STK17TA8-R25 datasheet(13/22 Pages) ETC

13 / 22 page

STK17TA8

February 2004

Document Control # ML0023 rev 0.3

nvTIME OPERATION

The STK17TA8 offers internal registers that contain

Clock, Alarm, Watchdog, Interrupt, and Control func-

tions. Internal double buffering of the clock and the

clock/timer information registers prevents accessing

transitional internal clock data during a read or write

operation. Double buffering also circumvents dis-

rupting normal timing counts or clock accuracy of

the internal clock while accessing clock data. Clock

and Alarm Registers store data in BCD format.

CLOCK OPERATIONS

The clock registers maintain time up to 9,999 years

in one second increments. The user can set the time

to any calendar time and the clock automatically

keeps track of days of the week and month, leap

years and century transitions. There are eight regis-

ters dedicated to the clock functions which are used

to set time with a write cycle and to read time during

a read cycle. These registers contain the Time of

Day in BCD format. Bits defined as “X” are currently

not used and are reserved for future use by Simtek.

READING THE CLOCK

While the double-buffered RTC register structure

reduces the chance of reading incorrect data from the

clock, the user should halt internal updates to the

STK17TA8 clock registers before reading clock data

to prevent the reading of data in transition. Stopping

the internal register updates does not affect clock

accuracy.

The updating process is stopped by writing a “1” to

the read bit (in the control register 1FFF0h), and will

not restart until a “0” is written to the read bit. The

RTC

registers can then be read while the internal

clock continues to run.

Within 10 msec after a “0” is written to the read bit,

all STK17TA8 registers are simultaneously updated.

SETTING THE CLOCK

Setting the write bit (in the control register 1FFF0h)

to a “1” halts updates to the STK17TA8 registers.

The correct day, date and time can then be written

into the registers in 24-hour BCD format. Resetting

the write bit to “0” transfers those values to the

actual clock counters, after which the clock resumes

normal operation.

BACKUP POWER

The STK17TA8 is intended for permanently pow-

ered operation, but when primary power, Vcc, fails

and drops below Vswitch the device will switch to

backup power from either Vbakcap or Vbakbat,

depending on whether a capacitor or battery is cho-

sen for the application.

The clock oscillator uses very little current, which

maximizes the backup time available from the

backup source. Regardless of clock operation with

the primary source removed, the data stored in

vSRAM is secure, having been stored in the nonvol-

atile elements as power was lost. Factors to be con-

sidered when choosing a backup power source

include: the expected duration of power outages and

the cost tradeoff of using a battery versus a capaci-

tor.

During backup operation the STK17TA8 consumes

a maximum of 300 nanoamps at 2 volts. Capacitor

or battery values should be chosen according to the

application. Backup time values based on maximum

current specs are shown below. Nominal times are

approximately 3 times longer.

Using a capacitor has the obvious advantage of

recharging the backup source each time the system

is powered up.

If a battery is used a 3V lithium is recommended and

the STK17TA8 will only source current from the bat-

tery when the primary power is removed. The bat-

tery will not, however, be recharged at any time by

the STK17TA8. The battery capacity should be cho-

sen for total anticipated cumulative down-time

required over the life of the system.

STOPPING AND STARTING THE OSCIL-

LATOR

The oscillator may be stopped at any time. This fea-

ture may be used to save battery or capacitor

energy during long-term storage to increase shelf

life. Setting the OSCEN bit in register 1FFF8h to 1

halts the oscillator. Setting the bit to 0 enables the

oscillator. The RTC does not run until the oscillator

Capacitor Value

Backup Time

0.1 F

72 hours

0.47 F

14 days

1.0 F

30 days

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