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ADE5166/ADE5169/ADE5566/ADE5569

Rev. B | Page 126 of 156

RTC CRYSTAL COMPENSATION

faster than in the 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 Bits

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 out by the RTC

circuitry. The compensation is fully digital and implemented

by adding or subtracting pulses from the crystal clock signal.

Calibration

Window (sec)

fRTCCAL

(Hz)

Normal Mode 0

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 com-

pensation SFR (TEMPCAL, Address 0xF7) and the RTCCOMP

SFR to determine how much compensation is required. The sum

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

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, with RTCCOMP and

TEMPCAL equal to zero, the nominal compensation required

to account for the error in the external crystal can be deter-

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

calibration window to determine the error in the pulse output.

Calculating the error in frequency between two consecutive

pulses on the RTC calibration pin is sufficient.

RTC Calibration

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 on up to four pins configured

by the four LSBs in the RTC calibration configuration register

(RTC_CAL, Address 0x0F). Enable the RTC output by setting

the CAL_EN bit (RTC_CAL[6]).

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

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

bit of the RTCCOMP SFR represents 2 ppm of correction, where

1 sec/day error is equal to 11.57 ppm.

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 the FSEL bits = 00 and 512 Hz with the FSEL bits = 01

(RTC_CAL[5:4]).

RTCCOMP = 5000 × (% Error)

RTCCOMP =

× (second/day Error)

During calibration, user software writes the current time to the

RTC. Refer to the Access to Internal RTC Registers section for

more information on how to read and write to the RTC time-

keeping registers.

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 the FSEL bits = 10 and

16 kHz with the FSEL bits = 11 (RTC_CAL[5:4]). Note that for

the 0.244 sec calibration window, the RTC is clocked 125 times

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