MC9S08SH32CWL Datasheet, PDF (26/328 Pages) Freescale Semiconductor, Inc – MC9S08SH32 Series Features

Chapter 2 Pins and Connections
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. Each pin must have a bypass capacitor for best noise
suppression.
V_DDA and V_SSA are the analog power supply pins for MCU. This voltage source supplies power to the
ADC module. A 0.1uF ceramic bypass capacitor should be located as near to the MCU power pins as
practical to suppress high-frequency noise. The V_REFH and V_REFL pins are the voltage reference high and
voltage reference low inputs, respectively for the ADC module. For this MCU, V_DDA shares the V_REFH
pin and these pins are available only in the 28-pin packages. In the 16-pin and 20-pin packages they are
double bonded to the V_DD pin. For this MCU, V_SSA shares the V_REFL pin and these pins are available only
in the 28-pin packages. In the 16-pin and 20-pin packages they are double bonded to the V_SS pin.
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 11, “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. 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).
MC9S08SH32 Series Data Sheet, Rev. 2
26
Freescale Semiconductor
PRELIMINARY