Chapter 2 Pins and Connections
MC9S08AW60 Data Sheet, Rev 2
28
Freescale Semiconductor
2.3.1
Power (VDD, 2 x VSS, VDDAD, VSSAD)
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 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 paired VDD and VSS
power pins as practical to suppress high-frequency noise. The MC9S08AW60 has a second VSS pin. This
pin should be connected to the system ground plane or to the primary VSS pin through a low-impedance
connection.
VDDAD and VSSAD are the analog power supply pins for the MCU. This voltage source supplies power to
the ADC module. A 0.1-
μF ceramic bypass capacitor should be located as near to the analog power pins
as practical to suppress high-frequency noise.
2.3.2
Oscillator (XTAL, EXTAL)
Out of reset, the MCU uses an internally generated clock (self-clocked mode — fSelf_reset) equivalent to
about 8-MHz crystal rate. This frequency source is used during reset startup and can be enabled as the
clock source for stop recovery to avoid the need for a long crystal startup delay. This MCU also contains
a trimmable internal clock generator (ICG) module that can be used to run the MCU. For more information
The oscillator amplitude on XTAL and EXTAL is gain limited for low-power oscillation. Typically, these
pins have a 1-V peak-to-peak signal. For noisy environments, the high gain output (HGO) bit can be set to
enable rail-to-rail oscillation.
The oscillator in this MCU is a Pierce oscillator that can accommodate a crystal or ceramic resonator in
either of two frequency ranges selected by the RANGE bit in the ICGC1 register. Rather than a crystal or
ceramic resonator, an external oscillator can be connected to the EXTAL input pin.
Refer to Figure 2-4 for the following discussion. RS (when used) and RF should be low-inductance resistors such as carbon composition resistors. Wire-wound resistors, and some metal lm resistors, have
too much inductance. C1 and C2 normally should be high-quality ceramic capacitors that are specically
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 and its
value is not generally critical. Typical systems use 1 M
Ω 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 specic
crystal or resonator. Be sure to take into account printed circuit board (PCB) capacitance and MCU pin
capacitance when sizing C1 and C2. The crystal manufacturer typically species a load capacitance which
is the series combination of C1 and C2 which are usually the same size. As a rst-order approximation,
use 10 pF as an estimate of combined pin and PCB capacitance for each oscillator pin (EXTAL and
XTAL).