ADE5166 datasheet(119/148 Pages) AD

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Preliminary Technical Data

ADE5166/ADE5169/ADE5566/ADE5569

Rev. PrB | Page 119 of 148

RTC CRYSTAL COMPENSATION

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 compensation register is ±2 ppm/LSB,

or 0.17 s/day/LSB. 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 and the sum of

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

RTC Calibration

The nominal crystal frequency can be calibrated by adjusting

the RTCCOMP register 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, 0x0F). Enable the RTC output by setting the

CAL_EN bit in the RTC calibration configuration register

(RTC_CAL, 0x0F).

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

calibration configuration register (RTC_CAL, 0x0F).

A shorter window of 0.244 sec 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 RTC calibration

configuration register (RTC_CAL, 0x0F). Note that for the

0.244 sec 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 132. 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

16384

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.

The value to write to the RTCCOMP register 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

where1 s/day error is equal to 11.57 ppm.

)

5000

Error

RTCCOMP

)

(

Error

day

RTCCOMP

During calibration, user software writes the RTC with the

current time. Refer to the Access to Internal RTC Registers

section for more information on how to read and write the RTC

timekeeping registers.

RTC TEMPERATURE COMPENSATION

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 tempera-

ture. By design, crystal frequency’s changes with temperature

according to Equation 35.

( )

( ) =

−

frequency

XTAL

frequency

XTAL

(

−

(35)

where k and T0 are given by the crystal manufacturer.

The ADE5166/ADE5169/ADE5566/ADE5569 provide the

unique feature of using its internal temperature measurement to

process the TEMPCAL register value based on the theoretical

inverted parabolic curve described in Equation 35. The coefficient

k and T0 are user programmable.

The crystal manufacturer coefficients need to be translated

to fit the format of crystal offset (XTALOS) and crystal gain

(XTALGain), which is a combination of the XTALG1 and

XTALG2 registers of the ADE5166/ADE5169/ADE5566/

ADE5569. The actual implementation of Equation 35 in the

ADE5166/ADE5169/ADE5566/ADE5569 is as follows:

[]

XTALGain

TEMPCAL

[]

(

XTALOS

TEMPADC

−

where XTALGain = XTALG2[6:0] × 28 + XTALG1[7:0]

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