Datasheet

Clock operation M41T66
20/34 Doc ID 15108 Rev 2
3.5 Square wave output
The M41T66 offers the user a programmable square wave function which is output on the
SQW pin. RS3-RS0 bits located in 04h establish the square wave output frequency. These
frequencies are listed in Table 4. Once the selection of the SQW frequency has been
completed, the SQW pin can be turned on and off under software control with the square
wave enable bit (SQWE) located in register 0Ah.
The SQW output is an open drain output driver. The initial power-up default for the SQW
output is 32 KHz.
Note: When the SQW is enabled and the ST bit is set (ST = 1), the square wave output could be
low or high and could drain current through the pull-up resistor.
Due to the output buffer circuitry used for the SQW output, this pin must not be taken to a
voltage greater than V
CC
. A diode is required on the SQW pin for SuperCap™ (or battery)
backup. A low threshold BAT42 diode is recommended (see Figure 4 on page 7).
3.6 Century bits
These two bits will increment in a binary fashion at the turn of the century, and handle all
leap years correctly. See Table 6 on page 22 for additional explanation.
Table 4. Square wave output frequency
Square wave bits Square wave
RS3 RS2 RS1 RS0 Frequency Units
0000None–
000132.768kHz
00108.192kHz
00114.096kHz
01002.048kHz
01011.024kHz
0110512Hz
0111256Hz
1000128Hz
100164Hz
101032Hz
101116Hz
11008Hz
11014Hz
11102Hz
11111Hz