Dynamic Oscillator Inaccuracy

Zynq UltraScale+ Device Technical Reference Manual (UG1085)

Document ID
UG1085
Release Date
2022-09-15
Revision
2.3 English

The frequency characteristic of a crystal depends on the type of crystal. The frequency is normally specified by a parabolic curve centered around 25 °C. A common parabolic coefficient for a 32.768 kHz tuning fork crystal is –0.04 ppm/°C. Therefore, the crystal frequency can be represented as shown in This Equation.

Equation 7-2      f = f0[1 – (0.04 x 10-6) x (T–T0)2]

For example, a clock built using a regular 32.768 kHz crystal that keeps time at room temperature loses two minutes per year at 10°C above or below room temperature and loses eight minutes per year at 20°C above or below room temperature.

The impact of temperature on the crystal oscillator can be analyzed and tabulated in advance. The example in Table: Impact of Temperature on a Crystal Oscillator analyzes how much the crystal frequency changes with every 10°C of temperature change, and shows the change in the value to program in the calibration and fractional calibration registers. If the system has a mechanism to read the ambient temperature of the crystal, it could access this table and calibrate the RTC accordingly.

Table 7-1:      Impact of Temperature on a Crystal Oscillator

Temperature (°C)

Frequency
(Hz)

Change
(PPM)

Change in Fractional Calibration

Change in Calibration

85

32,763.3

–144

5

–5

75

32,764.7

–100

12

–4

65

32,765.9

–64

14

–3

55

32,766.8

–36

13

–2

45

32,767.5

–16

8

–1

35

32,767.9

–4

14

–1

25

32,768.0

0

0

0

15

32,767.9

–4

14

–1