
M68HC11
REFERENCE MANUAL
RESETS AND INTERRUPTS
MOTOROLA
5-5
5.1.3 Mode of Operation Established
During reset, the basic mode of operation is established, which determines whether
the MCU will operate as a self-contained single-chip system or as an expanded sys-
tem that includes external memory resources. There are also special variations of
these two basic modes of operation. The bootstrap mode is the special variation of the
normal single-chip mode, and the special test mode is the special variation of the nor-
mal expanded mode. The levels on the two mode select pins during reset determine
which of these four modes of operation will be selected.
The hardware mode select mechanism begins with the logic levels on the MODA and
MODB pins while the MCU is in the reset state. The logic levels on the MODA and
MODB pins are fed into the MCU via a clocked pipeline path. The captured levels will
be those that were present part of a clock cycle before the RESET pin rose. This fact
assures a zero hold-time requirement on the mode select pins relative to the rising
edge at the RESET pin. The captured levels determine the logic state of the SMOD
and MDA control bits in the HPRIO register. These two control bits actually control the
logic circuits involved in hardware mode selection.
Table 5-1
summarizes the opera-
tion of the mode pins and mode control bits.
5.1.4 Program Counter Loaded with Reset Vector
As reset is released, the CPU program counter is loaded with the reset vector that
points to the first instruction in the user’s program. Depending on the cause of reset
and the mode of operation, the reset vector may be fetched from any of six possible
locations. In older Motorola MCUs, there was only one reset vector at $FFFE,FFFF.
5.2 Causes Of Reset
In the MC68HC11A8, there are on-chip systems that can detect MCU system failures
and generate a low level out the RESET pin to reinitialize other peripherals in the sys-
tem. To distinguish between these causes, separate reset Vectors are used. The pri-
mary reset vector is used when the cause of reset is the internal power-on reset circuit
or application of a low level to the RESET pin. In normal expanded and normal single-
chip modes, this vector is located at $FFFE,FFFF. If the oscillator input stops or is run-
ning too slow, the clock monitor circuit will generate a reset (provided the clock monitor
is enabled). Time-out of the internal COP watchdog timer will generate a reset (provid-
ed the COP system is enabled).
Table 5-2
summarizes the reset-vector locations ver-
sus the cause of reset and mode of operation.
Table 5-1 Hardware Mode Select Summary
Inputs
Mode Description
Control Bits in HPRIO (Latched at Reset)
RBOOT
SMOD
MODB
MODA
MDA
Inputs
1
0
Normal Single
Chip
Normal Expanded
Special Bootstrap
Special Test
0
0
0
0
1
0
0
1
0
1
0
0
0
0
0
1
0
1
1
0
0
1