NXP Semiconductors
IMXRT500HDG
i.MX RT500 Hardware Design Guide
There are many methods to calculate load capacitance, but none are accurate. At best,
they are approximations.
We recommend using a simple calculation to determine the initial values of the external
load capacitors. Then, after placing these values on the crystal, measure the frequency
with a CLKOUT pin. The load capacitors can be increased if the frequency is higher than
expected or decreased if the frequency is lower than expected.
Definitions:
•
C
L
- load capacitance of the crystal
•
C
x
, C
y
- value of the load capacitors on the crystal pins
•
C
Pin
- approximate pad capacitance of each crystal pin
•
C
Stray
- stray board capacitance
Load capacitance expression:
1. C
x
= C
y
≈ 2C
L
– C
Pin
– 2C
Stray
Or
2. C
x
= C
y
≈ 2(C
L
– C
Stray
) – C
Pin
Note:
It is an approximation.
Example 1:
Typical 32.768 kHz crystal:
C
L
= 12.5 pF
C
Pin
= 3 pF
C
Stray
= 0 (ignore for first pass calculation)
C
x
= C
y
≈ 2(12.5 pF – 0 pF) – 3 pF = 22 pF
Example 2:
Small 32.768 kHz crystal:
C
L
= 9 pF
C
Pin
= 3 pF
C
Stray
= 0 (ignore for first pass calculation)
C
x
= C
y
≈ 2 (9 pF – 0 pF) – 3 pF = 15 pF
Here are the definitions of the symbols used in the model and the simple calculation:
•
C
L
is the load capacitance of the crystal that you received from the vendor.
•
C
x
and C
y
are the physical load capacitors placed on the crystal pins.
•
C
Pin
is an approximate pad capacitance of each oscillator terminal, also called the
crystal pin of the MCU.
•
C
Stray
is the crystal industry term for the unknown board capacitance.
The load capacitance expression is derived from a standard crystal industry formula,
which involves C
L
and the unknown C
Stray
parameters.
Note:
The derived expression is an approximation. As a matter of fact, the industry
formula and other load capacitance calculations are also approximations, because
there are other dependencies that cannot be defined in simple models.
IMXRT500HDG
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User guide
Rev. 0 — 15 November 2022
19 / 48
