Clock Generator Module (CGM)

The lock detector compares the frequencies of the VCO feedback clock, CGMVDV, and the final reference clock, CGMRDV. Therefore, the speed of the lock detector is directly proportional to the final reference frequency, fRDV. The circuit determines the mode of the PLL and the lock condition based on this comparison.

4.3.2.2 Acquisition and Tracking Modes

The PLL filter is manually or automatically configurable into one of two operating modes:

1.Acquisition mode — In acquisition mode, the filter can make large frequency corrections to the VCO. This mode is used at PLL startup or when the PLL has suffered a severe noise hit and the VCO frequency is far off the desired frequency. When in acquisition mode, the ACQ bit is clear in the PLL bandwidth control register. See 4.5.2 PLL Bandwidth Control Register.

2.Tracking mode — In tracking mode, the filter makes only small corrections to the frequency of the VCO. PLL jitter is much lower in tracking mode, but the response to noise is also slower. The PLL enters tracking mode when the VCO frequency is nearly correct, such as when the PLL is selected as the base clock source. See 4.3.3 Base Clock Selector Circuit. The PLL is automatically in tracking mode when not in acquisition mode or when the ACQ bit is set.

4.3.2.3 Manual and Automatic PLL Bandwidth Modes

The PLL can change the bandwidth or operational mode of the loop filter manually or automatically.

In automatic bandwidth control mode (AUTO = 1), the lock detector automatically switches between acquisition and tracking modes. Automatic bandwidth control mode also is used to determine when the VCO clock, CGMVCLK, is safe to use as the source for the base clock, CGMOUT. See 4.5.2 PLL Bandwidth Control Register. If PLL interrupts are enabled, the software can wait for a PLL interrupt request and then check the LOCK bit. If interrupts are disabled, software can poll the LOCK bit continuously (during PLL startup, usually) or at periodic intervals. In either case, when the LOCK bit is set, the VCO clock is safe to use as the source for the base clock. See 4.3.3 Base Clock Selector Circuit. If the VCO is selected as the source for the base clock and the LOCK bit is clear, the PLL has suffered a severe noise hit and the software must take appropriate action, depending on the application. See 4.6 Interrupts for information and precautions on using interrupts.

These conditions apply when the PLL is in automatic bandwidth control mode:

•The ACQ bit (see 4.5.2 PLL Bandwidth Control Register) is a read-only indicator of the mode of the filter. For more information, see 4.3.2.2 Acquisition and Tracking Modes.

•The ACQ bit is set when the VCO frequency is within a certain tolerance, ∆TRK, and is cleared when the VCO frequency is out of a certain tolerance, ∆UNT. For more information, see 4.8 Acquisition/Lock Time Specifications.

•The LOCK bit is a read-only indicator of the locked state of the PLL.

•The LOCK bit is set when the VCO frequency is within a certain tolerance, ∆Lock, and is cleared when the VCO frequency is out of a certain tolerance, ∆UNL. For more information, see 4.8 Acquisition/Lock Time Specifications.

•CPU interrupts can occur if enabled (PLLIE = 1) when the PLL’s lock condition changes, toggling the LOCK bit. For more information, see 4.5.1 PLL Control Register.

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

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Freescale Semiconductor MC68HC908MR32 manual Acquisition and Tracking Modes, Manual and Automatic PLL Bandwidth Modes
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MC68HC908MR16, MC68HC908MR32 specifications

Freescale Semiconductor's MC68HC908MR32 and MC68HC908MR16 microcontrollers are part of the popular HC08 family, designed primarily for embedded applications. These microcontrollers are particularly favored in automotive, industrial, and consumer product sectors due to their reliability and versatility.

One of the standout features of the MC68HC908MR series is its CMOS technology, which enhances performance while minimizing power consumption. This makes these microcontrollers suitable for battery-operated devices. They operate at a maximum clock frequency of 2 MHz and offer a 16-bit architecture, providing a solid balance between processing power and efficiency.

The MC68HC908MR32 variant is equipped with 32KB of flash memory, which allows for the storage of complex programs and extensive data handling. In contrast, the MC68HC908MR16 features 16KB of flash memory, making it ideal for simpler applications. Both microcontrollers also come with 1KB of RAM, enabling efficient data processing and real-time operations.

Another significant characteristic of these microcontrollers is their integrated peripherals. They come with multiple input/output (I/O) pins, which allow for connectivity with various sensors and actuators. The built-in timer systems offer precise timing control for automotive and industrial applications, while the Analog-to-Digital Converter (ADC) provides essential conversion capabilities for various analog signals.

For communication purposes, the MC68HC908MR series includes a serial communication interface, enabling easy integration with other devices and systems. This versatility facilitates the development of complex systems that require interaction with external components.

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In summary, Freescale Semiconductor's MC68HC908MR32 and MC68HC908MR16 microcontrollers are key players in the embedded systems landscape. Their blend of power efficiency, integrated features, and scalability ensures they remain relevant for a wide array of applications, making them a favored choice among engineers and developers looking for dependable solutions in a competitive market.
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