MC9S08RC8 Datasheet, PDF (23/234 Pages) Freescale Semiconductor, Inc – Microcontrollers

Pins and Connections
2.3.1 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 a 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.
2.3.2 Oscillator
The oscillator in the MC9S08RC/RD/RE/RG is a traditional Pierce oscillator that can accommodate a
crystal or ceramic resonator in the range of 1 MHz to 16 MHz.
Refer to Figure 2-5 for the following discussion. R_F should be a low-inductance resistor such as a carbon
composition resistor. Wire-wound resistors, and some metal film resistors, have too much inductance. C1
and C2 normally should be high-quality ceramic capacitors 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 and its
value is not generally critical. Typical systems use 1 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 that
is the series combination of C1 and C2, which are usually the same size. As a first-order approximation,
use 5 pF as an estimate of combined pin and PCB capacitance for each oscillator pin (EXTAL and XTAL).
2.3.3 PTD1/RESET
The external pin reset function is shared with an output-only port function on the PTD1/RESET pin. The
reset function is enabled when RSTPE in SOPT is set. RSTPE is set following any reset of the MCU and
must be cleared in order to use this pin as an output-only port.
Whenever any reset is initiated (whether from an external signal or from an internal system), the reset pin
is driven low for about 34 cycles of f_{Self_reset}, released, and sampled again about 38 cycles of f_{Self_reset}
later. If reset was caused by an internal source such as low-voltage reset or watchdog timeout, the circuitry
expects the reset pin sample to return a logic 1. If the pin is still low at this sample point, the reset is
assumed to be from an external source. The reset circuitry decodes the cause of reset and records it by
setting a corresponding bit in the system control reset status register (SRS).
Never connect any significant capacitance to the reset pin because that would interfere with the circuit and
sequence that detects the source of reset. If an external capacitance prevents the reset pin from rising to a
valid logic 1 before the reset sample point, all resets will appear to be external resets.
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
Freescale Semiconductor
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