MOTOROLA
3-2
CONFIGURATION AND MODES OF OPERATION
M68HC11
REFERENCE MANUAL
3.1.1 Hardware Mode Select Pins
The hardware mode select mechanism starts 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 by way of a clocked pipeline path. The levels cap-
tured are those that were present part of a clock cycle before the RESET pin rose,
which assures there will be a zero hold-time requirement on the mode select pins rel-
ative to the rising edge at the RESET pin. The captured levels determine the logic state
of the special mode (SMOD) and mode A select (MDA) control bits in the highest pri-
ority interrupt (HPRIO) register. These two control bits actually control the logic circuits
involved in hardware mode selection.
Table 3-1
summarizes the operation of the
mode pins and mode control bits.
After RESET rises, the mode select pins no longer influence the MCU operating mode.
The MODA pin serves the alternate function of load instruction register (LIR) when the
MCU is not in reset. The open-drain active-low LIR output pin drives low during the first
E-clock cycle of each instruction. The MODB pin serves the alternate function of a
standby power supply (V
STBY
) to maintain RAM contents when V
DD
is not present.
The power-saving mode, STOP, is an alternate way to save RAM contents, which
does not require a separate standby power source.
3.1.2 Mode Control Bits in the HPRIO Register
The following register and paragraphs show the HPRIO register. The four low-order
bits (PSEL[3:0]) are not related to the mode select logic and will be discussed in
SEC-
TION 5 RESETS AND INTERRUPTS
. The HPRIO register may be read at any time,
but the four high-order bits may only be written under special circumstances. Usually,
control bits for unrelated on-chip systems would not be mixed in the same register.
RBOOT — Read Bootstrap ROM
Can be written only while SMOD equals one
1 = Bootstrap ROM enabled at $BF40–$BFFF
0 = Bootstrap ROM disabled and not present in memory map
The RBOOT control bit enables or disables the special bootstrap control ROM. This
Table 3-1 Hardware Mode Select Summary
Inputs
Mode Description
Control Bits in HPRIO (Latched at Reset)
RBOOT
SMOD
0
0
0
0
1
1
0
1
MODB
1
1
0
0
MODA
0
1
0
1
MDA
0
1
0
1
IRV
0
0
1
1
Normal Single Chip
Normal Expanded
Special Bootstrap
Special Test
HPRIO —
Highest Priority I-Bit Interrupt and Miscellaneous
$103C
BIT 7
RBOOT
6
5
4
3
2
1
BIT 0
PSEL0
SMOD
MDA
IRV
PSEL3
PSEL2
PSEL1
RESET:
(Refer to
Table 3-1
)