Pins and Connections
MCF51CN128 Reference Manual, Rev. 6
2-32
Freescale Semiconductor
2.4.1
Power
V
DD1.2.3.4
and V
SS1,2,3,4
are the primary power supply pins for the microcontroller. This voltage source
supplies power to all I/O buffer circuitry and to an internal voltage regulator. The internal voltage regulator
provides a regulated lower-voltage source to the CPU and other internal circuitry of the microcontroller.
Typically, application systems have two separate capacitor values across the power pins. In this case, there
should be:
•
A bulk electrolytic capacitor, such as a 10
μ
F tantalum capacitor, to provide bulk charge storage for
the overall system.
•
A 0.1
μ
F ceramic bypass capacitor located as close to the microcontroller power pins as practical
to suppress high-frequency noise. The MCF51CN128 has four V
DD
pins. Each pin must have a
bypass capacitor for best noise suppression.
V
DDA
and V
SSA
are the analog power supply pins for the microcontroller. This voltage source supplies
power to the ADC module. A 0.1
μ
F ceramic bypass capacitor should be located as close to the
microcontroller power pins as practical to suppress high-frequency noise.
2.4.2
Oscillator
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
for the following discussion. R
S
(when used) and R
F
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 that are specifically
designed for high-frequency applications.
R
F
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 M
Ω
to 10 M
Ω
. 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. 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).
