DS1338 I2C RTC with 56-Byte NV RAM
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whenever the seconds register is written. Write transfers occur on the acknowledge from the DS1338. Once the
countdown chain is reset, to avoid rollover issues the remaining time and date registers must be written within
1 second. The 1Hz square-wave output, if enabled, transitions high 500ms after the seconds data transfer,
provided the oscillator is already running.
The DS1338 runs in either 12-hour or 24-hour mode. Bit 6 of the hours register is defined as the 12-hour or 24-hour
mode-select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5 is the AM/PM bit, with logic
high being PM. In the 24-hour mode, bit 5 is the 20-hour bit (20鈥�23 hours). If the 12/24-hour mode select is
changed, the hours register must be re-initialized to the new format.
On an I2C START, the current time is transferred to a second set of registers. The time information is read from
these secondary registers, while the clock continues to run. This eliminates the need to re-read the registers in
case of an update of the main registers during a read.
Table 3. RTC and RAM Address Map
ADDRESS
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
FUNCTION
RANGE
00H
CH
10 Seconds
Seconds
00鈥�59
01H
0
10 Minutes
Minutes
00鈥�59
02H
0
12/24
AM
/PM
10
Hour
Hours
1鈥�12
+AM/PM
00鈥�23
20 Hour
03H
0
Day
1鈥�7
04H
0
10 Date
Date
01鈥�31
05H
0
10
Month
01鈥�12
06H
10 Year
Year
00鈥�99
07H
OUT
0
OSF
SQWE
0
RS1
RS0
Control
08H鈥�3FH
RAM 56 x 8
00H鈥揊FH
Note: Bits listed as 鈥�0鈥� always read as a 0.