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DS3231 Alarm

We discussed some of the alarm-related bits in the control register section of the DS3231 chapter. However, we have not looked at the alarm registers in detail yet.

Before getting into the implementation of the Alarm trait, let’s first understand the alarm registers and how alarms work in the DS3231.

Alarm 1 Registers

Alarm 1 uses four registers from 0x07 to 0x0A. These registers store the seconds, minutes, hours, and day/date values used for matching.

REGISTER BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Alarm 1 Seconds A1M1 Tens digit of seconds Ones digit of seconds
Alarm 1 Minutes A1M2 Tens digit of minutes Ones digit of minutes
Alarm 1 Hours A1M3 Select
12/24 format		 PM/AM Flag in 12 hour format
or
20 Hour bit in 24 hour format		 Tens digit of hours Ones digit of hours
Alarm 1 Day/Date A1M4 Select Day of The Week (e.g: Sunday)
or
Day of Month (e.g: 31st)		 Tens digit of day of the month Ones digit of Day/Date

Alarm 1 Seconds: Range 00-59

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

Bit 7 is the A1M1 mask bit. When it is 0, the alarm is triggered when the seconds value matches the current seconds. When it is 1, the seconds value is ignored.

If you want Alarm 1 to match seconds 30, the BCD value will be: 0011 0000. We will put 011 in bits 4-6 and 0000 in bits 0-3.

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 1 1 0 0 0 0
         │ └─┬─┘ └──┬──┘
         │   3      0
         A1M1

Alarm 1 Minutes: Range 00-59

This register works in the same way as the Seconds register.

Let’s say we want Alarm 1 to match minute 30. The BCD value will be 0011 0000. We put 011 in bits 4-6 and 0000 in bits 0-3. We should also set the A1M2 mask bit to 0.

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 1 1 0 0 0 0
         │ └─┬─┘ └──┬──┘
         │   3      0
         A1M2

Alarm 1 Hours: Range 1-12 + AM/PM or 00-23

The Alarm 1 Hours register uses the same 12-hour and 24-hour formats as the main Hours register.

Bit 7 is the A1M3 mask bit. When it is 0, the hour value must match the current hour. When it is 1, the hour value is ignored.

In 24-hour mode, bit 6 is 0, bit 5 is the 20-hour bit, bit 4 is the remaining tens digit, and bits 3-0 contain the ones digit.

For example, if we want Alarm 1 to match 07:00, the hour value will be 0000 0111 and A1M3 will be 0.

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 0 0 0 1 1 1
         │ │ └─┬─┘ └─┬─┘
         │ │   0     7
         │ 24-hour format
         A1M3

Alarm 1 Day/Date

The Alarm 1 Day/Date register can be used to match either the day of the month or the day of the week.

Bit 7 is the A1M4 mask bit. When it is 0, the day/date value is used for matching. When it is 1, the day/date value is ignored.

Bit 6 is the DY/DT bit. When it is 0, the value represents the day of the month (i.e., the date). When it is 1, the value represents the day of the week.

Note

The datasheet refers to the day of the month as the “date of the month”

For example, to match the 15th day of the month, DY/DT is 0 and the date value is 15.

Bit:     7 6 5 4 3 2 1 0
Value:   0 0 0 1 0 1 0 1
         │ │ └─┬─┘ └─┬─┘
         │ │   1     5
         │ Day of month
         A1M4

To match a particular weekday, DY/DT is set to 1 and bits 2-0 contain the weekday value.

For example, if you programmed the Day register with 1 = Sunday, 2 = Monday, and so on, Monday is represented by the value 2, and the alarm’s day value must match it.

Bit:     7 6 5 4 3 2 1 0
Value:   0 1 0 0 0 0 1 0
         │ │ └────┬────┘
         │ │      2
         │ Day of week
         A1M4

Alarm 2 Registers

Alarm 2 uses three registers from 0x0B to 0x0D. Unlike Alarm 1, it does not have a seconds register.

REGISTER BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Alarm 2 Minutes A2M2 Tens digit of minutes Ones digit of minutes
Alarm 2 Hours A2M3 Select
12/24 format		 PM/AM Flag in 12 hour format
or
20 Hour bit in 24 hour format		 Tens digit of hours Ones digit of hours
Alarm 2 Day/Date A2M4 Select Day of The Week (e.g: Sunday)
or
Day of Month (e.g: 31st)		 Tens digit of day of the month Ones digit of Day/Date

Alarm Matching

The A1Mx and A2Mx bits are mask bits. When a mask bit is 0, the corresponding field is used for matching. When it is 1, the field is ignored.

However, these bits cannot be combined arbitrarily. The DS3231 supports only specific combinations of mask bits. Configurations that are not listed in the datasheet can result in illogical operation.

For Alarm 1, the supported configurations are:

Note

Here, X means “don’t care”. The value of that bit can be either 0 or 1 because it does not affect the alarm matching for that configuration.

DY/DTA1M4A1M3A1M2A1M1Alarm Rate
X1111Once per second (nothing is required to match)
X1110When seconds match
X1100When minutes and seconds match
X1000When hours, minutes, and seconds match
00000When date, hours, minutes, and seconds match
10000When day, hours, minutes, and seconds match

For example, an Alarm 1 for every day at 07:30 needs the hour, minute, and second fields to match. We can set the hour to 07, the minute to 30, and the second to 00, with A1M1, A1M2, and A1M3 set to 0 and A1M4 set to 1.

Alarm 2 has similar restrictions, but it does not have a seconds field:

DY/DTA2M4A2M3A2M2Alarm Rate
X111Once per minute (at 00 seconds)
X110When minutes match
X100When hours and minutes match
0000When date, hours, and minutes match
1000When day, hours, and minutes match

Alarm Behavior

When the RTC register values match the configured alarm values, the corresponding alarm flag, A1F or A2F, is set to 1.

If the corresponding alarm interrupt enable bit, A1IE or A2IE, is also set to 1, and the INTCN bit is set to 1, the alarm condition activates the INT/SQW signal.

When an alarm triggers the INT/SQW output, the signal itself does not indicate which alarm triggered it because either alarm can activate the output. We need to check the A1F and A2F flags to determine which alarm was triggered.

Note

The alarm flags A1F and A2F are in the Control/Status register (0x0F), while the alarm interrupt enable bits A1IE and A2IE are in the Control register (0x0E). Refer to the Control Register section for more details.

The A1F and A2F flags remain set after the alarm is triggered. To clear an alarm flag, we write 0 to the corresponding flag. Writing 1 does not change the flag.

The alarm match is checked once every second when the RTC updates the time and date registers.