M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-11
Figure 2-8 RAM Standby MODB/V
STBY
Connections
2.2.3 Crystal Oscillator and Clock Pins (EXTAL, XTAL, and E)
The oscillator pins can be used with an external crystal network or an externally gen-
erated CMOS-compatible clock source. The frequency applied to these pins is four
times higher than the desired bus frequency (E-clock rate). The E clock is the bus fre-
quency clock output, which is used as a basic timing reference signal. When the E
clock is low (address portion of a bus cycle), an internal process is occurring; when E
is high, data is being addressed. The E clock is free running at one-fourth the crystal
frequency as long as the oscillator is active (STOP stops all clocks).
The oscillator in the MC68HC11A8 consists of a large two-input NAND gate. One of
the inputs to this gate is driven by an internal signal that disables the oscillator when
the MCU is in the STOP mode. The other input is the EXTAL input pin of the MCU.
The output of this NAND gate is the XTAL output pin of the MCU.
The XTAL pin is normally left unterminated when using an external CMOS-compatible
clock input to the EXTAL pin. However, a 10 k
-100 k
load resistor to ground may
be used to reduce generated radio frequency interference (RFI) noise emission. The
XTAL output is normally intended to drive only a crystal, but XTAL can be used as a 4
x clock output if special care is taken to avoid undesirable loading. The XTAL output
may be buffered with a high-impedance buffer such as the 74HC04, or it may be used
to drive the EXTAL input of another M68HC11 MCU. In all cases, the circuit-board lay-
out around the oscillator pins is critical. Load capacitances specified in the data sheets
and technical summary include all stray layout capacitances. Thus, the physical ca-
pacitors connected to these pins should always be less than the specified load capac-
itances by the estimated interconnection capacitances.
Figure 2-9
and
Figure 2-10
show the internal and external components that form the
crystal oscillator, called a Pierce oscillator (also known as a parallel resonant crystal
oscillator).
Figure 2-9
shows the connections for high-frequency crystals (greater than 1 MHz),
and
Figure 2-10
shows connections for low-frequency operation (less than 1 MHz).
OUT
4.7K
TO MODB/VSTBY
OF M68HC11
MAX
690
V
VBATT
VDD
+
4.8 V
NiCd
VDD