MOTOROLA
5-2
RESETS AND INTERRUPTS
M68HC11
REFERENCE MANUAL
SET signal resets the clock divider circuitry so the on-chip oscillator will start. If an ap-
plication includes external clock circuitry driving the EXTAL pin, the RESET signal
should force this external clock to resume oscillation.
5.1.1 System Initial Conditions
Once the reset condition is recognized, internal registers and control bits are forced to
an initial state. These initial states, in turn, control on-chip peripheral systems to force
them to known start-up states. Most of the initial conditions are independent of the op-
erating mode. The following paragraphs summarize the initial conditions of the MCU
as it leaves reset.
5.1.1.1 CPU
After reset, the CPU fetches the restart vector from locations $FFFE,FFFF
($BFFE,BFFF if in special test or bootstrap mode) during the first three cycles and be-
gins executing instructions. The stack pointer and other CPU registers are indetermi-
nate immediately after reset; however, the X and I interrupt mask bits in the CCR are
set to mask any interrupt requests. Also, the S bit in the CCR is set to disable the
STOP mode.
5.1.1.2 Memory Map
After reset, the RAM and I/O mapping (INIT) register is initialized to $01, putting the
256 bytes of random-access memory (RAM) at locations $0000–$00FF and the con-
trol registers at locations $1000–$103F. The 8-Kbyte read-only memory (ROM) and/
or the 512-byte EEPROM may or may not be present in the memory map because the
two bits that enable them in the configuration control (CONFIG) register are EEPROM
cells not affected by reset or power-down.
5.1.1.3 Parallel I/O
When a reset occurs in expanded-multiplexed operating mode, the 18 pins used for
parallel I/O are dedicated to the expansion bus. If a reset occurs in the single-chip op-
erating mode, the strobe A flag (STAF), strobe A interrupt (STAI), and handshake
(HNDS) control bits in the parallel input/output control (PIOC) register are cleared so
that no interrupt is pending or enabled, and the simple strobed mode (rather than full-
handshake mode) of parallel I/O is selected. The port C wired-OR mode (CWOM) bit
in PIOC is cleared. Port C is initialized as an input port (data direction register for port
C, DDRC $00); port B is a general-purpose output port with all bits cleared. STRA is
the edge-sensitive strobe A input, and the active edge is initially configured to detect
rising edges (edge select for strobe A (EGA) bit in PIOC is set). Port C, port D (bits
[5:0]), port A (bits 0, 1, 2, and 7), and port E are configured as general-purpose high-
impedance inputs. Port B and bits [6:3] of port A have their directions fixed as outputs,
and their reset state is logic zero.
5.1.1.4 Timer
During reset, the timer system is initialized to a count of $0000. The prescaler bits are
cleared, and all output-compare registers are initialized to $FFFF. All input-capture