Z86E3016PSG Datasheet, PDF (56/348 Pages) Zilog, Inc. – Z86E3016PSG

Z8 Family of Microcontrollers
User Manual
38
data lines, system ground) to reduce cross talk and noise injection. This is
usually accomplished by keeping other traces and system ground trace
planes away from the oscillator circuit and by placing a Z8^® CPU device
V_SS ground ring around the traces/components. The ground side of the
oscillator lead caps should be connected to a single trace to the Z8^®
CPU’s V_SS (GND) pin. It should not be shared with any other system
ground trace or components except at the Z8^® CPU’s V_SS pin. This is to
prevent differential system ground noise injection into the oscillator (see
Figure 17).
Indications of an Unreliable Design
Start-up time and output level are two major indicators that are used in
working designs to determine their reliability over full lot and tempera-
ture variations. These two indicators are described below.
Start-Up Time. If start-up time is excessive, or varies widely from unit to
unit, there is probably a gain problem. C1/C2 must be reduced; the ampli-
fier gain is not adequate at frequency, or crystal resistance is too large.
Output Level. The signal at the amplifier output should swing from
ground to V_CC. This indicates there is adequate gain in the amplifier. As
the oscillator starts up, the signal amplitude grows until clipping occurs,
at which point the loop gain is effectively reduced to unity and constant
oscillation is achieved. A signal of less than 2.5 volts peak-to-peak is an
indication that low gain may be a problem. Either C₁ or C₂ should be
made smaller or a low-resistance crystal should be used.
Circuit Board Design Rules
The following circuit board design rules are suggested:
• To prevent induced noise the crystal and load capacitors should be
physically located as close to the Z8^® CPU as possible.
Clock
UM001602-0904