P R E L I M I N A R Y
BUS OPERATION
The industry-standard 80C186 and 80C188 microcon-
trollers use a multiplexed address and data (AD) bus.
The address is present on the AD bus only during the
t1 clock phase. The Am186EM and Am188EM microcon-
trollers 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 (t1–t4).
cesses, thus preserving the industry-standard 80C186
and 80C188 microcontrollers’ multiplexed address bus
and providing support for existing emulation tools.
The following diagrams show the Am186EM and
Am188EM microcontroller bus cycles when the ad-
dress bus disable feature is in effect.
Figure 3 shows the affected signals during a normal
read or write operation for an Am186EM microcontrol-
ler. The address and data will be multiplexed onto the
AD bus.
For systems where power consumption is a concern, it
is possible to disable the address from being driven on
the AD bus on the Am186EM microcontroller and on
the AD and AO buses on the Am188EM 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 nor-
mal bus cycles to the associated address space is re-
duced, decreasing power consumption and reducing
processor switching noise. On the Am188EM micro-
controller, the address is driven on A015–A08 during
the data portion of the bus cycle, regardless of the set-
ting of the DA bits.
Figure 4 shows an Am186EM microcontroller bus cycle
when address bus disable is in effect. This results in
having the AD bus operate in a nonmultiplexed ad-
dress/data mode. The A bus will have the address dur-
ing a read or write operation.
Figure 5 shows the affected signals during a normal
read or write operation for an Am188EM microcontrol-
ler. The multiplexed address/data mode is compatible
with the 80C186 and 80C188 microcontrollers and
might be used to take advantage of existing logic or pe-
ripherals.
Figure 6 shows an Am188EM microcontroller bus cycle
when address bus disable is in effect. The address and
data is not multiplexed. The AD7–AD0 signals will have
only data on the bus, while the AO bus will have the ad-
dress during a read or write operation.
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 ac-
t1
t2
t3
Data
Phase
t4
Address
Phase
CLKOUTA
A19–A0
Address
AD15–AD0
(Read)
Address
Address
Data
AD15–AD0
(Write)
Data
LCS or UCS
MCSx, PCSx
Figure 3. Am186EM Microcontroller Address Bus—Normal Read and Write Operation
Am186/188EM and Am186/188EMLV Microcontrollers
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