M68HC11
REFERENCE MANUAL
ON-CHIP MEMORY
MOTOROLA
4-3
The position of RAM in the 64-Kbyte address space is controlled by the RAM and I/O
mapping (INIT) register. The upper four bits of INIT (RAM[3:0]) specify the upper four
bits of the 16-bit RAM addresses. At reset, the RAM[3:0] bits are forced to zero so the
RAM is initially located at $0000–$00FF. By writing some other value to the INIT reg-
ister, the RAM can be relocated to the beginning of any 4-Kbyte page in the 64-Kbyte
address space. In normal operating modes, the INIT resister is protected so that it can
only be changed within the first 64 cycles after reset. For more detailed information
about the INIT register, see
3.3.1 RAM and I/O Mapping Register (INIT)
.
4.2.2 RAM Standby
There are several purposes for a RAM standby function. In battery operated systems,
the RAM standby function provides a way to conserve limited battery power during
times of MCU inactivity, which increases the effective time the system can operate
without battery charging or replacement. In systems using a municipal electric system
as the primary source of power, operating power is not usually a major issue, but pow-
er interruptions can be. There may be enough energy stored in regulator filter capac-
itors to allow a system to operate for some period of time after primary power is lost.
The system current drain determines how long the stored energy can maintain the sys-
tem. By detecting the loss of primary power and changing to a low-power standby
mode, the MCU system can be maintained through longer power interruptions. After
the interruption, the system can decide whether to continue operation or to perform a
complete reset initialization. In other municipal-powered systems, it may be useful to
maintain a limited amount of information during very long interruptions of primary pow-
er. In such cases, a separate standby power source based on a battery could be used
to maintain the contents of RAM while the system is non-operational.
The on-chip RAM of the M68HC11 Family is fully static; there are two ways RAM con-
tents can be maintained while reducing system power consumption to very low levels.
The easiest method for low-power RAM standby is the software-based STOP mode.
The alternate method uses the MODB/V
STBY
pin for standby power in a mostly hard-
ware approach. Since the entire MCU, including RAM, is fully static, there is no mini-
mum oscillator clock frequency. In complementary metal oxide semiconductor
(CMOS) integrated circuits, power supply current is directly proportional to operating
frequency; thus, only very small leakage currents exist when clocks are stopped. This
is the basis for the STOP method of RAM standby. When the MCU is stopped, all CPU
registers, control and I/O registers, and all RAM contents remain unchanged as long
as V
DD
is present. I
DD
for the MCU is reduced to a few microamps when MCU clocks
are stopped.
In some systems, there may be other circuitry powered from V
DD
that cannot be easily
placed in a low-power standby mode. In these systems, V
DD
must be turned off to re-
duce system power drain. The MODB/V
STBY
method of RAM standby allows V
DD
to
be removed without losing the contents of on-chip RAM. This method is more hard-
ware intensive because it involves a second power supply and associated problems.
In CMOS systems, it is possible to power an integrated circuit through an I/O pin be-
cause, on some I/O pins, there is an inherent diode between the pin and the internal
V
DD
. In some CMOS systems, even the sequencing of power supplies is critical, which