DS1556 1M, Nonvolatile, Y2K-Compliant Timekeeping RAM

7 of 18

READING THE CLOCK

When reading the RTC data, it is recommended to halt updates to the external set of double-buffered RTC

Registers. This puts the external registers into a static state allowing data to be read without register

values changing during the read process. Normal updates to the internal registers continue while in this

state. External updates are halted when a 1 is written into the read bit, B6 of the Control Register

(1FFF8h). As long as a 1 remains in the Control Register read bit, updating is halted. After a halt is

issued, the registers reflect the RTC count (day, date, and time) that was current at the moment the halt

command was issued. Normal updates to the external set of registers will resume within 1 second after the

read bit is set to a 0 for a minimum of 500

s. The read bit must be a zero for a minimum of 500s to

ensure the external registers will be updated.

SETTING THE CLOCK

The MSB 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 DS1556 (1FFF8h to 1FFFFh) 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 DS1556 is guaranteed to keep time accuracy to within

1 minute per month at 25C. The RTC is

calibrated at the factory by Maxim using nonvolatile tuning elements, and does not require additional

calibration. For this reason, methods of field clock calibration are not available and not necessary. The

electrical environment also affects clock accuracy, and caution should be taken to place the RTC in the

lowest-level EMI section of the PC board layout. For additional information, refer to Application Note

58.

CLOCK ACCURACY (PowerCap MODULE)

The DS1556 and DS9034PCX are each individually tested for accuracy. Once mounted together, the

module will typically keep time accuracy to within

1.53 minutes per month (35 ppm) at 25°C. The

electrical environment also affects clock accuracy, and caution should be taken to place the RTC in the

lowest-level

EMI

section

of

the

PC

board

layout.

For

additional

information,

refer

to

Application Note 58.

FREQUENCY TEST MODE

The DS1556 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 1FFF7h = 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 that requires a pullup resistor for

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