L = round

Clock Generator Module (CGM)

5. Calculate the bus frequency, fBUS, and compare fBUS with fBUSDES.

fBUS =

fVCLK

4

Example: N = 32 MHz = 8 MHz

4MHz

6.If the calculated fBUS is not within the tolerance limits of the application, select another fBUSDES or another fRCLK.

7.Using the value 4.9152 MHz for fNOM, calculate the VCO linear range multiplier, L. The linear range multiplier controls the frequency range of the PLL.

( fVCLK ) fNOM

32 MHz

Example: L = 4.9152 MHz = 7 MHz

8.Calculate the VCO center-of-range frequency, fVRS. The center-or-range frequency is the midpoint between the minimum and maximum frequencies attainable by the PLL.

fVRS = L x fNOM

Example: fVRS = 7 x 4.9152 MHz = 34.4 MHz

For proper operation,

fVRS – fVCLK ≤

fNOM

2

 

CAUTION

Exceeding the recommended maximum bus frequency or VCO frequency can crash the MCU.

9.Program the PLL registers accordingly:

a.In the upper four bits of the PLL programming register (PPG), program the binary equivalent of N.

b.In the lower four bits of the PLL programming register (PPG), program the binary equivalent of L.

4.3.2.5 Special Programming Exceptions

The programming method described in 4.3.2.4 Programming the PLL does not account for possible exceptions. A value of 0 for N or L is meaningless when used in the equations given. To account for these exceptions:

•A 0 value for N is interpreted exactly the same as a value of 1.

•A 0 value for L disables the PLL and prevents its selection as the source for the base clock. See 4.3.3 Base Clock Selector Circuit.

4.3.3 Base Clock Selector Circuit

This circuit is used to select either the crystal clock, CGMXCLK, or the VCO clock, CGMVCLK, as the source of the base clock, CGMOUT. The two input clocks go through a transition control circuit that waits up to three CGMXCLK cycles and three CGMVCLK cycles to change from one clock source to the other. During this time, CGMOUT is held in stasis. The output of the transition control circuit is then divided by

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

62

Freescale Semiconductor

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Freescale Semiconductor MC68HC908MR32, MC68HC908MR16 manual Base Clock Selector Circuit, Special Programming Exceptions
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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.

Security is another crucial aspect of these microcontrollers. They have built-in fail-safe mechanisms to ensure reliable operation under various conditions, making them suitable for critical systems. Additionally, their robust architecture helps to safeguard against potential disruptions or attacks.

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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