Datasheet

Chapter 2 Pins and Connections
MC9S08DN60 Series Data Sheet, Rev 3
Freescale Semiconductor 27
2.2.1 Power
V
DD
and V
SS
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 MCU power pins as
practical to suppress high-frequency noise. The MC9S08DN60 Series has two V
DD
pins except on the
32-pin package. Each pin must have a bypass capacitor for best noise suppression.
V
DDA
and V
SSA
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 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 multi-purpose clock
generator (MCG) module. For more information on the MCG, see Chapter 8, “Multi-Purpose Clock
Generator (S08MCGV1).”
The oscillator (XOSC) in this MCU is a Pierce oscillator that can accommodate a crystal or ceramic
resonator. 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. 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 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).
2.2.3 RESET
RESET is a dedicated pin with a pull-up device built in. It has input hysteresis, a high current output driver,
and no output slew rate control. Internal power-on reset and low-voltage reset circuitry typically make
external reset circuitry unnecessary. This pin is normally connected to the standard 6-pin background
debug connector so a development system can directly reset the MCU system. If desired, a manual external
reset can be added by supplying a simple switch to ground (pull reset pin low to force a reset).