Low-Current, SPI-Compatible Real-Time Clock

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9

Setting the Clock

Writing to the Timekeeping Registers

The time and date are set by writing to the timekeeping

registers (Seconds, Minutes, Hours, Date, Month, Day,

Year, and Century). During a write operation, an input

buffer accepts the new time data while the timekeeping

registers continue to increment normally, based on the

crystal counter. The buffer also keeps the timekeeping

registers from changing as the result of an incomplete

write operation, and collision-detection circuitry

ensures that a time write does not occur coincident with

a Seconds register increment. The updated time is

loaded into the timekeeping registers after the rising

edge of CS, at the end of the SPI write operation. An

incomplete write operation aborts the update proce-

dure, and the contents of the input buffer are discard-

ed. The timekeeping registers reflect the new time

beginning with the first Seconds register increment

after the rising edge of CS.

Although both single writes and burst writes are possi-

ble, the best way to write to the timekeeping registers is

with a burst write. With a burst write, the main time-

keeping registers (Seconds, Minutes, Hours, Date,

Month, Day, Year) and the Control register are written

sequentially following the address/command byte. They

must be written as a group of eight registers, with 8 bits

each, for proper execution of the burst write function.

All seven timekeeping registers are simultaneously

loaded into the clock counters by the rising edge of CS,

at the end of the SPI write operation.

If single write operations are used to enter data into the

timekeeping registers, error checking is required. If not

writing to the Seconds register, begin by reading the

Seconds register and save it as initial-seconds. Then

write to the required timekeeping registers, and finally

read the Seconds register again (final-seconds). Check

to see that final-seconds is equal to initial-seconds. If

not, repeat the write process. If writing to the Seconds

register, update the Seconds register first, and then

read it back and store its value (initial-seconds).

Update the remaining timekeeping registers and then

read the Seconds register again (final-seconds). Check

to see that final-seconds is equal to initial-seconds. If

not, repeat the write process.

AM/PM and 12Hr/24Hr Mode

Bit 7 of the Hours register selects 12hr or 24hr mode.

When high, 12hr mode is selected. In 12hr mode, bit 5

is the AM/PM bit, logic-high for PM. In 24hr mode, bit 5

is the 20hr bit, logic-high for hours 20 through 23.

Write-Protect Bit

Bit 7 of the Control register is the write-protect bit.

When high, the write-protect bit prevents write opera-

tions to all registers except itself. After initial settings

are written to the timekeeping registers, set the write-

protect bit to logic 1 to prevent erroneous data from

entering the registers during power glitches or inter-

rupted serial transfers. The lower 7 bits (bits 0–6) are

unusable, and always read zero. Any data written to

bits 0–6 are ignored. Bit 7 must be set to zero before a

single write to the clock, before a write to RAM, or dur-

ing a burst write to the clock.

Example: Setting the Clock

with a Burst Write

To set the clock to 10:11:31PM, Thursday July 4th,

2002, with a burst write operation, write 3Fh as the

address/command byte, followed by 8 bytes, 31h, 11h,

B0h, 04h, 07h, 05h, 02h, and 00h (Figure 2). 3Fh is the

clock burst write address/command. The first data

byte, 31h, sets the Seconds register to 31. The second

data byte, 11h, sets the Minutes register to 11. The

third data byte, B0h, sets the Hours register to 12hr

mode, and 10PM. The fourth data byte, 04h, sets the

Date register (day of the month) to the 4th. The fifth

data byte, 07h, sets the Month register to July. The

sixth data byte, 05h, sets the Day register (day of the

week) to Thursday. The seventh data byte, 02h, sets

the Year register to 02. The eighth data byte, 00h,

clears the write-protect bit of the Control register to

allow writing to the device. The Century register is not

accessed with a burst write and therefore must be writ-

ten to separately to set the century to 20. Note the

Century register corresponds to the thousand and hun-

dred digits of the current year and defaults to 19.