ACT5880
Rev 2, 03-Sep-13
REAL TIME CLOCK
The real time clock or namely the RTC circuit in the
ACT5880 is powered by the VALIVE output of the
always-on LDO. The circuit includes a crystal
oscillator running with 32.768kHz quartz resonator, a
rank of counters and manipulation interface. The
RTC time counts for seconds, minutes, hours and
days, alarms against comparison of second, minute
and hour, outputs a square wave at its oscillator's
frequency, asserts interrupt or wakes up the system
if it is configured so. The data settings and the
configuring are through I2C operation. Refer to the
REGISTER MAP AND DESCRIPTIONS section for
details.
Figure 13:
The Crystal Oscillation Circuit.
CL2
XTAL+
RSH
ACT5880
XTAL-
CL1
A bit in the control register, RTCEN[ ], enables the
RTC circuit when it set 1, whose default is 0.
Whenever power loss happens, it returns to 0 to
disable the RTC counter. It is necessary to check
this bit to assure the RTC runs normally before
taking the time from the RTC. The oscillator and the
square wave output are not affected by this bit,
which keeps operation until it could works.
time set in respective registers, interrupt asserts in
normal operation or wake-up the ACT5880, then to
wake up the host system, just as if a power-on
button press event happens. To enable the alarm
function, 2 control bytes, 0x3C and 0x3D need to be
set to 0x80 and 0x08 respectively first, then set the
RAAI[ ] bit and clear the nWKALM[ ] bit.
Crystal Selection
When an interrupt occurs, its assertion is kept until
the 0x3C is read for clearing; an internal latch circuit
enabled by the nWKALM[ ] bit (=0) forces an on-
state for possible wake-up, which also generates
and holds both nIRQ and nPBSTAT low. The
nWKALM[ ] bit has to be set 1 to release the forced
on state and the nIRQ low state after the alarm, to
allow other signals like PWRHLD to take control of
power state again.
A precise 32.768k clock crystal is required to run
the oscillator. The oscillation circuit works properly
with large variety of load capacitances, CL1 and CL2
in range of 5pF to 30pF. The maximum serial
resistance RS of the crystal for having stable
oscillation is about 60kꢀ to this device, while the
less serial resistance is the easier to starting and
maintaining oscillation. Commonly, the bigger size
crystals have less serial resistance. For small size
SMD crystal, an external shut resistor is required.
Table 9 lists few crystal models and the external
component parameter suggested. Refer to Figure
13 for the circuit diagram.
Following is a sample of setting RTC alarm or wake
up functions:
To enable the RTC wake-up alarm:
Set RTCEN[ ]=1 to enable the RTC
running;
Table 9:
Typical Crystal Parameters.
Set low 8 bits of Day by writing 0xB0,
high 8 bits of Day by writing 0xB1;
MODEL
CL1, CL2 (pF)
RSH (MΩ)
Set Seconds/Minutes/Hours by writing
0xA4, 0xA8, 0xAC;
MC-146
TXC 9H03 2000 10
AB38T
Set RCL[ ]=1 to load counters;
Set byte 0x3C to 0x80;
10 (7~15)
5.6 (4.7~8.2)
Set byte 0x3D to 0x08;
Set alarm Seconds/Minutes/Hours by
writing 0x90, 0x94, 0x98;
RTC Interrupt and Wake-up
The RTC can be configured to assert interrupt and/
or to wake-up the ACT5880 from disabled state or
deep sleep state. When the hour counts, the minute
counts and the second counts all match the alarm
Set nWKALM[ ]=0, RAAI[ ]=1 to enable the
alarm.
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