• Connect the output of the inverter to the 8QM/8QXP RTC_XTALI ball.
• Connect the power pin of the inverter to the 1.8 V always on system rail and the inverter ground pin to the system ground.
3.9.3 Internal load capacitor trimming (24 MHz and 32.768 kHz)
See the appropriate i.MX8 datasheet for guidelines on selecting the 24 MHz and 32.768 kHz crystals.
Choosing the appropriate crystal element for an oscillator is important. NXP recommends starting the selection process by looking
at the parts chosen for NXP’s MEK reference designs. Whilst many different crystals can work to create a successful oscillator,
NXP has the most experience with the crystal model on the MEK.
Start-up time for the 24 MHz oscillator is monitored by the ROM code. It must be stable within 5 milliseconds of POR_B
de-asserting, or the boot process will fail. If a crystal start-up violates this limit, a reset will occur. Longer start-up times are
associated with crystals with higher series resistance. It is importance to select a crystal with an ESR of less than 60 Ω. Higher
load capacitances will also result in longer start-up times. It is important to consider parasitic capacitances present in the PCB and
device pin when selecting the desired internal load capacitor. However, a correct operating frequency is also dependent on load
capacitance. Choosing too small a value for start-up reasons may result in an incorrect operating frequency.
Selecting the correct crystal load capacitance (C
L
) is an important part of any oscillator design, because it impacts the clock
performance, stability, and start-up time. Typically, higher C
L
values should be used for higher clock performance. However, as
many factors (including board layout) can impact performance, NXP recommends characterization testing to be carried out with
the selected crystal vendor.
i.MX8 QM and i.MX8 QXP provide programmable internal load capacitors for both the 24 MHz and 32.768 kHz oscillator designs.
Although external load capacitors can be used, NXP has designed this module to use internal capacitors, because they are less
impacted by external factors, whilst providing an overall cost saving.
Default internal C
L
values are provided for both the 24 MHz (20 pF) and 32.768 kHz (16 pF) oscillators to enable the initial boot and
testing. If it is determined that the default values are not appropriate after the crystal characterization is completed, fuse settings
(Fuse Row Index 768) can be used to trim the internal C
L
value.
The default values were selected specifically for the MEK reference designs and should prove good enough for other designs
in general.
Note: The impact of stray capacitance C
stray
1)
, typically 16 pF
2)
, should be included when programming trimmed values, for
example C
L
= (C
L1
× C
L2
)/ (C
L1
+ C
L2
) + C
stray
/2. Then the needed trim value can be estimated
3)
as C
trim
= 2 × C
L
- C
stray
. For the
24 MHz crystal, usage of the internal capacitors is strongly recommended to reduce the cost and simplify the PCB layout. A 24
MHz crystal with C
L
equal to or larger than 12 pF is recommended.
The following is an example of calculation for a crystal with C
L
= 12 pF:
C
trim
= 2 × C
L
– C
stray
= 2 × 12 – 16 = 8 pF
1)
C
stray
is understood as the cumulative effect of the typical PCB parasitic capacitance and the on-chip and package capacitances.
2)
The value is per-pin and applicable only for the pins of the 24 MHz crystal.
3)
Characterization must be performed to confirm the correct trim setting.
Table 28. Fuse row index 768
Bits Description
31 BRD_OSC_24M_TRIM_VALID
0 – use 20 pF default value
1 – use TRIM_VALUE_24M
30 Reserved (MUST BE 0)
Table continues on the next page...
NXP Semiconductors
i.MX8 layout/routing recommendations
i.MX8 QM / i.MX8 QXP Hardware Developer’s Guide, Rev. 2.4p, 06/2021
User's Guide 54 / 89
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