MC68HC908AT32 Datasheet, PDF (101/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

8.4.5 Oscillator Enable Signal (SIMOSCEN)
The SIMOSCEN signal enables the oscillator and PLL.
CGM Registers
8.4.6 Crystal Output Frequency Signal (CGMXCLK)
CGMXCLK is the crystal oscillator output signal. It runs at the full speed of the crystal (f_XCLK) and comes
directly from the crystal oscillator circuit. Figure 8-3 shows only the logical relation of CGMXCLK to OSC1
and OSC2 and may not represent the actual circuitry. The duty cycle of CGMXCLK is unknown and may
depend on the crystal and other external factors. Also, the frequency and amplitude of CGMXCLK can be
unstable at startup.
8.4.7 CGM Base Clock Output (CGMOUT)
CGMOUT is the clock output of the CGM. This signal is used to generate the MCU clocks. CGMOUT is
a 50 percent duty cycle clock running at twice the bus frequency. CGMOUT is software programmable to
be either the oscillator output, CGMXCLK, divided by two or the VCO clock, CGMVCLK, divided by two.
8.4.8 CGM CPU Interrupt (CGMINT)
CGMINT is the CPU interrupt signal generated by the PLL lock detector.
8.5 CGM Registers
Three registers control and monitor operation of the CGM:
• PLL control register (PCTL)
• PLL bandwidth control register (PBWC)
• PLL programming register (PPG)
8.5.1 PLL Control Register
The PLL control register contains the interrupt enable and flag bits, the on/off switch, and the base clock
selector bit.
Address: $001C
Bit 7
6
5
4
3
2
1
Bit 0
Read:
PLLF
1
1
1
1
PLLIE
PLLON
BCS
Write:
Reset: 0
0
1
0
1
1
1
1
= Unimplemented
Figure 8-4. PLL Control Register (PCTL)
PLLIE — PLL Interrupt Enable Bit
This read/write bit enables the PLL to generate a CPU interrupt request when the LOCK bit toggles,
setting the PLL flag, PLLF. When the AUTO bit in the PLL bandwidth control register (PBWC) is clear,
PLLIE cannot be written and reads as logic 0. Reset clears the PLLIE bit.
1 = PLL CPU interrupt requests enabled
0 = PLL CPU interrupt requests disabled
MC68HC908AT32 Data Sheet, Rev. 3.1
Freescale Semiconductor
101