HCS08 datasheet(27/290 Pages) MOTOROLA

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Data Sheet — MC9S08GB60/D V1.5

MOTOROLA

MC9S08GB/GT

2.3.1 Power

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 close to the MCU power pins as

practical to suppress high-frequency noise.

VDDAD and VSSAD are the analog power supply pins for the MCU. This voltage source supplies power to

the ATD. A 0.1-

µF ceramic bypass capacitor should be located as close to the MCU power pins as practical

to suppress high-frequency noise.

2.3.2 Oscillator

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

on the ICG, see the Internal Clock Generator section.

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, and the XTAL output

pin must be left unconnected.

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 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 specific

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 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).

2.3.3 Reset

RESET is a dedicated pin with a pullup 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

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