ADE7166_15 datasheet(125/152 Pages) AD

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ADE7116/ADE7156/ADE7166/ADE7169/ADE7566/ADE7569

Rev. B | Page 125 of 1

Take care when changing the interval timer time base. The

recommended procedure is as follows:

If the alarm interval SFR (INTVAL, Address 0xA6) is going

to be modified, write to this register first. Then, wait for

one 128 Hz clock cycle to synchronize with the RTC,

64,000 cycles at a 4.096 MHz instruction cycle clock.

Disable the interval timer by clearing the ITEN bit in the

RTC configuration SFR (TIMECON, Address 0xA1).

Then, wait for one 128 Hz clock cycle to synchronize with

the RTC, 64,000 cycles at a 4.096 MHz instruction cycle

clock.

Read the TIMECON SFR to ensure that the ITEN bit is

clear. If it is not, wait for another 128 Hz clock cycle.

Set the time-base bits (ITS) in the TIMECON SFR to

configure the interval. Wait for a 128 Hz clock cycle for this

change to take effect.

The RTC alarm event wakes the 8052 MCU core if the MCU is

in PSM2 mode when the alarm event occurs.

RTC CALIBRATION

The RTC provides registers to compensate for the tolerance of

the crystal frequency and its variation over temperature.

Up to ±248 ppm frequency error can be calibrated with the RTC

circuitry. The compensation is fully digital and implemented by

adding or subtracting pulses from the crystal clock signal.

The resolution of the RTC nominal compensation SFR

(RTCCOMP, Address 0xF6) is ±2 ppm/LSB or 0.17 sec/day/LSB.

The RTC compensation circuitry adds the RTC temperature

compensation SFR (TEMPCAL, Address 0xF7) and the RTC

nominal compensation SFR (RTCCOMP, Address 0xF6) to

determine how much compensation is required. The sum of

these two registers is limited to ±248 ppm or 42.85 sec/day.

Calibrating the RTC

The nominal crystal frequency can be calibrated by adjusting

the RTCCOMP SFR so that the clock going into the RTC is

precisely 32.768 kHz at 25°C.

Calibration Flow

An RTC calibration pulse output is provided on the P0.2/CF1/

RTCCAL pin. Enable the RTC output by setting the RTCCAL

bit in the interrupt pins configuration SFR (INTPR,

Address 0xFF).

The RTC calibration is accurate to within ±2 ppm over a 30.5 sec

window in all operational modes: PSM0, PSM1, and PSM2. Two

output frequencies are offered for the normal RTC mode: 1 Hz

with FSEL = 00 and 512 Hz with FSEL = 01 in the interrupt pins

configuration SFR (INTPR, Address 0xFF).

A shorter window of 0.244 sec is offered for fast calibration

during PSM0 or PSM1 mode. Two output frequencies are

offered for this RTC calibration output mode: 500 Hz with

FSEL = 10 and 16 kHz with FSEL = 11 in the INTPR SFR.

Note that for the 0.244 second calibration window, the RTC is

clocked 125 times faster than in normal mode, resulting in

timekeeping registers that represent seconds/125, minutes/125,

and hours/125 instead of seconds, minutes, and hours.

Therefore, this mode should be used for calibration only.

Table 138. RTC Calibration Options

Option

FSEL

Calibration

Window (sec)

fRTCCAL

(Hz)

Normal Mode 0

30.5

Normal Mode 1

30.5

512

Calibration Mode 0

0.244

500

Calibration Mode 1

0.244

16,000

When no RTC compensation is applied, RTCCOMP and

TEMPCAL are equal to zero, the nominal compensation

required to account for the error in the external crystal can be

determined. In this case, it is not necessary to wait for an entire

calibration window to determine the error in the pulse output.

Calculating at the error in frequency between two consecutive

pulses on the P0.2/CF1/RTCCAL pin is sufficient.

The value to write to the RTCCOMP SFR is calculated from the

% error or seconds per day error on the frequency output. Each

LSB of the RTCCOMP SFR represents 2 ppm of correction

where 1 sec/day error is equal to 11.57 ppm.

)

5000

Error

RTCCOMP

)

(

Error

sec/day

RTCCOMP

During calibration, user software writes the current time. Refer

to the Read and Write Operations section for more information

on how to read and write to the RTC timekeeping registers.

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