Technical Data
MC68HC11E Family
—
Rev. 4
82
Operating Modes and On-Chip Memory
MOTOROLA
Operating Modes and On-Chip Memory
4.4.2 Mode Selection
The four mode variations are selected by the logic states of the MODA
and MODB pins during reset. The MODA and MODB logic levels
determine the logic state of SMOD and the MDA control bits in the
highest priority I-bit interrupt and miscellaneous (HPRIO) register.
After reset is released, the mode select pins no longer influence the
MCU operating mode. In single-chip operating mode, the MODA pin is
connected to a logic level 0. In expanded mode, MODA is normally
connected to V
DD
through a pullup resistor of 4.7 k
. The MODA pin
also functions as the load instruction register LIR pin when the MCU is
not in reset. The open-drain active low LIR output pin drives low during
the first E cycle of each instruction. The MODB pin also functions as
standby power input (V
STBY
), which allows RAM contents to be
maintained in absence of V
DD
.
Refer to
Table 4-1
, which is a summary of mode pin operation, the mode
control bits, and the four operating modes.
A normal mode is selected when MODB is logic 1 during reset. One of
three reset vectors is fetched from address $FFFA
–
$FFFF, and program
execution begins from the address indicated by this vector. If MODB is
logic 0 during reset, the special mode reset vector is fetched from
addresses $BFFA
–
$BFFF, and software has access to special test
features. Refer to
Section 5. Resets and Interrupts
.
Table 4-1. Hardware Mode Select Summary
Input Levels
at Reset
Mode
Control Bits in HPRIO
(Latched at Reset)
MODB
MODA
RBOOT
SMOD
MDA
1
0
Single chip
0
0
0
1
1
Expanded
0
0
1
0
0
Bootstrap
1
1
0
0
1
Special test
0
1
1