mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 34

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

Pins and Connections
2.1.5
V
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 microcontroller.
Typically, application systems have two separate capacitors across the power pins. In this case, there must
be a bulk electrolytic capacitor, such as a 10μF tantalum capacitor, to provide bulk charge storage for the
overall system and a 0.1 μF ceramic bypass capacitor located as close to the microcontroller power pins
as practical to suppress high-frequency noise. The MCF51AC256 in 80-pin package has two V
Each pin must have a bypass capacitor for best noise suppression.
V
power to the ADC module. A 0.1 μF ceramic bypass capacitor must be located as close to the
microcontroller analog power pins as practical to suppress high-frequency noise.
2.1.6
Immediately after reset, the microcontroller uses an internally generated clock provided by the
multipurpose clock generation (MCG) module.
The oscillator (XOSC) in this microcontroller 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
such as carbon composition resistors. Wire-wound resistors and some metal film resistors, have too much
inductance. C1 and C2 normally must be high-quality ceramic capacitors that are specifically designed for
high-frequency applications.
R
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 5 pF to 25 pF 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
microcontroller pin capacitance when selecting C1 and C2. The crystal manufacturer typically specifies a
load capacitance which is the series combination of C1 and C2 (which are usually the same size). As a
first-order approximation, use 10 pF as an estimate of combined pin and PCB capacitance for each
oscillator pin (EXTAL and XTAL).
2-8
F
DD
DDA
is used to provide a bias path to keep the EXTAL input in its linear range during crystal startup; its value
and V
and V
Figure 2-3
SS
Power (V
Oscillator (XTAL and EXTAL)
SSA
are the primary power supply pins for the microcontroller. This voltage source supplies
are the analog power supply pins for the microcontroller. This voltage source supplies
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
for the following discussion. R
DD
, V
SS
, V
DDA
, and V
S
(when used) and R
SSA
)
F
must be low-inductance resistors
Freescale Semiconductor
DD
pins.