MOTOROLA
3-10
CONFIGURATION AND MODES OF OPERATION
M68HC11
REFERENCE MANUAL
after reset. The COP system is discussed in detail in
SECTION 5 RESETS AND IN-
TERRUPTS
.
3.4 Normal MCU Operating Modes
The normal modes of operation are selected by having a logic one on the MODB pin
during reset. The reset vector is fetched from addresses $FFFE,FFFF, and program
execution begins from the address indicated by this vector. In normal single-chip
mode, the internal 8-Kbyte program memory is enabled in this memory space so the
reset vector is fetched from this internal ROM. In normal expanded mode, the internal
8-Kbyte ROM may or may not be enabled, depending on the ROMON bit in the CON-
FIG register. If the internal ROM is on, the reset vector is fetched from within this ROM;
otherwise, it is fetched from external memory addresses $FFFE,FFFF.
3.4.1 Normal Single-Chip Mode
The normal single-chip mode is selected by a logic one on the MODB pin and a logic
zero on the MODA pin during reset. Because the single-chip modes do not require any
external address and data bus functions, port B, port C, strobe A (STRA), and strobe
B (STRB) pins are available for general-purpose parallel I/O. In this mode, all software
needed to control the MCU is contained in internal memories.
The ROMON control bit in the EEPROM-based CONFIG register is overridden in nor-
mal single-chip mode to force the internal 8-Kbyte ROM on. This procedure is required
because there must be a valid reset vector for the MCU to operate in a logical manner.
3.4.2 Normal Expanded Mode
The normal expanded mode is selected by having a logic one on both the MODB pin
and MODA pin during reset. This mode of operation allows external memory and pe-
ripheral devices to be accessed by a time-multiplexed address/data bus. By multiplex-
ing the low-order eight bits of address with data on the port C pins, only 18 pins are
needed to provide an 8-bit data bus, a 16-bit address bus, and two bus control lines.
The low-order address lines are separated from data with an external transparent latch
such as a 74HC373, which is clocked by the address strobe (AS) signal. All bus cycles,
whether internal or external, execute at the E-clock frequency (no throughput penalty
for external devices). The maximum bus frequency for the MC68HC11A8 is 2.1 MHz,
which is comparable to the fastest external EPROMs available at the time the
M68HC11 was introduced.
SECTION 2 PINS AND CONNECTIONS
gives more de-
Table 3-2 Watchdog Rates vs. Crystal Frequency
CR1
CR0
E + 2
15
Divided by
Crystal Frequency
8 MHz
Nominal Time-Out
16.384 ms
65.536 ms
262.14 ms
1.049 s
2 MHz
Bus Frequency (E Clock)
2
23
Hz
4 MHz
0
0
1
1
0
1
0
1
1
4
15.625 ms
62.5 ms
250 ms
1 s
2.1 MHz
32.768 ms
131.07 ms
524.29 ms
2.1 s
1 MHz
16
64