Rainbow Electronics DS1543 User Manual

Page 6

DS1543

6 of 17

SETTING THE CLOCK

The 8

bit, B7 of the control register is the write bit. Setting the write bit to a 1, like the read bit, halts

updates to the DS1543 (1FF8h-1FFFh) registers. After setting the write bit to a 1, RTC registers can be
loaded with the desired RTC count (day, date, and time) in 24-hour BCD format. Setting the write bit to
a 0 then transfers the values written to the internal RTC registers and allows normal operation to resume.

CLOCK ACCURACY (DIP MODULE)

The DS1543 is guaranteed to keep time accuracy to within

1 minute per month at 25

C. The RTC is

calibrated at the factory by Dallas Semiconductor using nonvolatile tuning elements. The DS1543 does
not require additional calibration and, in most applications, temperature deviations will have a negligible
effect on accuracy. For this reason, methods of field clock calibration are not available and not necessary.
Attempts to calibrate the RTC that may be used with similar device types (M48T5x family) will not have
any effect even though the DS1543 appears to accept calibration data.

CLOCK ACCURACY (POWERCAP MODULE)

The DS1543 and DS9034PCX are each individually tested for accuracy. Once mounted together, the
module is guaranteed to keep time accuracy to within

1.53 minutes per month (35 ppm) at 25

FREQUENCY TEST MODE

The DS1543 frequency test mode uses the open-drain

IRQ

/FT output. With the oscillator running, the

IRQ

/FT output will toggle at 512 Hz when the FT bit is a 1, the Alarm Flag Enable bit (AE) is a 0, and

the Watchdog Steering bit (WDS) is a 1 or the Watchdog Register is reset (register 1FF7h = 00h). The

IRQ

/FT output and the frequency test mode can be used as a measure of the actual frequency of the

32.768 kHz RTC oscillator. The

IRQ

/FT pin is an open-drain output which requires a pullup resistor for

proper operation. The FT bit is cleared to a 0 on power-up.

USING THE CLOCK ALARM

The alarm settings and control for the DS1543 reside within registers 1FF2h - 1FF5h. Register 1FF6h
contains two alarm enable bits: Alarm Enable (AE) and Alarm in Backup Enable (ABE). The AE and
ABE bits must be set as described below for the

IRQ

/FT output to be activated for a matched alarm

condition.

The alarm can be programmed to activate on a specific day of the month or repeat every day, hour,
minute, or second. It can also be programmed to go off while the DS1543 is in the battery-backed state of
operation to serve as a system wake-up. Alarm mask bits AM1-AM4 control the alarm mode. Table 3
shows the possible settings. Configurations not listed in the table default to the once per second mode to
notify the user of an incorrect alarm setting.

ALARM MASK BITS Table 3

AM4

AM3

AM2

AM1

ALARM RATE

Once per second

When seconds match

When minutes and seconds match

When hours, minutes, and seconds match

When date, hours, minutes, and seconds match