ADE7166 datasheet(120/144 Pages) AD

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ADE7566/ADE7569/ADE7166/ADE7169

Rev. A | Page 120 of 144

Take care when changing the interval timer time base. The

recommended procedure is as follows:

If the Alarm Interval SFR (INTVAL, 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, 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[1:0]) 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 when the alarm event occurs.

RTC CALIBRATION

The RTC provides registers to calibrate the nominal external

crystal frequency and its variation over temperature. A frequency

error up to ±248 ppm can be calibrated out by the RTC circuitry,

which adds or subtracts pulses from the external crystal signal.

The nominal crystal frequency should be calibrated with the

RTC nominal compensation register so that the clock going into

the RTC is precisely 32.768 kHz at 25°C. The RTC Temperature

Compensation SFR (TEMPCAL, 0xF7) is used to compensate

for the external crystal drift over temperature by adding or

subtracting additional pulses based on temperature.

The LSB of each RTC compensation register represents a

±2 ppm frequency error. The RTC compensation circuitry adds

the RTC Temperature Compensation SFR (TEMPCAL, 0xF7)

and the RTC Nominal Compensation SFR (RTCCOMP, 0xF6)

to determine how much compensation is required. Note that

the sum of these two registers is limited to ±248 ppm.

Calibration Flow

A 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, 0xFF).

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

second window in all operational modes: PSM0, PSM1, and

PSM2. Two output frequencies are offered for the normal RTC

mode: 1 Hz with FSEL[1:0] = 00 and 512 Hz with FSEL[1:0] =

01 in the Interrupt Pins Configuration SFR (INTPR, 0xFF).

A shorter window of 0.244 seconds is offered for fast calibration

during PSM0 or PSM1. Two output frequencies are offered for

this RTC calibration output mode: 500 Hz with FSEL[1:0] = 10

and 16.384 kHz with FSEL[1:0] = 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 130. RTC Calibration Options

Option

FSEL[1:0]

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

16,384

When no RTC compensation is applied, that is, when RTC

Nominal Compensation SFR (RTCCOMP, 0xF6) and RTC

Temperature Compensation SFR (TEMPCAL, 0xF7) are equal

to 0, 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 enough.

The value to write to the RTC Nominal Compensation

SFR (RTCCOMP, 0xF6) 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 second/day error is equal to 11.57 ppm.

)

5000

Error

RTCCOMP

)

(

Error

sec/day

RTCCOMP

During calibration, user software writes the RTC with the

current time. Refer to the Read and Write Operations section

for more information on how to read and write the RTC

timekeeping registers.

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