Figure 5-2. oscillator startup flow -13, 2 external clock (crystal/resonator) – Maxim Integrated MAXQ7666 User Manual
Page 210
Figure 5-2. Oscillator Startup Flow
YES
YES
POWER-ON RESET
START UP 7.6MHz
INTERNAL RC OSCILLATOR
8.6ms POWER-UP COUNTER DELAY
RESET 16-BIT
POWER-UP COUNTER
INTERNAL RC-BASED
CODE EXECUTION
INTERNAL 7.6MHz RC OSCILLATOR STARTUP FLOW
A) POWER-UP COUNTER
= 65,535?
B) DVDD > DEFAULT POR
THRESHOLD LEVEL
(2.7V–2.99V)
DVDD > ˜1.2V
NO
NO
In all other cases, setting XT = 0 switches the system clock source to the internal RC immediately. The RGMD (CKCN.5) bit indicates
the current system clock source. If the 7.6MHz RC oscillator is providing the system clock, RGMD equals 1; otherwise, RGMD equals
0. When used as the system clock, the 7.6MHz RC oscillator clock is divided down according to the PMME, CD1, CD0 bit selections
(CKCN.2:CKCN.0) just the same as the external crystal/clock possibilities. The WDT alone is always clocked by the undivided 7.6MHz
RC oscillator clock.
When the system clock source is switched from the 7.6MHz RC oscillator to the high-frequency external crystal/clock 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-frequen-
cy oscillator. This is reflected by the value of the RGMD bit, which remains at 1 until the warmup for the high-frequency oscillator is
complete and the clock switches over, at which point RGMD switches to 0.
Note that in order to use the 7.6MHz RC oscillator, the RCE bit in the OSCC register must be set to logic 1, which enables the 7.6MHz
RC oscillator. The 7.6MHz RC oscillator is enabled (RCE = 1, XT = 0) by default after a power-on reset.
5.3.2 External Clock (Crystal/Resonator)
An external quartz crystal or a ceramic resonator can be connected from XIN to XOUT as the device determining the frequency of
operation. The MAXQ7665/MAXQ7666 are designed to operate at a maximum frequency of 8.12MHz. For details of the high-frequen-
cy crystal oscillator specification, refer to the MAXQ7665/MAXQ7666 data sheet.
MAXQ7665/MAXQ7666 User’s Guide
5-13
Maxim Integrated