Power; Oscillator; Freescale Semiconductor - ROHS MC9S08QE128 Reference Manual

2.2.1

Power

V and V are the primary power supply pins for the MCU. This voltage source supplies power to all
DD SS
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 10-μF tantalum capacitor, to provide bulk charge storage
for the overall system and a 0.1-μF ceramic bypass capacitor located as near to the MCU power pins as
practical to suppress high-frequency noise. Actual decoupling capacitor values and number will vary
according to layout and application. The MC9S08QE128 Series has two V
package. Each pin must have a bypass capacitor for best noise suppression.
V and V are the analog power supply pins for the MCU. This voltage source supplies power to the
DDA SSA
ADC module. A 0.1-μF ceramic bypass capacitor should be located as near to the MCU power pins as
practical to suppress high-frequency noise.
2.2.2

Oscillator

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
The oscillator (XOSCVLP) 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. The oscillator
can be configured to run in stop2 or stop3 modes.
Refer to Figure 2-6 for the following discussion. R
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 is used to provide a bias path to keep the EXTAL input in its linear range during crystal startup; its value
F
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 MCU 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).
When using the oscillator in low range and low gain mode, the external components R
not required.
2.2.3 RESET and RSTO
After a power-on reset (POR), the PTA5/IRQ/TCLK/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,

Freescale Semiconductor

S
MC9S08QE128 MCU Series Reference Manual, Rev. 2
Chapter 11, "Internal Clock Source
(when used) and R should be low-inductance
F
Chapter 2 Pins and Connections
pins except on the 32-pin
DD
(S08ICSV3)."
, R , C and C
S F 1
are
2
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