ACT5880 datasheet(76/87 Pages) ACTIVE-SEMI

76 / 87 page

ACT5880

Rev 2, 03-Sep-13

- 76 -

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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

details.

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.

Crystal Selection

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 60kΩ 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.

Table 9:

Typical Crystal Parameters.

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

Figure 13:

The Crystal Oscillation Circuit.

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.

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.

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;

Set low 8 bits of Day by writing 0xB0,

high 8 bits of Day by writing 0xB1;

Set Seconds/Minutes/Hours by writing

0xA4, 0xA8, 0xAC;

Set RCL[ ]=1 to load counters;

Set byte 0x3C to 0x80;

Set byte 0x3D to 0x08;

Set alarm Seconds/Minutes/Hours by

writing 0x90, 0x94, 0x98;

Set nWKALM[ ]=0, RAAI[ ]=1 to enable the

alarm.

REAL TIME CLOCK

MODEL

CL1, CL2 (pF)

RSH (MΩ)

MC-146

10 (7~15)

TXC 9H03 2000 10

AB38T

5.6 (4.7~8.2)

CL2

CL1

XTAL+

XTAL-

RSH

ACT5880

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