P R E L I M I N A R Y
CLKOUTB operate at either the processor frequency or
External Source Clock
the crystal input frequency. The output drivers for both
clocks are individually programmable for disable.
Figure 9 shows the organization of the clocks.
Alternately, the internal oscillator can be driven from an
external clock source. This source should be con-
nected to the input of the inverting amplifier (X1), with
the output (X2) not connected.
The second clock output (CLKOUTB) allows one clock
to run at the crystal input frequency and the other clock
to run at the power-save frequency. Individual drive en-
able bits allow selective enabling of just one or both of
these clock outputs.
System Clocks
The base system clock of the 80C186 and 80C188
microcontrollers is renamed CLKOUTA and the
additional output is called CLKOUTB. CLKOUTA and
Processor Internal Clock
Power-Save
Divisor
(/2 to /128)
PLL
CLKOUTA
Mux
X1, X2
Drive
Enable
Time
Mux
Delay
CLKOUTB
6 ±2.5ns
Drive
Enable
Figure 9. Clock Organization
After RES becomes inactive and an internal processing in-
terval elapses, the microcontroller begins execution with
the instruction at physical location FFFF0h. RES also sets
some registers to predefined values.
Power-Save Operation
The power-save mode of the Am186EM and
Am188EM microcontrollers reduces power consump-
tion and heat dissipation, thereby extending battery life
in portable systems. In power-save mode, operation of
the CPU and internal peripherals continues at a slower
clock frequency. When an interrupt occurs, the micro-
controller automatically returns to its normal operating
frequency on the internal clock’s next rising edge of t3.
In order for an interrupt to be recognized, it must be
valid before the internal clock’s rising edge of t3.
The Reset Configuration Register
When the RES input is asserted Low, the contents of the
address/data bus (AD15–AD0) are written into the Reset
Configuration register. The system can place configura-
tion information on the address/data bus using weak ex-
ternal pullup or pulldown resistors, or using an external
driver that is enabled during reset. The processor does not
drive the address/data bus during reset.
Note: Power-save operation requires that clock-de-
pendent devices be reprogrammed for clock frequency
changes. Software drivers must be aware of clock fre-
quency.
For example, the Reset Configuration register could be
used to provide the software with the position of a con-
figuration switch in the system. Using weak external
pullup and pulldown resistors on the address and data
bus, the system would provide the microcontroller with
a value corresponding to the position of the jumper dur-
ing a reset.
Initialization and Processor Reset
Processor initialization or startup is accomplished by
driving the RES input pin Low. RES must be held Low for
1 ms during power-up to ensure proper device initializa-
tion. RES forces the Am186EM and Am188EM microcon-
trollers to terminate all execution and local bus activity. No
instruction or bus activity occurs as long as RES is active.
42
Am186/188EM and Am186/188EMLV Microcontrollers
AMD [ AMD ]
