Table of Contents
Advertisement
CLOCK GENERATION AND POWER MANAGEMENT
An important consideration when using crystals is that the oscillator start correctly over the volt-
age and temperature ranges expected in operation. Observe oscillator startup in the laboratory.
Varying the load capacitors (within about ± 50%) can optimize startup characteristics versus sta-
bility. In your experiments, consider stray capacitance and scope loading effects.
For help in selecting external oscillator components for unusual circumstances, count on the crys-
tal manufacturer as your best resource. Using low-cost ceramic resonators in place of crystals is
possible if your application will tolerate less precise frequencies.
5.1.2
Using an External Oscillator
The microprocessor's on-board clock oscillator allows the use of a relatively low cost crystal.
However, the designer may also use a "canned oscillator" or other external frequency source.
Connect the external frequency input (EFI) signal directly to the oscillator CLKIN input. Leave
OSCOUT unconnected. This oscillator input drives the internal divide-by-two counter directly,
generating the CPU clock signals. The external frequency input can have practically any duty cy-
cle, provided it meets the minimum high and low times stated in the data sheet. Selecting an ex-
ternal clock oscillator is more straightforward than selecting a crystal.
5.1.3
Output from the Clock Generator
The crystal oscillator output drives a divide-by-two circuit, generating a 50% duty cycle clock for
the processor's integrated components. All processor timings refer to this clock, available exter-
nally at the CLKOUT pin. CLKOUT changes state on the high-to-low transition of the CLKIN
signal, even during reset and bus hold. CLKOUT is also available during Idle mode, but not dur-
ing Powerdown mode. (See "Idle Mode" on page 5-11 and "Powerdown Mode" on page 5-16.)
In a CMOS circuit, significant current flows only during logic level transitions. Since the micro-
processor consists mostly of clocked circuitry, the clock distribution is the basis of power man-
agement.
5.1.4
Reset and Clock Synchronization
The clock generator provides a system reset signal (RESOUT). The RESIN input generates RE-
SOUT and the clock generator synchronizes it to the CLKOUT signal.
A Schmitt trigger in the RESIN input ensures that the switch point for a low-to-high transition is
greater than the switch point for a high-to-low transition. The processor must remain in reset a
minimum of 4 CLKOUT cycles after V
RC circuit to drive the RESIN input (see Figure 5-5). Typical applications can use about 100 mil-
liseconds as an RC time constant.
5-6
and CLKOUT stabilize. The hysteresis allows a simple
CC
Advertisement
Table of Contents
