i.MX 6ULL Applications Processors for Industrial Products, Rev. 1.2, 11/2017
18 NXP Semiconductors
Modules List
3.1 Special Signal Considerations
Table 3 lists special signal considerations for the i.MX 6ULL processors. The signal names are listed in
alphabetical order.
The package contact assignments can be found in Section 6, “Package Information and Contact
Assignments".” Signal descriptions are provided in the i.MX 6ULL Reference Manual (IMX6ULLRM).
Table 3. Special Signal Considerations
Signal Name Remarks
CCM_CLK1_P/
CCM_CLK1_N
One general purpose differential high speed clock Input/output is provided.
It can be used:
• To feed external reference clock to the PLLs and further to the modules inside SoC.
• To output internal SoC clock to be used outside the SoC as either reference clock or as a
functional clock for peripherals.
See the i.MX 6ULL Reference Manual (IMX6ULLRM) for details on the respective clock trees.
Alternatively one may use single ended signal to drive CLK1_P input. In this case corresponding
CLK1_N input should be tied to the constant voltage level equal 1/2 of the input signal swing.
Termination should be provided in case of high frequency signals.
After initialization, the CLK1 input/output can be disabled (if not used). If unused, either or both of
the CLK1_N/P pairs may remain unconnected.
RTC_XTALI/RTC_XTALO If the user wishes to configure RTC_XTALI and RTC_XTALO as an RTC oscillator, a 32.768 kHz
crystal, (100 k ESR, 10 pF load) should be connected between RTC_XTALI and RTC_XTALO.
Keep in mind the capacitors implemented on either side of the crystal are about twice the crystal
load capacitor. To hit the exact oscillation frequency, the board capacitors need to be reduced to
account for board and chip parasitics. The integrated oscillation amplifier is self biasing, but
relatively weak. Care must be taken to limit parasitic leakage from RTC_XTALI and RTC_XTALO
to either power or ground (>100 M). This will debias the amplifier and cause a reduction of startup
margin. Typically RTC_XTALI and RTC_XTALO should bias to approximately 0.5 V.
If it is desired to feed an external low frequency clock into RTC_XTALI the RTC_XTALO pin should
be remain unconnected or driven with a complimentary signal. The logic level of this forcing clock
should not exceed VDD_SNVS_CAP level and the frequency should be <100 kHz under typical
conditions.
In case when high accuracy real time clock are not required, system may use internal low
frequency ring oscillator. It is recommended to connect RTC_XTALI to GND and keep RTC_XTALO
unconnected.
XTALI/XTALO A 24.0 MHz crystal should be connected between XTALI and XTALO.
The crystal must be rated for a maximum drive level of 250 W. An ESR (equivalent series
resistance) of typical 80 is recommended. NXP BSP (board support package) software requires
24 MHz on XTALI/XTALO.
The crystal can be eliminated if an external 24 MHz oscillator is available in the system. In this
case, XTALO must be directly driven by the external oscillator and XTALI is disconnected.
If this clock is used as a reference for USB, then there are strict frequency tolerance and jitter
requirements. See OSC24M chapter and relevant interface specifications chapters for details.
DRAM_VREF When using DDR_VREF with DDR I/O, the nominal reference voltage must be half of the
NVCC_DRAM supply. The user must tie DDR_VREF to a precision external resistor divider. Use a
1k 0.5% resistor to GND and a 1 k 0.5% resistor to NVCC_DRAM. Shunt each resistor with a
closely-mounted 0.1 µF capacitor.
To reduce supply current, a pair of 1.5 k 0.1% resistors can be used. Using resistors with
recommended tolerances ensures the ± 2% DDR_VREF tolerance (per the DDR3 specification) is
maintained when two DDR3 ICs plus the i.MX 6ULL are drawing current on the resistor divider.