
MOTOROLA
5-4
RESETS AND INTERRUPTS
M68HC11
REFERENCE MANUAL
5.1.1.11 Other System Controls
The EEPROM programming controls are all disabled so the memory system is config-
ured for normal read operation. The highest priority I bit interrupt defaults to being the
external interrupt request (IRQ) pin by PSEL[3:0] equal to 0:1:0:1. The IRQ pin is con-
figured for level-sensitive operation (for wired-OR systems). The read bootstrap ROM
(RBOOT), special mode (SMOD), and mode A (MDA) bits in the HPRIO register reflect
the status of the mode B (MODB) and MODA inputs at the rising edge of reset. The
enable oscillator start-up delay (DLY) control bit is set to specify that an oscillator start-
up delay is imposed upon recovery from STOP mode. The clock monitor system is dis-
abled by clock monitor enable (CME) equals zero.
The MC68HC11A8 has three internal sources that can cause reset as well as the ex-
ternal application of a low level to the RESET pin. No matter which of these sources
causes reset, the entire MCU is reset. The RESET pin is driven low as a result of any
of the reset sources. The only distinction that is made between the causes of reset is
the reset vector, which is used to tell the CPU the starting address for execution when
reset is released.
A few registers are not forced to a start-up condition as a result of reset. Since these
registers do not affect the starting conditions at MCU pins, it is not important to force
them to a start-up state during reset. One such example is the main-timer input-cap-
ture registers. Since these registers are not useful until after an input capture occurs,
it is not important to force them to a start-up state during reset.
5.1.2 CONFIG Register Allows Flexible Configuration
The M68HC11 includes a nonvolatile CONFIG register, which controls a number of
options typically controlled by mask options or by additional mode selection choices in
other MCUs. By using a nonvolatile EEPROM-based register, it is possible to achieve
the same effects as if the options were mask programmed and, at the same time, allow
users to change these features after the MCU is manufactured. The most important
aspect of this method of selecting options is that the selections automatically take ef-
fect on any power-up or reset without any software intervention. Two classes of fea-
tures can be controlled in this manner. First, there are configuration choices that must
inherently be made before the reset vector is even fetched. For example, the ROM en-
able must be decided so that the reset vector can be fetched out of the correct memory
as the MCU comes out of reset. The COP watchdog timer enable is an example of the
second class of features that can be controlled by an EEPROM bit. The COP watch-
dog timer is intended to detect software failures; thus, it is important to enable or dis-
able this feature without any software intervention. If software could disable or was
required to enable the COP watchdog, the COP watchdog timer could not detect a fail-
ure of that software.
The CONFIG register controls the presence or absence of ROM and/or EEPROM, en-
ables/disables the COP watchdog timer, and engages/disengages the security option.
The CONFIG register and mechanism are described in greater detail in
3.2.1 Opera-
tion of CONFIG Mechanism
. The features enabled by the CONFIG register can be
thought of as mask-programmed options that do not require software service.