HCS08 datasheet(28/316 Pages) NXP

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Chapter 2 Pins and Connections

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

for the overall system and a 0.1-

F ceramic bypass capacitor located as near to the MCU power pins as

practical to suppress high-frequency noise. Each pin must have a bypass capacitor for best noise

suppression.

2.2.2

Oscillator (XOSC)

Immediately after reset, the MCU uses an internally generated clock provided by the clock source

generator (ICS) module. For more information on the ICS, see Chapter 10, “Internal Clock Source

(S08ICSV2).”

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. RS (when used) and RF 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.

RF 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

 to 10 M. 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 Pin

RESET is a dedicated pin with open-drain drive containing an internal pull-up device. 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).

Whenever any reset is initiated (whether from an external signal or from an internal system), the RESET

pin is driven low for about 66 bus cycles. The reset circuitry decodes the cause of reset and records it by

setting a corresponding bit in the system reset status register (SRS).

NOTE

This pin does not contain a clamp diode to VDD and should not be driven

above VDD.

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