CY14B101K
Document Number: 001-06401 Rev. *I
Page 7 of 28
Real Time Clock Operation
nvTIME Operation
The CY14B101K offers internal registers that contain clock,
alarm, watchdog, interrupt, and control functions. RTC registers
use the last 16 address locations of the SRAM.
Internal double
buffering of the clock and the clock or timer information registers
prevents accessing transitional internal clock data during a
READ or WRITE operation. Double buffering also circumvents
disrupting normal timing counts or clock accuracy of the internal
clock while accessing clock data. Clock and Alarm Registers
store data in BCD format.
The RTC register addresses for CY14B101K range from
0x1FFF0 to 0x1FFFF. Refer to
RTC Register Map[5, 6]
on page
11 and
Register Map Detail
on page 12 for detailed description.
Clock Operations
The clock registers maintain time up to 9,999 years in one
second increments. The user sets the time to any calendar time
and the clock automatically keeps track of days of the week,
month, leap years, and century transitions. There are eight
registers dedicated to the clock functions that 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 double buffered RTC register structure reduces the chance
of reading incorrect data from the clock. The user should stop
internal updates to the CY14B101K time keeping registers
before reading clock data, to prevent 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
‘R’ (in the flags register at 0x1FFF0), and does not restart until a
‘0’ is written to the read bit. The RTC registers are then read while
the internal clock continues to run. After a ‘0’ is written to the read
bit (‘R’), all CY14B101K registers are simultaneously updated
within 20 ms.
Setting the Clock
Setting the write bit ‘W’ (in the flags register at 0x1FFF0) to a ‘1’
stops updates to the time keeping registers and enables the time
to be set. The correct day, date, and time are then written into
the 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. Resetting the WRITE
bit to ‘0’ transfers those values to the actual clock counters, after
which the clock resumes normal operation.
Backup Power
The RTC in the CY14B101K is intended for permanently
powered operations. Either the V
RTCcap
or V
RTCbat
pin is
connected depending on whether a capacitor or battery is
chosen for the application. When the primary power, V
CC
, fails
and drops below V
SWITCH
, the device switches to the backup
power supply.
The clock oscillator uses very little current to maximize the
backup time available from the backup source. Regardless of
clock operation with the primary source removed, the data stored
in nvSRAM is secure, as it is stored in the nonvolatile elements
when power was lost.
During backup operation, the CY14B101K consumes a
maximum of 300 nA at 2V. The user should choose capacitor or
battery values according to the application.
Backup time values, based on maximum current specifications,
are shown in the following table. Nominal times are approxi-
mately three 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, use a 3V lithium; the CY14B101K only sources current
from the battery when the primary power is removed. However,
the battery is not recharged at any time by the CY14B101K. The
battery capacity is chosen for total anticipated cumulative
downtime required over the life of the system.
Stopping and Starting the Oscillator
The OSCEN bit in the calibration register at 0x1FFF8 controls
the enable and disable of the oscillator. This active LOW bit is
nonvolatile and is shipped to customers in the “enabled” (set to
0) state. To preserve the battery life when the system is in
storage, OSCEN bit must be set to ‘1’. This turns off the oscillator
circuit, extending the battery life. If the OSCEN bit goes from
disabled to enabled, it takes approximately 5 seconds
(10
seconds maximum) for the oscillator to start.
While system power is off, if the voltage on the backup supply
(V
RTCcap
or V
RTCbat
) falls below their respective minimum level,
the oscillator may fail.The CY14B101K has the ability to detect
oscillator failure when system power is restored. This is recorded
in 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 to “1”. The system must check
for this condition and then write ‘0’ to clear the flag. Note that in
addition to setting the OSCF flag bit, the time registers are reset
to the “Base Time” (see
“Setting the Clock”
on page 7), which is
the value last written to the time keeping registers. The Control
or Calibration registers and the OSCEN bit are not affected by
the “oscillator failed” condition.
The value of OSCF must be reset to ‘0’ when the time 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 “1” to enable writes to the Flag register. Write a “0”
to the OSCF bit and then reset the write bit to “0” to disable
writes.
Table 3. RTC Backup Time
Capacitor Value
Backup Time
0.1F
72 hours
0.47F
14 days
1.0F
30 days
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