit day clock counter, the next 20 bits will

be output (LSB first) as a binary count of the amount of

time elapsed in the current day (see Figure 5). The time

can be calculated by dividing this count reading by 220

(20 bits is equal to 1,048,576 counts). One minus this re-

sult is the fraction of a day remaining. The 20-bit day

clock counter is driven by an internal oscillator that has a

period of 82.4 ms. If the command register is properly

loaded with the function code for reading the 9-bit num-

ber of days counter, the next 9 bits will be output (LSB

first) as a binary count of the days remaining (see

Figure 6). This count is decremented each time the day

clock counter rolls over to zero. When the number of

days remaining counter rolls through zero, normal and

program mode write cycles are inhibited. If the program

mode read cycle to the number of days counter is at-

tempted, the nine bits will be returned as all ones.

If the command register is properly loaded with the func-

tion code for writing the 9-bit number of days counter,

the next nine bits will be input (LSB first) as a binary

count of the desired number of days in which the Time-

Key will be fully functional (see Figure 7). The number of

days counter can be changed by writing over an entered

value as often as required until the lock command is en-

tered. The lock command is given when the command

locking up the number of days counter. The lock com-

mand consists of the 24-bit command word only (see

Figure 8). Once the lock command is given, all future

write cycles to the number of days register are ignored.

After the correct value has been written and locked into

the number of days counter, the DS1207 will start count-

ing the time from the entered value to zero after the first

access to the TimeKey is executed, provided the arm

oscillator bit is set. The arm oscillator bit is set when the

command register has been properly loaded with the

function code for arming the oscillator. The arm oscilla-

tor command consists of the 24-bit command word only

(see Figure 9). One other command is also available for

use in setting and reading time remaining. A stop oscil-

lator command is given when the command register is

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