DS1207 datasheet(3/17 Pages) DALLAS

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DS1207

021798 3/17

properly loaded with the function code for stopping the

oscillator. The stop oscillator command consists of the

24-bit command word only (see Figure 10). This com-

mand will only execute prior to issuing a lock command.

After the lock command is issued, stop oscillator com-

mands are ignored.

A sequence for properly setting the expiration time of

the DS1207 is as follows (see Figure 11). First, program

the identification and security match bits to the desired

value. Use normal mode operation to write the appropri-

ate secure data. Second, write the number days remain-

ing register to the desired value. This number can be im-

mediately verified by reading the number of days

remaining. Next, arm the oscillator by writing the appro-

priate command. Then do a normal mode read. This ac-

tion will start the internal oscillator. Now read the 20-bit

day clock counter several times to verify that the oscilla-

tor is running. After oscillator activity has been verified,

issue the stop oscillator command. The lock command

should be issued, followed by the arm oscillator com-

mand. The TimeKey will start the countdown to expira-

tion on the next access. To guarantee security, a locked

TimeKey cannot be unlocked. The key cannot be re-

programmed after expiration. The oscillator verification

portion of this sequence is not required and can be de-

leted when speed in setting time remaining is important.

COMMAND WORD

Each data transfer for normal and program mode begins

with a 3-byte command word as shown in Figure 12. As

defined, the first byte of the command word specifies the

function code. Eight function codes are acceptable

(Figure 13). If any one of the bits of the first byte of the

command word fails to meet one of the exact patterns

for function codes, the data transfer will be aborted.

The first two bits of the second byte of the command

word specify whether the data transfer to follow is pro-

gram or normal mode. The bit pattern for program mode

is 0 in bit 0 and 1 in bit 1. The bit pattern for normal mode

is a 1 in bit 0 and a 0 in bit 1. The other two possible com-

binations for the first two bits of byte 2 will cause the

transfer to abort. The program mode can be invoked

with one of seven function codes: program identification

and security match, read the 20-bit day clock counter,

read the number of days count, write the number of days

count, lock number of days count, arm oscillator, and

stop oscillator.

The remaining six bits of byte 2 and the first four bits of

byte 3 must be written to match one of the five patterns

as indicated in Figure 12 or data transfer will abort. Un-

der special contract with Dallas Semiconductor, these

bits can be defined by the user as any bit pattern other

than those specified as unavailable. The bit pattern as

defined by the user must be written exactly or data

transfer will abort. The last four bits of byte 3 of the com-

mand word must be written 1011 or data transfer will

abort. Table 1 provides a summary of the command

words in hexadecimal as they apply to all function codes

for both program mode and normal mode.

RESET AND CLOCK CONTROL

All data transfers are initiated by driving the RST input

high. The reset input serves three functions. First, it

turns on control logic which allows access to the com-

mand register for the command sequence. Second, the

RST signal provides a power source for the cycle to fol-

low. To meet this requirement, a drive source for RST of

2 mA at 3.5 volts is required. Third, the RST signal pro-

vides a method of terminating data transfer.

A clock cycle is a sequence of a falling edge followed by

a rising edge. For data inputs, the data must be valid

during the rising edge of the clock cycle. Command bits

and data bits are input on the rising edge of the clock.

Data bits are output on the falling edge of the clock. The

rising edge of the clock returns the DQ pin to a high im-

pedance state. All data transfer terminates if the RST

pin is low and the DQ pin goes to a high impedance

state. Data transfer is illustrated in Figure 14 for normal

mode and Figure 15 for program mode.

TIMEKEY CONNECTIONS

The TimeKey is designed to be plugged into a standard

5-pin 0.1 inch center SIP receptacle. A guide is provided

to prevent the TimeKey from being plugged in back-

wards and aid in alignment of the receptacle. For porta-

ble applications, contact to the TimeKey pins can be de-

termined to ensure connection integrity before data

transfer begins. CLK, RST, and DQ all have 20K ohm

pulldown resistors to ground that can be sensed by a

reading device.

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