MC9S08JM16 Datasheet, PDF (27/386 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 2 Pins and Connections
2.3.1 Power (V_DD, V_SS, V_SSOSC, V_DDAD, V_SSAD, V_USB33)
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 is 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 as practical to the paired V_DD and
V_SS power pins to suppress high-frequency noise. The MC9S08JM16 has a V_SSOSC pin. This pin must be
connected to the system ground plane or to the primary V_SS pin through a low-impedance connection.
V_DDAD and V_SSAD are the analog power supply pins for the MCU. This voltage source supplies power to
the ADC module. A 0.1 μF ceramic bypass capacitor must be located as near to the analog power pins as
practical to suppress high-frequency noise.
V_USB33 is connected to the internal USB 3.3 V regulator. V_USB33 maintains an output voltage of 3.3 V
and only can source enough current for internal USB transceiver and USB pullup resistor. Two separate
capacitors (4.7 F bulk electrolytic stability capacitor and 0.47 F ceramic bypass capacitors) must be
connected across this pin to ground to decrease the output ripple of this voltage regulator when it is
enabled.
2.3.2 Oscillator (XTAL, EXTAL)
Immediately after reset, the MCU uses an internally generated clock provided by the multi-purpose clock
generator (MCG) module. For more information on the MCG, see Chapter 12, “Multi-Purpose Clock
Generator (S08MCGV1).”
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.
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. C1 and C2 normally must
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).
MC9S08JM16 Series Data Sheet, Rev. 2
Freescale Semiconductor
27