MC68HC912B32 Datasheet, PDF (11/128 Pages) Motorola, Inc – 16-Bit Microcontroller

3.3 Signal Descriptions
3.3.1 Crystal Driver and External Clock Input (XTAL, EXTAL)
These pins provide the interface for either a crystal or a CMOS compatible clock to control the internal
clock generator circuitry. Out of reset the frequency applied to EXTAL is twice the desired E-clock rate.
All the device clocks are derived from the EXTAL input frequency.
EXTAL
MCU
XTAL
10 MΩ
C
2xE
CRYSTAL
C
Figure 4 Common Crystal Connections
COMMON XTAL CONN
EXTAL
MCU
XTAL _NC
2xE
CMOS-COMPATIBLE
EXTERNAL OSCILLATOR
Figure 5 External Oscillator Connections
EXT EXTAL CONN
XTAL is the crystal output. The XTAL pin must be left unterminated when an external CMOS compatible
clock input is connected to the EXTAL pin. The XTAL output is normally intended to drive only a crystal.
The XTAL output can be buffered with a high-impedance buffer to drive the EXTAL input of another de-
vice.
In all cases take extra care in the circuit board layout around the oscillator pins. Load capacitances
shown in the oscillator circuits include all stray layout capacitances. Refer to Figure 4 and Figure 5 for
diagrams of oscillator circuits.
3.3.2 E-Clock Output (ECLK)
ECLK is the output connection for the internal bus clock and is used to demultiplex the address and data
and is used as a timing reference. ECLK frequency is equal to 1/2 the crystal frequency out of reset. E-
clock output can be turned off in single-chip modes to reduce the effects of RFI. In special peripheral
mode the E clock is an input to the MCU. All clocks, including the E-clock, are halted when the MCU is
in STOP mode. It is possible to configure the MCU to interface to slow external memory. ECLK can be
stretched for such accesses.
MC68HC912B32
MC68HC912B32TS/D
MOTOROLA
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