Details of Time Data Bits

Now let's look at how the data is stored inside each register. Remember, each register holds 8 bits (bit 7 to bit 0), and the time values are stored in BCD format.

REGISTER	BIT 7	BIT 6	BIT 5	BIT 4	BIT 3	BIT 2
Seconds	CH	Tens digit of seconds			Ones digit of seconds
Minutes	0	Tens digit of minutes			Ones digit of minutes
Hours	0	Select 12/24 format	PM/AM Flag in 12 hour format or Part of Tens in 24 hour format	Tens digit of hours	Ones digit of hours
Day	0	0	0	0	0	Day value (1-7)
Date	0	0	Tens digit of date		Ones digit of date
Month	0	0	0	Tens digit of month	Ones digit of month
Year	Tens digit of year				Ones digit of year

Seconds: Range 00-59

The bits 0-3 represent the ones digit part of the seconds. The bits 4-6 represent the tens digit part of the seconds.

If you want to represent seconds 30, the BCD format will be: 0011 0000. So, we will put 011 in bits 4-6 (tens digit) and 0000 in bits 0-3 (ones digit).

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 1 1 0 0 0 0
         │ └─┬─┘ └──┬──┘
         │   3      0
         CH bit

CH bit (bit 7) controls clock halt - it's like an on/off switch for the entire clock (1 = clock stopped, 0 = clock running).

Hours: Range 1-12 + AM/PM (12-hour mode) or 00-23 (24-hour mode)

In 12-hour mode:

Bit 6 = 1 (selects 12-hour format)
Bit 5 = AM/PM bit (0=AM, 1=PM)
Bit 4 = tens digit of hour (0 or 1)
Bits 3-0 = ones digit of hour

In 24-hour mode:

Bit 6 = 0 (selects 24-hour format)
Bits 5-4 = tens digit of hour
Bits 3-0 = ones digit of hour

Important: The hours value must be re-entered whenever the 12/24-hour mode bit is changed.

If you want to represent 2 PM in 12-hour mode, the format will be: 0110 0010. So bit 6=1 (12-hour), bit 5=1 (PM), bit 4=0 (tens digit), bits 3-0 will be 0010 (ones digit).

Bit:     7 6 5 4 3 2 1 0
Value:   0 1 1 0 0 0 1 0
         │ │ │ │ └──┬──┘
         │ │ │ │    2
         │ │ │ tens digit
         │ │ PM bit
         │ 12-hour format
         Always 0

If you want to represent 14:00 in 24-hour mode, the format will be: 0001 0100. So bit 6=0 (24-hour), bits 5-4 will be 01 (tens digit), bits 3-0 will be 0100 (ones digit).

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 0 1 0 1 0 0
         │ │ └┬┘ └──┬──┘
         │ │  1     4
         │ 24-hour format
         Always 0

Day: Range 01-07 (1=Sunday, 2=Monday, etc.)

The bits 0-2 represent the day value. Bits 3-7 are always 0.

If you want to represent Tuesday (day 3), the format will be: 0000 0011.

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 0 0 0 0 1 1
         └───┬───┘ └─┬─┘
         Always 0     3

Date: Range 01-31 (day of month)

The bits 0-3 represent the ones digit part of the date. The bits 4-5 represent the tens digit part of the date.

If you want to represent date 25, the BCD format will be: 0010 0101. So, we will put 10 in bits 4-5 (tens digit) and 0101 in bits 0-3 (ones digit).

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 1 0 0 1 0 1
         └┬┘ └┬┘ └──┬──┘
          0   2     5
         Always 0

Bits 6-7 are always 0.

Year: Range 00-99 (represents 2000-2099)

The bits 0-3 represent the ones digit part of the year. The bits 4-7 represent the tens digit part of the year.

If you want to represent year 23 (2023), the BCD format will be: 0010 0011. So, we will put 0010 in bits 4-7 (tens digit) and 0011 in bits 0-3 (ones digit).

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 1 0 0 0 1 1
         └──┬──┘ └──┬──┘
            2      3

Remember when I first mentioned the DS1307 and how it could only represent dates until 2100? I was curious about that limitation too. Well, Now we've identified the culprit: the DS1307's 8-bit year register can only store two digits (00-99) in BCD format, and the chip assumes these always represent years in the 2000s century.

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