DP8570A datasheet(21/26 Pages) NSC

21 / 26 page

Application Hints

Suggested Initialization Procedure for DP8570A in bat-

tery backed applications that use the VBB pin

1 Enter the test mode by writinga1tobit D7 in the Period-

ic Flag Register

2 Write zero to the RAMTEST mode Register located in

page 0 address HEX 1F

3 Leave the test mode by writinga0tobit D7 in the Period-

ic Flag Register Steps 1 2 3 guarantee that if the test

mode had been entered during power on (due to random

pulses from the system) all test mode conditions are

cleared Most important is that the OSC Fail Disable bit is

cleared Refer to AN-589 for more information on test

mode operation

4 After power on (VCC and VBB powered) select the cor-

rect crystal frequency bits (D7 D6 in the Real Time Mode

TABLE I

Frequency

32768 kHz

4194304 MHz

49152 MHz

320 kHz

5 Enter a software loop that does the following

Set a 3 second (approx) software counter The crystal

oscillator may take 1 second to start

51 Writea1tobit D3 in the Real Time Mode Register

(try to start the clock) Make sure the crystal select

bits remain the same as in step 1 Under normal

operation this bit can be set only if the oscillator is

running During the software loop RAM real time

counters output configuration interrupt control and

timer functions may be initialized

6 Test bit D6 in the Periodic Flag Register

IFa1

go to 51 If this bit remains a 1 after 3 seconds

then abort and check hardware The crystal may be de-

fective or not installed There may be a short at OSC IN

or OSC OUT to VCC or GND or to some impedance that

is less than 10 MX

IFa0

then the oscillator is running go to step 7

7 Writea0tobit D6 in the Periodic Flag Register This

action puts the clock chip in the battery backed mode

This mode can be entered only if the osc fail flag (bit D6

of the Periodic Flag Register) is a 0 Reminder Bit D6 is

a dual function bit When read D6 returns oscillator

status

When written

D6 causes either the Battery

Backed Mode or the Single Supply Mode of operation

The only method to ensure the chip is in the battery

backed mode is to measure the waveform at the OSC

OUT pin If the battery backed mode was selected suc-

cessfully then the peak to peak waveform at OSC OUT

is referenced to the battery voltage If not in battery

backed mode the waveform is referenced to VCC The

measurement should be made with a high impedance

low capacitance probe (10 MX 10 pF oscilloscope

probe or better) Typical peak to peak swings are within

06V of VCC and ground respectively

8 Writea1tobit D7 of Interrupt Control Register 1 This

action enables the PFAIL pin and associated circuitry

9 Writea1tobit D4 of the Real Time Mode Register This

action ensures that bit D7 of Interrupt Control Register 1

remains a 1 when VBB l VCC (Standby Mode)

10 Initialize the rest of the chip as needed

Typical Application

TLF8638 – 22

These components may be necessary to meet UL requirements

for lithium batteries Consult battery manufacturer

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