MC9S08SH8CTG Datasheet, PDF (30/341 Pages) Freescale Semiconductor, Inc – HCS08 Microcontrollers

Chapter 2 Pins and Connections
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 2-5 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
After a power-on reset (POR), the PTA5/IRQ/TCLK/RESET pin defaults to a general-purpose I/O port
pin, PTA5. Setting RSTPE in SOPT1 configures the pin to be the RESET pin with an open-drain drive
containing an internal pull-up device. After configured as RESET, the pin will remain RESET until the
next POR. The RESET pin when enabled can be used to reset the MCU from an external source when the
pin is driven low.
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 non-POR reset is initiated (whether from an external signal or from an internal system), the
RESET pin if enabled 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 V_DD and should not be driven
above V_DD.
MC9S08SH8 MCU Series Data Sheet, Rev. 3
26
Freescale Semiconductor