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MINIMUM HARDWARE CONSIDERATIONS
If the A/D converter will be used, connect V
to a separate reference supply to minimize noise
REF
during A/D conversions. Even if the A/D converter will not be used, V
and ANGND must be
REF
connected to provide power to port 0. Refer to "Analog Ground and Reference Voltages" on page
11-13 for a detailed discussion of A/D power and ground recommendations.
Multilayer printed circuit boards with separate V
and ground planes also help to minimize
CC
noise. For more information on noise protection, refer to AP-125, Designing Microcontroller Sys-
tems for Noisy Environments and AP-711, EMI Design Techniques for Microcontrollers in Auto-
motive Applications.
12.4 PROVIDING THE CLOCK
The device can either use the on-chip oscillator to generate the clocks or use an external clock
input signal. The following paragraphs describe the considerations for both methods.
12.4.1 Using the On-chip Oscillator
The on-chip oscillator circuit (Figure 12-3) consists of a crystal-controlled, positive reactance os-
cillator. In this application, the crystal operates in a parallel resonance mode. The feedback resis-
tor, Rf, consists of paralleled n-channel and p-channel FETs controlled by the internal powerdown
signal. In powerdown mode, Rf acts as an open and the output drivers are disabled, which disables
the oscillator. Both the XTAL1 and XTAL2 pins have built-in electrostatic discharge (ESD) pro-
tection.
12-5
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