AMD Am186 ES, Руководство пользователя, Страница: 59 / 192

System Overview 3-27
3.4.3 External Source Clock
Alternately, the internal oscillator can be driven from an external clock source. This source
should be connected to the input of the inverting amplifier (X1) with the output (X2) not
connected.
3.4.4 System Clocks
Figure 3-6 shows the organization of the clocks. The original 80C186/188 microcontroller
system clock has been renamed CLKOUTA. CLKOUTB is provided as an additional output.
Figure 3-6 Clock Organization
CLKOUTA and CLKOUTB operate at either the fundamental processor frequency or the
PLL frequency. The output drivers for both clocks are individually programmable for drive
enable or disable.
The second clock output (CLKOUTB) allows one clock to run at the PLL frequency and the
other clock to run at the power-save frequency. Individual drive enable bits allow selective
enabling of just one or both of these clock outputs.
3.4.5 Power-Save Operation
The power-save mode reduces power consumption and heat dissipation, which can reduce
power supply costs and size in all systems and extend 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 microcontroller automatically returns to its
normal operating frequency on the internal clock’s next rising edge of t
3
. For an interrupt
to be recognized, it must be valid before the internal clock’s rising edge of t
3
.
Note: Power-save operation requires that clock-dependent devices be reprogrammed for
clock frequency changes. Software drivers must be aware of clock frequency.
PLL
Power-Save
Divisor
(/2 to /128)
Mux
CLKOUTA
CLKOUTB
Drive
Enable
Drive
Enable
X1, X2
Processor Internal Clock
Time
Delay
6
±
2.5ns
Mux