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Rtc operations, Real time clock, Reading the clock – Cypress AutoStore STK17TA8 User Manual

Page 14: Setting the clock, Backup power, Stopping and starting the rtc oscillator, Stk17ta8

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STK17TA8

Document #: 001-52039 Rev. **

Page 14 of 23

Figure 15. Current versus Cycle Time

RTC Operations

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

The user should halt internal updates to the real time clock

registers before reading clock data to prevent the 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 0x1FFF0)

captures the current time in holding registers. Clock updates 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 20ms 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 0x1FFF0) to a "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 in intended to keep time even when system power is

lost. When primary power, V

CC

, 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 2 volts

to maximize the backup time available from the backup source.

You 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.
If you select 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 specs

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 real time

clock continues to have the same backup time over years of

operation.
If you select a battery power source, connect the battery to the

V

RTCbat

pin and leave the V

RTCcap

pin unconnected. A 3V lithium

battery 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 Calibration register at 0x1FFF8 enables RTC

oscillator operation. This bit is nonvolatile and shipped to

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

to a 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 max) for the oscillator to start.
The STK17TA8 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 0x1FFF0).

When the device is powered on (V

CC

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 the first 5 ms, the

OSCF bit is set. The user should check for this condition and then

write a 0 to clear the flag. When the OSCF flag bit is set, the real

time clock registers are reset to the “Base Time” (see the section

"Setting the Clock"), 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 which may have become set when the system was first

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

0x1FFF0) to a "1" to enable writes to the Flag register. Write a

“0” to the OSCF bit. and thenreset the write bit to "0" to disable

writes.

Capacitor Value

Backup Time

0.1 F

72 hours

0.47 F

14 days

1.0 F

30 days

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