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System Overview
3-18
3.2 BUS OPERATION
The industry-standard 80C186 and 80C188 microcontrollers use a multiplexed address
and data (AD) bus. The address is present on the AD bus only during the t
1
clock phase.
The Am186ER and Am188ER microcontrollers continue to provide the multiplexed AD bus and,
in addition, provide a nonmultiplexed address (A) bus. The A bus provides an address to the
system for the complete bus cycle (t
1
–t
4
).
For systems where power consumption is a concern, it is possible to disable the address
from being driven on the AD bus on the Am186ER microcontroller and on the AD and AO
buses on the Am188ER microcontroller during the normal address portion of the bus cycle
for accesses to UCS and/or LCS address spaces. In this mode, the affected bus is placed
in a high-impedance state during the address portion of the bus cycle. This feature is
enabled through the DA bits in the UMCS and LMCS registers. When address disable is
in effect, the number of signals that assert on the bus during all normal bus cycles to the
associated address space is reduced, thus decreasing power consumption, reducing
processor switching noise, and preventing bus contention with memory devices and
peripherals when operating at high clock rates. On the Am188ER microcontroller, the
address is driven on A015–A08 during the data portion of the bus cycle, regardless of the
setting of the DA bits.
If the ADEN
pin is pulled Low during processor reset, the value of the DA bits in the UMCS and
LMCS registers is ignored and the address is driven on the AD bus for all accesses, thus
preserving the industry-standard 80C186 and 80C188 microcontrollers’ multiplexed address bus
and providing support for existing emulation tools.
Figure 3-1 on page 3-19 shows the affected signals during a normal read or write operation
for an Am186ER microcontroller. The address and data will be multiplexed onto the AD bus.
Figure 3-2 on page 3-19 shows an Am186ER microcontroller bus cycle when address bus
disable is in effect. This results in the AD bus operating in a nonmultiplexed data-only mode.
The A bus will provide the address during a read or write operation.
Figure 3-3 on page 3-20 shows the affected signals during a normal read or write operation
for an Am188ER microcontroller. The multiplexed address/data mode is compatible with
80C188 microcontrollers and might be used to take advantage of existing logic or
peripherals.
Figure 3-4 on page 3-20 shows an Am188ER microcontroller bus cycle when address bus
disable is in effect. The address and data are not multiplexed. The AD7–AD0 signals will
have only data on the bus, while the A bus will have the address during a read or write
operation. The AO bus will also have the address during t
2
–t
4
.