NXP Semiconductors MC9S08LG16, Reference Manual

Page: 33 / 373

Chapter 2 Pins and Connections
MC9S08LG32 MCU Series, Rev. 5
Freescale Semiconductor 33
2.3.1 Power
V
DD
and V
SS
are 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 a regulated
lower-voltage source for the CPU and other internal circuitry of the MCU.
The LCD/GPIO can be powered differently. For additional information, see Chapter 6, “Parallel
Input/Output Control .”
Typically, application systems have two separate capacitors across the power pins. In this case, there must
be a bulk electrolytic capacitor, such as a 10-
μ
F tantalum capacitor, to provide bulk charge storage for the
overall system, and a 0.1-
μ
F ceramic bypass capacitor located as near to the MCU power pins as practical
to suppress high-frequency noise.
V
DDA
and V
SSA
are the analog power supply pins for the MCU. This voltage source supplies power to the
ADC modules.
V
REFH
and V
REFL
pins are the voltage reference high and the voltage reference low inputs, respectively,
for the ADC module. For this MCU, V
DDA
shares the V
REFH
pin and V
SSA
shares the V
REFL
pin.
2.3.2 Oscillator
Immediately after reset, the MCU uses an internally generated clock provided by the internal clock source
(ICS) module. The ICS can be configured to run off the on-chip oscillator (ICSERCLK). The output of the
oscillator (OSCOUT) is used to run the RTC and LCD bypassing the ICS. The oscillator can be configured
to run in stop2 or stop3 modes. For more information, see
Section 1.3, “System Clock Distribution ,” and
Chapter 11, “Internal Clock Source (S08ICSV3) .”
The oscillator (XOSC) in this MCU is a Pierce oscillator that can accommodate a crystal or ceramic
resonator. An external clock source can optionally be connected to the EXTAL input pin.
Refer to Figure 2-4 for the following discussion. R
S
(when used) and R
F
must be low-inductance resistors,
such as carbon composition resistors. Wire-wound resistors and some metal film resistors have too much
inductance. C
1
and C
2
normally must be high-quality ceramic capacitors that are specifically designed for
high-frequency applications.
R
F
provides 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
Ω
to 10 M
Ω
. Higher values are sensitive to humidity and lower
values reduce gain and (in extreme cases) could prevent startup.
C
1
and C
2
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 consider the printed circuit board (PCB) capacitance and the MCU pin
capacitance when selecting C
1
and C
2
. The crystal manufacturer typically specifies a load capacitance,
which is the series combination of C
1
and C
2
(which are usually of 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).