Propeller Manual

1: Introducing the Propeller Chip
Propeller Manual v1.1 · Page 29
Table 1-7: CLK Register PLLENA (Bit 6)
Bit Effect
0
Disables the PLL circuit. The
RCFAST and RCSLOW settings of the _CLKMODE declaration
configure PLLENA this way.
1
Enables the PLL circuit. Each of the PLLxx settings of the _CLKMODE declaration
configures PLLENA this way at compile time. The Clock PLL internally multiplies the
XIN pin frequency by 16. OSCENA must also be ‘1’ to propagate the XIN signal to the
Clock PLL. The Clock PLL's internal frequency must be kept within 64 MHz to 128 MHz
– this translates to an XIN frequency range of 4 MHz to 8 MHz. Allow 100 µs for the
Clock PLL to stabilize before switching to one of its outputs via the CLKSELx bits.
Once the Crystal Oscillator and Clock PLL circuits are enabled and stabilized, you can
switch freely among all clock sources by changing the CLKSELx bits.
Table 1-8: CLK Register OSCENA (Bit 5)
Bit Effect
0
Disables the Crystal Oscillator circuit. The
RCFAST and T RCSLOW settings of the
_CLKMODE declaration configure OSCENA this way.
1
Enables the Crystal Oscillator circuit so that a clock signal can be input to XIN, or so
that XIN and XOUT can function together as a feedback oscillator. The XINPUT and
XTALx settings of the _CLKMODE declaration configure OSCENA this way. The OSCMx
bits select the operating mode of the Crystal Oscillator circuit. Note that no external
resistors or capacitors are required for crystals and resonators. Allow a crystal or
resonator 10 ms to stabilize before switching to a Crystal Oscillator or Clock PLL output
via the CLKSELx bits. When enabling the Crystal Oscillator circuit, the Clock PLL may
be enabled at the same time so that they can share the stabilization period.
Table 1-9: CLK Register OSCMx (Bits 4:3)
OSCMx
1 0
_CLKMODE
Setting
XOUT
Resistance
XIN/XOUT
Capacitance
Frequency Range
0 0
XINPUT
Infinite 6 pF (pad only) DC to 80 MHz Input
0 1
XTAL1
2000  36 pF 4 to 16 MHz Crystal/Resonator
1 0
XTAL2
1000  26 pF 8 to 32 MHz Crystal/Resonator
1 1
XTAL3
500  16 pF 20 to 60 MHz Crystal/Resonator