Z8F6421PM020SC Datasheet, PDF (225/299 Pages) Zilog, Inc. – High Performance 8-Bit Microcontrollers Z8 Encore-R 64K Series

Z8 Encore!^® 64K Series
Product Specification
205
On-Chip Oscillator
Overview
The products in the 64K Series feature an on-chip oscillator for use with external crystals
with frequencies from 32KHz to 20MHz. In addition, the oscillator can support external
RC networks with oscillation frequencies up to 4MHz or ceramic resonators with oscilla-
tion frequencies up to 20MHz. This oscillator generates the primary system clock for the
internal eZ8 CPU and the majority of the on-chip peripherals. Alternatively, the X_IN input
pin can also accept a CMOS-level clock input signal (32KHz–20MHz). If an external
clock generator is used, the X_OUT pin must be left unconnected.
When configured for use with crystal oscillators or external clock drivers, the frequency of
the signal on the X_IN input pin determines the frequency of the system clock (that is, no
internal clock divider). In RC operation, the system clock is driven by a clock divider
(divide by 2) to ensure 50% duty cycle.
Operating Modes
The 64K Series products support 4 different oscillator modes:
• On-chip oscillator configured for use with external RC networks (<4MHz).
• Minimum power for use with very low frequency crystals (32KHz to 1.0MHz).
• Medium power for use with medium frequency crystals or ceramic resonators
(0.5MHz to 10.0MHz).
• Maximum power for use with high frequency crystals or ceramic resonators (8.0MHz
to 20.0MHz).
The oscillator mode is selected through user-programmable Option Bits. Refer to the
Option Bits chapter for information.
Crystal Oscillator Operation
Figure 40 illustrates a recommended configuration for connection with an external funda-
mental-mode, parallel-resonant crystal operating at 20MHz. Recommended 20MHz crys-
tal specifications are provided in Table 103. Resistor R1 is optional and limits total power
dissipation by the crystal. The printed circuit board layout must add no more than 4pF of
PS019915-1005
On-Chip Oscillator