8 REAL-TIME CLOCK (RTC)
S1C33L26 TECHNICAL MANUAL
Seiko Epson Corporation
8-9
Precautions
All RTC interrupt control bits described above are indeterminate when power is turned on. Moreover, these bits
are not initialized to specific values by an initial reset.
After power-on, be sure to set RTCIEN to 0 (interrupt disabled) to prevent the occurrence of unwanted RTC in-
terrupts. Also be sure to write 1 to RTCIRQ to reset it.
When a software reset is performed (RTCRST
→
1
→
0), RTCIRQ and RTCIEN are reset to 0 to disable the
interrupt request output. Also RTCT[2:0] is reset to 0x1.
WAKEUP and #STBY Pins
8.5
The S1C33L26 has a battery backup function that allows the system to turn the system power (LV
DD
, PLLV
DD
,
HV
DD
, AV
DD
) off with the RTC (including the OSC1 oscillator circuit) kept active and the BBRAM data maintained
by supplying RTCV
DD
. The RTC provides the WAKEUP and #STBY pins used for controlling this function.
The #STBY pin is used to disconnect the circuits driven with RTCV
DD
(RTC, OSC1, and BBRAM) from the other
circuits driven with LV
DD
, PLLV
DD
, HV
DD
, and AV
DD
(including the control registers for RTC and OSC1). The
#STBY pin must be set to a high level during normal operation. Setting the #STBY pin to a low level from outside
the IC disconnects the RTCV
DD
circuits from the system allowing the system power (LV
DD
, PLLV
DD
, HV
DD
, AV
DD
)
turned off.
The WAKEUP pin is an output pin of which the output can be controlled by the RTC interrupt or software. This
output can control the external regulator to turn the system power (LV
DD
, PLLV
DD
, HV
DD
, AV
DD
) on and off. Note
that leakage current flows to the RTCV
DD
system if the system power is turned off when the #STBY pin is set to a
high level. Therefore, the #STBY pin must be set to a low level before the system power is turned off.
Figure 8.5.1 shows an example of system standby/wakeup circuit using the WAKEUP and #STBY pins.
RTC
OSC1
BBRAM
Regulator
(When WAKEUP polarity is set to active high)
S1C33L26
Active high
WAKEUP
#RESET
GPIO (1)
GPIO (2)
#STBY
POWER SW
RESET SW
*
*
LV
DD
, PLLV
DD
, HV
DD
, AV
DD
EN
3.3 V DC IN
3.3 V
1.8 V, 3.3 V
RTCV
DD
(Battery)
5.1 Example of System Standby/Wakeup Circuit
Figure 8.
Note: The WAKEUP pin does not exist in the TQFP15-128pin package model.
Selecting the WAKEUP signal polarity
Use WUP_POL/RTC_WAKEUP register to select the WAKEUP output level when it is asserted by an RTC in-
terrupt or software control.
The WAKEUP output is configured to active high signal when WUP_POL is set to 0 or active low signal when
WUP_POL is set to 1. WUP_POL is not initialized at initial reset, therefore, it must be initialized with software
when using the WAKEUP output.
Controlling the WAKEUP output
Controlling by an RTC interrupt
When the cause of RTC interrupt that has been selected with software (see Section 8.4) occurs, the WAKEUP
signal is asserted similar to the interrupt request signal. The RTC maintains the WAKEUP signal at the active
level until the system resumes operating and clears the RTC interrupt status bit RTCIRQ/RTC_INTSTAT regis-
ter. The WAKEUP signal will be negated after RTCIRQ is cleared.