ST AN1216 Application note

AN1216

APPLICATION NOTE

Implementing a Periodic Alarm with TIMEKEEPER

The TIMEKEEPER M48T59 and M48T59Y, from STMicroele ctronics, each provide an 8K x 8 bit non
volatile static RAM, and an integrated real time clock. Each also provides an alarm which can be set either
for a given time and day, or to repeat at a certain day in every month, or at a certain hour in every day, or
at a certain minute of every hour, or at a certain second of every minute. With this functionality already
provided in the hardware, the software to implement an alarm of any given period is greatly simplified, as
described in this document.

Although specifically tailored for the M48T59 (or M48T59Y) device, the ideas can be adapted easily to use

any of ST’s other TIMEKEEPER devices that have the alarm feature. Some modifications in the MCU
memory mapping (the TIMEKEEPER address space) and in the MCU register mapp ing (such as the
pointer to register address) would need to be made.

Table 1. Typical TIMEKEEPER (M48T59) Register M ap

Address

Data

Function

D7 D6 D5 D4 D3 D2 D1 D0

Range
(in BCD
Format)

1FFFh 10 Years Year Year 00-99
1FFEh 0 0 0 10M Month Month 01-12
1FFDh 0 0 10 Date Date Date 01-31
1FFCh 0 FT 0 0 0 Day Day 01-7
1FFBh 0 0 10 Hours Hours Hour 00-23
1FFAh 0 10 Minutes 10 Minutes Minute 00-59
1FF9h ST 10 Seconds Seconds Second 00-59
1FF8h W R S Calibration Control
1FF7h WDS BMB4 BMB3 BMB2 BMB1 BMB0 RB1 RB0 Watch
1FF6hAFE0ABE00000Interrupt
1FF5h RPT4 0 AI 10 Date Alarm Date A Date 01-31
1FF4h RPT3 0 AI 10 Hour Alarm Hour A Hour 00-23
1FF3h RPT2 Alarm 10 Minutes Alarm Minutes A Minute 00-59
1FF2h RPT1 Alarm 10 Seconds Alarm Seconds A Second 00-59
1FF1h unused unused unused
1FF0hWDFAF0BLZZZZFlags

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AN1216 - APPLICATION NOTE

TIMEKEEPER CONFIGURATION

The TIMEKEEPER register mapping is shown in Table 1. This is divided in two parts: the clock registers
and the alarm registers.

Clock Registers

The clock registers can be configured in the C language computer program as follows:

*TIMEKEEPER_CAL |= 0x80
*TIMEKEEPER_SEC= 00 //user clock setting : seconds parameter
*TIMEKEEPER_MIN= 00 //user clock setting : minutes parameter
*TIMEKEEPER_HOUR= 00 //user clock setting : hours parameter
*TIMEKEEPER_CAL&= 0x7F

The process for starting the clock and making the calibration adjustments are described in the
data sheet, and in application notes

AN925

and

AN934

.

Alarm Registers

It is necessary to set the Write bit, W, at the top of the control register (at address offset 1FF8h) before
proceeding to any c lock modification. Modi fications to the alarm regist ers, though, can be made at any
time, with no prior changes to the control register being necessary.

The program listing, at the end of this document, contains statements to perform the following functions:

1. The Stop bit (ST, bit 7 of the register at offset 1FF9h) has to be reset to start the TIMEKEEPER oscil­lator

*TIMEKEEPER_SEC &= 0x7F; // reset bit D7 using a mask 0x7F

2. The Alarm Flag Enable (AFE) bit (bit 7 of the register at offset 1FF6h) is set, thereby allowing the IRQ
pin (pin 26) to output the interrupt signal (active low).

*TKPER_AL_IT |= 0x80; // set bit D7 using a mask 0x80

3. The flag register (a t offset 1F F0h) mus t be read at the beginning of the alarm updat ing rou tine. If not,
the AF flag will never be released, and the TIMEKEEPER will continuously output an inte rrupt to the
MCU, and the system will become jammed.

