M68HC08 datasheet(73/172 Pages) FREESCALE

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I/O Signals

MC68HC908JL3E Family Data Sheet, Rev. 3

Freescale Semiconductor

6.4 I/O Signals

The following paragraphs describe the oscillator I/O signals.

6.4.1 Crystal Amplifier Input Pin (OSC1)

OSC1 pin is an input to the crystal oscillator amplifier or the input to the RC oscillator circuit.

6.4.2 Crystal Amplifier Output Pin (OSC2/PTA6/RCCLK)

For the X-tal oscillator device, OSC2 pin is the output of the crystal oscillator inverting amplifier.

For the RC oscillator device, OSC2 pin can be configured as a general purpose I/O pin PTA6, or the

output of the internal RC oscillator clock, RCCLK.

6.4.3 Oscillator Enable Signal (SIMOSCEN)

The SIMOSCEN signal comes from the system integration module (SIM) and enables/disables the X-tal

oscillator circuit or the RC-oscillator.

6.4.4 X-tal Oscillator Clock (XTALCLK)

XTALCLK is the X-tal oscillator output signal. It runs at the full speed of the crystal (fXCLK) and comes

directly from the crystal oscillator circuit. Figure 6-1 shows only the logical relation of XTALCLK to OSC1

and OSC2 and may not represent the actual circuitry. The duty cycle of XTALCLK is unknown and may

depend on the crystal and other external factors. Also, the frequency and amplitude of XTALCLK can be

unstable at start-up.

6.4.5 RC Oscillator Clock (RCCLK)

RCCLK is the RC oscillator output signal. Its frequency is directly proportional to the time constant of the

external R and C. Figure 6-2 shows only the logical relation of RCCLK to OSC1 and may not represent

the actual circuitry.

6.4.6 Oscillator Out 2 (2OSCOUT)

2OSCOUT is same as the input clock (XTALCLK or RCCLK). This signal is driven to the SIM module and

is used to determine the COP cycles.

6.4.7 Oscillator Out (OSCOUT)

The frequency of this signal is equal to half of the 2OSCOUT, this signal is driven to the SIM for generation

of the bus clocks used by the CPU and other modules on the MCU. OSCOUT will be divided again in the

SIM and results in the internal bus frequency being one fourth of the XTALCLK or RCCLK frequency.

Device

Oscillator

OSC2 pin function

MC68HC908JL3E/JK3E/JK1E

X-tal

Inverting OSC1

MC68HRC908JL3E/JK3E/JK1E

Controlled by PTA6EN bit in PTAPUER ($0D)

PTA6EN = 0: RCCLK output

PTA6EN = 1: PTA6 I/O

M68HC08 Datasheet(PDF) 73 Page - Freescale Semiconductor, Inc