Z86E3016PSG Datasheet, PDF (54/348 Pages) Zilog, Inc. – Z86E3016PSG

Z8 Family of Microcontrollers
User Manual
36
A 1 in bit D7 configures the oscillator with standard drive, while a 0 con-
figures the oscillator with Low EMI drive. This only affects the drive
capability of the oscillator and does not affect the relationship of the
XTAL clock frequency to the internal system clock (SCLK).
PCON (Fh) 00h
D7 D6 D5 D4 D3 D2 D1 D0
Low EMI Oscillator
0 Low EMI
1 Standard
Figure 15. Port Configuration Register (Write-Only)
Oscillator Operation
The Z8^® CPU uses a Pierce oscillator with an internal feedback (see
Figure 16). The advantages of this circuit are low cost, large output signal,
low-power level in the crystal, stability with respect to V_CC and tempera-
ture, and low impedances (not disturbed by stray affects).
One drawback is the requirement for high gain in the amplifier to com-
pensate for feedback path losses. The oscillator amplifies its own noise at
start-up until it settles at the frequency that satisfies the gain/phase
requirements A x B = 1, where A = V₀/V_I is the gain of the amplifier and
B = V_I/V₀ is the gain of the feedback element. The total phase shift
around the loop is forced to zero (360 degrees). Because VIN must be in
phase with itself, the amplifier/inverter provides 180 degree phase shift
and the feedback element is forced to provide the other 180 degrees of
phase shift.
R1 is a resistive component placed from output to input of the amplifier.
The purpose of this feedback is to bias the amplifier in its linear region
and to provide the start-up transition.
Clock
UM001602-0904