MOTOROLA
2-10
PINS AND CONNECTIONS
M68HC11
REFERENCE MANUAL
Figure 2-7 Reduced I
DD
MODA/LIR Connections
The V
STBY
function is accomplished by a transistor switch that connects either V
STBY
or V
DD
to the RAM and reset logic, depending upon the relative levels of V
STBY
and
V
DD
. The switch connects V
DD
unless V
STBY
is more than a threshold higher than V
DD
.
A threshold is approximately a diode drop (0.7 V) but varies from lot to lot due to pro-
cessing variations. During normal operation of the MCU, V
DD
is supplying power to the
RAM. In a standby situation, V
STBY
should be maintained at a valid level, and RESET
should be activated (pulled low) when V
DD
drops below legal limits. RESET should al-
ways be held low whenever V
DD
is below its operating limit. If the MCU is to be oper-
ated in a special mode (MODB low before applying reset) and the MODB/V
STBY
pin is
being used to back up the RAM, the MODB/V
STBY
pin should not be driven low unless
V
DD
is at (has returned to) a legal level. Some logic may be required in systems that
use MODB/V
STBY
as a standby supply and wish to use one of the special modes of
operation. In most applications, the MODB pin would be connected to V
DD
through a
4.7 k
pull-up resistor for normal modes or directly to ground for special modes.
There are two ways to maintain the contents of on-chip RAM with minimal power con-
sumption (as in a battery-based application). The preferred method uses the STOP
mode of operation, and the second method uses the MODB/V
STBY
pin (see
Figure 2-
8
). Each of these methods has advantages. The STOP method is preferred because
it is much simpler than the separate power-supply method in terms of hardware costs
and complexity. The STOP method saves power by stopping all MCU clocks, which
reduces the V
DD
current to a few microamps. No external logic is needed, and the con-
tents of internal registers are maintained in addition to the contents of internal RAM.
The MODB/V
STBY
pin method would be used in cases where there is a significant
amount of external circuitry operating from V
DD
so that the added complexity of two
supplies and added logic is justified by the power savings.
4.7K
RESET
74HC04
TO MODA/LIR
OF M68HC11