M48T59

SOFTWARE CONFIGURATION

The program is listed on page 4. To unders tand its operation, it is important to distinguish between t he
three pointer variables, pointing to physical addresses in the hardware:

*TKPER_AL_HOUR, *TKPER_AL_MIN, *TKPER_AL_SEC

and the three integer variables, used as work-space by the software:

alarm_hour, alarm_minute, alarm_second

The first three variables are poin ters to the phy sical a ddress of the values that are stored in the M48T59
memory device.

The three software variables are used to hold the user’s data (they specify the period of the alarm in hours,

minutes and seconds). T his is not the same information as is stored in the TIMEKEEPER registers, as
pointed to by the pointer variables, but is used in their calculation.

The program does make use of the four Repeat bits (RPT4, RPT3, RPT2 and RPT1) that are physically
located i n the TIM EKEEPER d evice. These should a ll be se t, exc ept fo r those corre sponding to fields that
contain significant data. For instance, to set an alarm that repe ats every 3 minutes and 45 s econds, the

alarm_minutes

and

alarm_seconds

variables would be loa ded with these two values. Then a ppropriate
values would be calculated for load ing in the “Alarm Minutes” and “Alarm Seconds” fields of the alarm
registers (at addresses 1 FF3h a nd 1FF2h, *TKPER_AL_MIN and *TKPER_AL_SEC, respectively), and
their Repeat bits (RPT2 and RPT1, respectively) would be reset to ‘0’. Meanwhile, the

alarm_hour

variable, and the “Alarm D ate” and “Alarm H our” fields of the alarm reg isters (at address es 1FF5h and
1FF4h, *TKPER_AL_DATE and *TKPER_AL_HOUR, respectively) would be treated as “Don’t Care”, as

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AN1216 - APPLICATION NOTE

indicated by their Repea t bits (RPT4 and RPT3, respectively) being set. This is s ummarized i n Table 2,
with the three local integer variables,
period.

Table 2. Bit Setting to Control the Period of the Repeated Alarm

RPT4 RPT3 RPT2 RPT1 Periodic alarm activated every

11111 second
1 1 1 0 alarm_second seconds (less than 1 minute)
1 1 0 0 alarm_minute minutes alarm_second seconds (less than 1 hour)
1 0 0 0 alarm_hour hours alarm_minute minutes alarm_second seconds (less than 1 day)

For example, to set a period of 1 hour 49 minute 35 seconds, the procedure is as follows:

RPT4 = 1
RPT3 = RPT2 = RPT1 = 0
alarm_second = 0x35
alarm_minute = 0x49
alarm_hour = 0x01

Or, to set a period of 49 minute 35 seconds, the procedure is as follows:

RPT4 = RPT3 = 1
RPT2 = RPT1 = 0
alarm_second = 0x35
alarm_minute = 0x49
alarm_hour = DontCare

alarm_second, alarm_minute

and

alarm_hour

, used to represent the

SOFTWARE IMPLEMENTATION
TIMEKEEPER Data Format

TIMEKEEPER data is held as BCD (binary coded decimal). This is handled in the C programming

language using th e ‘ uns igne d char’ data type. Th is can be converted within the C prog ra m to other data
types, such as ‘integer’, for numeric processing. Two functions are prov ide d in the program at the end of
this document for making this conversion.

– Char_To_Int: to take a BCD parameter, and to return the equivalent integer value
– Int_To_Char: to take an integer parameter, and to return the equivalent BCD value.
The valid ranges for the alarm fields are summarized in Table 3.

Tabl e 3. TIMEKEEPER Data Format

Data C language type

Alarm second 0-59 00-3B 0-59
Alarm minute 0-59 00-3B 0-59
Alarm hour 0-12 00-0B 0-12

Int (integer)

Decimal

Char (character)

Hexadecimal

Char (character)

BCD

Alarm Update Management

When the alarm signal is g enerated by the TIME KEEPER device, it is communicated to the MCU. The
MCU can monit or for th is event either by po lling, or b y usi ng interr upts. T here ar e two v arian ts of eac h
method:

■ Polling

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