M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-9
After reset is released, 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 dur-
ing the first E 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.
The LIR function is intended for monitoring on a logic analyzer during debug of a sys-
tem. Since this status indicator shows where each instruction begins, programs can
be followed easily. The mode A select levels and the LIR status levels were selected
to prevent interference between the shared functions of the pin. In single-chip applica-
tions, this pin is simply connected to V
SS
. Since the LIR output is open-drain, there is
no conflict between the direct V
SS
connection and the LIR signal that drives the pin low
during the first E cycle of each instruction. There is no practical reason to monitor LIR
during single-chip modes because there is no visibility to internal data and address
buses. In expanded-mode systems, the MODA/LIR pin is normally pulled up to V
DD
by
a 4.7 k
resistor. During reset, the pull-up resistor instructs the MODA pin to select
expanded modes. During-program execution, the pin is driven low during the first cycle
of each instruction by the LIR signal and is pulled up between LIR signals by the ex-
ternal 4.7 k
pull-up.
In expanded-mode systems where it is important to minimize power-supply current,
logic could be used to drive the MODA/LIR pin rather than just using a simple pull-up
(see
Figure 2-7
). During reset, the MODA pin would be driven high to select expanded
mode. After reset, the LIR pin would be driven low by logic. The logic should not be
operating against a pull-up, but rather it should be a logic-gate-type output with some
series resistance to protect against the unlikely event of a conflict between an active-
low LIR signal and an active-high logic-gate output signal. Such a conflict could only
occur briefly at the falling edge of reset. Since LIR is active for about one out of every
three cycles during normal execution (average instructions take about three cycles),
I
DD
could be reduced by about 350
μ
A (5 V
÷
4.7 k
x 33% duty cycle).
Table 2-1 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