Maxim Integrated MAXQ7666 - Manual (Page 211)

MAXQ7665/MAXQ7666 User’s Guide
5-14
The crystal oscillator/resonator is disabled upon power-up, as the default mode for the MAXQ7665/MAXQ7666 is to run from the inter-
nal 7.6MHz RC oscillator. To use the external crystal/resonator, select the input (HFIC1:HFIC0) and output capacitance (HFOC1:HFOC0)
of the internal high-frequency oscillator (to match the external crystal/resonator load capacitance requirement) in the OSCC register.
Figure 5-3 shows the possible options. The HFE bit (OSCC.0) must be set to 1 to enable the internal high-frequency oscillator.
Note: For MAXQ7666, the HFIC1:HFIC0 bits select both the input and output capacitance. The HFOC1:HFOC0 bits are used to select
the crystal drive strength as explained in the oscillator control register subsection.
To select the crystal/resonator as the system clock source, the XT bit must be set to 1. When the system clock source is switched from
the 7.6MHz RC oscillator to the high-frequency external crystal/resonator by setting XT = 1, the 7.6MHz RC oscillator will still be used as
the system clock source until the warmup period has completed for the high-frequency oscillator. It is important that the 7.6MHz RC oscil-
lator is not disabled (RCE = 0) before the system clock is switched to external crystal/resonator. The RGMD bit may be monitored by
application software to determine when this switchover has actually occurred, as code execution continues during the warmup delay.
The switchover to the high-frequency oscillator requires a warmup delay of 4096 crystal/resonator clock cycles under the following
circumstances:
• When resuming execution from stop mode with HFE = 1 and XT = 1.
• When the high-frequency oscillator has been shutdown (HFE = 0) or has not been started since the last POR or stop mode exit.
Setting the HFE bit to 1 allows the high-frequency oscillator to keep running even when the internal RC oscillator has been selected as the
system clock. Once the high-frequency oscillator has been started and allowed to warm up for the first time (following a POR, exit from
stop mode, shutdown of the oscillator), application code can switch to the high-frequency oscillator with a warmup delay of only four clock
cycles instead of 4096. The XHFRY (ASR.11) bit can be monitored by application software to determine if the high-frequency oscillator
warmup is complete (XHFRY = 1). Figure 5-4 illustrates the steps to select external crystal/resonator as the system clock source.
When the high-frequency oscillator (using external crystal/resonator) is used as the system clock source, the clock is divided down
according to the PMME, CD1, CD0 bit selections (CKCN.2:CKCN.0).
5.3.2.1 High-Frequency Oscillator Application Configuration
The MAXQ7665/MAXQ7666 high-frequency oscillator is optimized for 8MHz operation. Figure 5-5 shows an example application con-
figuration using an external crystal, two low-end capacitors, and an optional series resistor to limit current through the crystal. Pins XIN
and XOUT connect the oscillator to the external crystal. By using the internal built-in capacitor (selected by HFIC1:HFIC0 and
HFOC1:HFOC0 bits), the user can save board space and avoid the external end capacitors. The user should review the crystal spec-
ifications for appropriate values of the external components. Additionally, the user should verify the oscillator performance across the
required DVDD and temperature range.
Note: For MAXQ7666, the HFIC1:HFIC0 bits select both the input and output capacitance. The HFOC1:HFOC0 bits are used to select
the crystal drive strength as explained in the oscillator control register subsection.
Figure 5-3. High-Frequency Crystal Oscillator Configuration
11pF
STOP
AGC
11pF
7pF
6pF
XTAL
9pF
9pF
7pF
7pF
FAIL
DETECT
HFCI1
HFFINT
HFCI0
HFCO1 HFCO0
XOUT
XIN
DV DD
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