ADE5166 datasheet(119/148 Pages) AD

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ADE5166/ADE5169

Rev. 0 | Page 119 of 148

RTC CRYSTAL COMPENSATION

calibration window, the RTC is clocked 125 times 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[1:0]

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.

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 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.

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

16000

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 at 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 (Bit 6) in the RTC calibration configuration

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.

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[1:0] = 00 and 512 Hz with FSEL[1:0] = 01 in the RTC

calibration configuration register (RTC_CAL, Address 0x0F).

RTCCOMP = 5000 × (% Error)

A shorter window of 0.244 sec is offered for fast calibration during

PSM0 or PSM1mode . Two output frequencies are offered for this

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

16 kHz with FSEL[1:0] = 11 in the RTC calibration configuration

RTCCOMP =

× (sec/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.

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