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STK17T88

Document Number: 001-52040 Rev. *C Page 15 of 24

Real Time Clock

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 registers 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 ‘0’ are currently not used and are reserved for

future use by Cypress.

Reading the Clock

Halt internal updates to the real time clock registers before

reading clock data to prevent reading of data in transition.

Stopping the internal register updates does not affect clock

accuracy.

Write a ‘1’ to the read bit “R” (in the Flags register at 0x7FF0) to

capture the current time in holding registers. Clock updates do

not restart until a ‘0’ is written to the read bit. The RTC registers

can now be read while the internal clock continues to run.

Within 20 ms after a ‘0’ is written to the read bit, all real time clock

registers are simultaneously updated.

Setting the Clock

Set the write bit “W” (in the Flags register at 0x7FF0) to ‘1’ to

enable the time to be set. The correct day, date, and time can

then be written into the real time clock registers in 24-hour BCD

format. The time written is referred to as the “Base Time.” This

value is stored in nonvolatile registers and used in calculation of

the current time. Reset the write bit to ‘0’ to transfer the time to

the actual clock counters, The clock starts counting at the new

base time.

Backup Power

The RTC is intended to keep time even when system power is

lost. When primary power, V

, drops below V

SWITCH

, the real

time clock switches to the backup power supply connected to

either the V

RTCcap

or V

RTCbat

pin.

The clock oscillator uses a maximum of 300 nanoamps at 2V to

maximize the backup time available from the backup source.

The user can power the real time clock with either a capacitor or

a battery. Factors to be considered when choosing a backup

power source include the expected duration of power outages

and the cost and reliability trade-off of using a battery versus a

capacitor.

When selecting a capacitor power source, connect the capacitor

to the V

RTCcap

pin and leave the V

RTCbat

pin unconnected.

Capacitor backup time values based on maximum current speci-

fications are shown below. Nominal times are approximately

three times longer.

A capacitor has the obvious advantage of being more reliable

and not containing hazardous materials. The capacitor is

recharged every time the power is turned on so that the real time

clock continues to have the same backup time over years of

operation

When selecting a battery power source, connect the battery to

the V

RTCbat

pin and leave the V

RTCcap

pin unconnected. A 3V

lithium is recommended for this application. The battery capacity

should be chosen for the total anticipated cumulative down-time

required over the life of the system.

The real time clock is designed with a diode internally connected

to the V

RTCbat

pin. This prevents the battery from ever being

charged by the circuit.

Stopping and Starting the RTC Oscillator

The OSCEN bit in the Calibration register at 0x7FF8 enables

RTC oscillator operation. This bit is nonvolatile and shipped to

customers in the “enabled” state (set to ‘0’). OSCEN should be

set to ‘1’ to preserve battery life while the system is in storage.

This turns off the oscillator circuit extending the battery life. If the

OSCEN bit goes from disabled to enabled, it typically takes 5

seconds (10 seconds maximum) for the oscillator to start.

The STK17T88 has the ability to detect oscillator failure due to

loss of backup power. The failure is recorded by the OSCF

(Oscillator Failed bit) of the Flags register (at address 0x7FF0).

When the device is powered on (V

goes above V

SWITCH

) the

OSCEN bit is checked for “enabled” status. If the OSCEN bit is

enabled and the oscillator is not active within 5 ms, the OSCF bit

is set. Check for this condition and then write a ‘0’ to clear the

flag. When the OSCF flag bit, the real time clock registers are

reset to the “Base Time” (see the section Setting the Clock on

page 15, the value last written to the real time clock registers.

The value of OSCF should be reset to ‘0’ when the real time clock

registers are written for the first time. This initializes the state of

this bit because it may have become set when the system was

first powered on.

To reset OSCF, set the write bit “W” (in the Flags register at

0x7FF0) to ‘1’ to enable writes to the Flags register. Write ‘0’ to

the OSCF bit and then reset the write bit to ‘0’ to disable writes.

Calibrating The Clock

The RTC is driven by a quartz controlled oscillator with a nominal

frequency of 32.768 kHz. Clock accuracy depends on the quality

of the crystal specified (usually 35 ppm at 25

°C). This error can

equate to 1.53 minutes gain or loss per month. The STK17T88

employs a calibration circuit that can improve the accuracy to

+1/-2 ppm at 25

°C. The calibration circuit adds or subtracts

counts from the oscillator divider circuit.

The number of time pulses added or subtracted depends upon

the value loaded into the five calibration bits found in Calibration

subtracting counts slows the clock down. The calibration bits

occupy the five lower order bits of the register. These bits can be

set to represent any value between 0 and 31 in binary form. Bit

D5 is a Sign bit, where a ‘1’ indicates positive calibration and a

‘0’ indicates negative calibration. Calibration occurs during a 64

minute period. The first 62 minutes in the cycle may, once per

minute, have one second either shortened by 128 or lengthened

by 256 oscillator cycles.

Capacitor Value Backup Time

0.1 F 72 hours

0.47 F 14 days

1.0 F 30 days

Not Recommended for New Designs

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