AM186EM-33VCSLASHW datasheet(42/98 Pages) AMD

42 / 98 page

Am186/188EM and Am186/188EMLV Microcontrollers

PRELIMINA R Y

External Source Clock

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.

System Clocks

The base system clock of the 80C186 and 80C188

microcontrollers is renamed CLKOUTA and the

additional output is called CLKOUTB. CLKOUTA and

CLKOUTB operate at either the processor frequency or

the crystal input frequency. The output drivers for both

clocks are individually programmable for disable.

Figure 9 shows the organization of the clocks.

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.

Figure 9.

Clock Organization

Power-Save Operation

T h e power - s av e mod e of the A m186E M 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.

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.

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.

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.

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.

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.

PLL

Power-Save

Divisor

(/2 to /128)

Mux

CLKOUTA

CLKOUTB

Drive

Enable

Drive

Enable

X1, X2

Processor Internal Clock

Time

Delay

±2.5ns

Mux

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