Freescale Semiconductor MCF51QE128RM manual Power, Oscillator, Reset and Rsto

Chapter 2 Pins and Connections

2.2.1Power

VDD and VSS are the primary power supply pins for the MCU. This voltage source supplies power to all I/O buffer circuitry and to an internal voltage regulator. The internal voltage regulator provides regulated lower-voltage source to the CPU and other internal circuitry of the MCU.

Typically, application systems have two separate capacitors across the power pins. In this case, there should be a bulk electrolytic capacitor, such as a 10μF tantalum capacitor, to provide bulk charge storage for the overall system and a 0.1μF ceramic bypass capacitor located as close to the MCU power pins as practical to suppress high-frequency noise. The MCF51QE128/64/32has two VDD pins. Each pin must have a bypass capacitor for best noise suppression.

VDDAD and VSSAD are the analog power supply pins for the MCU. This voltage source supplies power to the ADC module. A 0.1μF ceramic bypass capacitor should be located as close to the MCU power pins as practical to suppress high-frequency noise.

2.2.2Oscillator

Immediately after reset, the MCU uses an internally generated clock provided by the internal clock source (ICS) module. For more information on the ICS, see Chapter 12, “Internal Clock Source (S08ICSV3)”.

The oscillator (XOSC) in this MCU is a Pierce oscillator that can accommodate a crystal or ceramic resonator. Optionally, an external clock source can be connected to the EXTAL input pin.

Refer to Figure 2-3for the following discussion. RS (when used) and RF should be low-inductance resistors such as carbon composition resistors. Wire-wound resistors, and some metal film resistors, have too much inductance. C1 and C2 normally should be high-quality ceramic capacitors specifically designed for high-frequency applications.

RF is used to provide a bias path to keep the EXTAL input in its linear range during crystal startup; its value is not generally critical. Typical systems use 1 MΩ to 10 MΩ . Higher values are sensitive to humidity and lower values reduce gain and (in extreme cases) could prevent startup.

C1 and C2 are typically in the 5pF to 25pF range and are chosen to match the requirements of a specific crystal or resonator. Be sure to take into account printed circuit board (PCB) capacitance and MCU pin capacitance when selecting C1 and C2. The crystal manufacturer typically specifies a load capacitance that is the series combination of C1 and C2 (which are usually the same size). As a first-order approximation, use 10pF as an estimate of combined pin and PCB capacitance for each oscillator pin (EXTAL and XTAL).

2.2.3RESET and RSTO

After a power-on reset (POR), the PTA5/IRQ/TPM1CLK/RESET pin defaults to a general-purpose input port pin, PTA5. Setting RSTPE in SOPT1 configures the pin to be the RESET pin. After configured as RESET, the pin remains RESET until the next POR. The RESET pin can be used to reset the MCU from an external source when the pin is driven low. When enabled as the RESET pin (RSTPE = 1), the pin is configured as an input with an internal pullup device automatically enabled.

MCF51QE128 MCU Series Reference Manual, Rev. 3

Freescale Semiconductor 37

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