Intel PXA26X User Manual download pdf (Page 145)

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Intel® PXA26x Processor Family Developer’s Manual 4-37

System Integration Unit

4.3.3.2.2 Trim Example #2 – Measured Value Has a Fractional Component

This example is more common in that the measured frequency of the oscillator has a fractional

component. Again, the desired Hz-clock-output frequency is 1 Hz. If the oscillator output is

measured as 32768.92 cycles/s (Hz), an integer trim is necessary so that the average number of

cycles counted before generating one 1 Hz clock is 32768.92. Similar to the previous example, the

integer field RTTR[15:0] is loaded with the hexadecimal equivalent of 32768-1 or 0x0000_7FFF

(reset value).

Because the actual clock frequency is 0.92 cycles per second faster than the integer value, the Hz

clock generated by just the integer trimming is slightly faster than needed and must be slowed

down. Accordingly, program the fractional trim to delete 0.92 cycles per second on average to

bring the Hz-output frequency down to the proper value. Since the trimming procedure is

performed every 1023 (2

-1) seconds, the trim must be set to delete 941.16 clocks every 1023

seconds (.92 x 1023 = 941.16). Load the counter with the hexadecimal equivalent of 941, or

0x3AD. The fractional component of this value cannot be trimmed out and constitutes the error in

trimming, described below.

This trim setting leaves an error of. 16 cycles per 1023 seconds. The error calculation yields (in

parts-per-million or ppm):

4.3.3.2.3 Maximum Error Calculation Versus Real-Time Clock Accuracy

As seen from trim example #2, the maximum possible error approaches 1 clock per 2

-1 seconds.

Calculating the ppm error for this scenario yields:

To maintain an accuracy of +/- 5 seconds per month, the required accuracy is calculated to be:

This calculation indicates that the Hz-clock output can be made very accurate through the use of

the trim procedure. Likewise, use the trim procedure to compensate for a range of factors that can

affect crystal oscillators. Such factors can include, but are not limited to:

• Manufacturing and supplier variance in the crystals

• Crystal aging effects

• System voltage differences

• System manufacturing variance

Error

0.16 cycles

1023 sec

---------------------------X

1 sec

32768 cycles

------------------------------- 0.002 ppm==

Error (maximum)

1 cycle

1023 sec

---------------------

1 sec

32768 cycles

---------------------------------- 0 . 0 3 p p m==

Error

5 sec

month

---------------

1 month

2592000 sec

------------------------------ 1.9 ppm==

1 2 ... 140 141 142 143 144 145 146 147 148 149 150 ... 623 624

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