AN-1058: Using CBC348xx EnerChip RTC in High Accuracy Applications

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Doc AN-72-1058 Rev B

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In applications where high rates of temperature change rates are expected, the system could be operated in RC

Autocalibrate mode for the main part of the year to save power and switch to Crystal (XT) mode when a large or

fast temperature excursion is detected by the system microcontroller.

Initial Timing Calibration Algorithm

To initially calibrate out any crystal frequency inaccuracies and any parasitic load inaccuracies, the following

procedure should be run using a timing measurement device that can resolve better than one ppm in less than

a half-second. This device could be a frequency counter with a good calibration, a custom circuit built on the

device-under-test (DUT) board of the system tester that includes an accurate oscillator of 10MHz or better and

a counter that can count how many tenths of microseconds between the edges of the clock output from the

CBC348xx, or possibly the system tester itself if it can be calibrated to resolve tenths of microseconds between

edges of the output clock. The procedure is as follows:

1. Set the OFFSETX, CMDX, and XTCAL register fields to 0 to make sure the oscillator is running

without any calibration taking place. The CMDX and OFFSETX fields are both in the CAL_XT register.

The XTCAL bits are in the Osc Control register.

2. Select the XT oscillator by setting the OSEL bit of the Osc register to 0.

3. Configure a square wave output on one of the output pins of frequency Fnom (for example 16Hz).

See register SQW in the datasheet for information on how to do this.

4. Measure the pulse width of several cycles of the square wave with a resolution of better than

0.25ppm (resolve edges to 15ns or better using averaging if needed) and convert to a frequency

Fmeas by taking the reciprocal of the pulse width.

5. Compute the pulse adjustment value (PADJ) required in ppm as ((32,768 - Fmeas)*1000000) /

32,768 = PADJ.

6. Compute the adjustment value in steps as PADJ / (1000000 / 2^19) = Padj / (1.90735) = ADJ.

7. If ADJ < -320 the XT frequency is too high to be calibrated. Contact Cymbet for design assistance.

8. Compensate by starting at the top of this table and proceeding down until the pertinent condition is

met and set the fields XTCAL, CMDX, and OFFSETX as indicated:

If ADJ is as below

set XTCAL =

set CMDX =

set OFFSETX =

ADJ < -256

3

1

(ADJ + 192)/2

-256 < ADJ < -192

3

0

(ADJ + 192)

-192 < ADJ < -128

2

0

(ADJ + 128)

-128 < ADJ < -64

1

0

(ADJ + 64)

-64 < ADJ < 64

0

0

(ADJ)

64 < ADJ < 128

0

1

(ADJ)/2

Otherwise, XT is too low to calibrate. Contact Cymbet.

The calibration register values can be saved in nonvolatile memory in the final system’s microprocessor for

reloading when system power is recovered.

Suggested Crystals for CBC348xx devices

The CBC348xx should operate well with any standard 32.768kHz tuning fork crystal with 0-12pF capacitance

and 0-90KΩ ESR. Some crystals that can be used with the CBC348xx are:

Microcrystal

CC7V-T1A or CM7V-T1A

Cardinal

CPSZ-A2 C1 70 -32.768 D9

Epson

C-002RX, FC-135, FC-12D, or FC-12M