Datasheet

ADuC836
–38–
ADuC836
–39–
ON-CHIP PLL
The ADuC836 is intended for use with a 32.768 kHz watch
crystal. A PLL locks onto a multiple (384) of this to provide a
stable 12.582912 MHz clock for the system. The core can
operate at this frequency, or at binary submultiples of it, to allow
power saving in cases where maximum core performance is not
required. The default core clock is the PLL clock divided by
8 or 1.572864 MHz. The ADC clocks are also derived from the
PLL clock, with the modulator rate being the same as the crystal
oscillator frequency. This choice of frequencies ensures that the
modulators and the core will be synchronous, regardless of the
core clock rate. The PLL control register is PLLCON.
Table XVII. PLLCON SFR Bit Designations
Bit Name Description
7 OSC_PD Oscillator Power-Down Bit.
Set by user to halt the 32 kHz oscillator in Power-Down mode.
Cleared by user to enable the 32 kHz oscillator in Power-Down mode.
This feature allows the TIC to continue counting even in Power-Down mode.
6 LOCK PLL Lock Bit. This is a read-only bit.
Set automatically at power-on to indicate the PLL loop is correctly tracking the crystal clock. After
power-down, this bit can be polled to wait for the PLL to lock.
Cleared automatically at power-on to indicate the PLL is not correctly tracking the crystal clock. This may
be due to the absence of a crystal clock or an external crystal at power-on. In this mode, the PLL output
can be 12.58 MHz ± 20%. After the ADuC836 wakes up from power-down, user code may poll this bit
to wait for the PLL to lock. If LOCK = 0, then the PLL is not locked.
5 ––– Reserved for Future Use. Should be written with 0.
4 LTEA Reading this bit returns the state of the external EA pin latched at reset or power-on.
3 FINT Fast Interrupt Response Bit.
Set by user enabling the response to any interrupt to be executed at the fastest core clock frequency,
regardless of the conguration of the CD2–0 bits (see below). After user code has returned from an interrupt,
the core resumes code execution at the core clock selected by the CD2–0 bits. Cleared by user to disable
the fast interrupt response feature.
2 CD2 CPU (Core Clock) Divider Bits.
1 CD1 This number determines the frequency at which the microcontroller core will operate.
0 CD0 CD2 CD1 CD0 Core Clock Frequency (MHz)
0 0 0 12.582912
0 0 1 6.291456
0 1 0 3.145728
0 1 1 1.572864 (Default Core Clock Frequency)
1 0 0 0.786432
1 0 1 0.393216
1 1 0 0.196608
1 1 1 0.098304
PLLCON PLL Control Register
SFR Address D7H
Power-On Default Value 03H
Bit Addressable No
REV. A
REV. A