Z8F082ASH020SC Datasheet, PDF (183/244 Pages) Zilog, Inc. – Z8 Encore XP-R F08xA Series with eXtended Peripherals

Z8 Encore! XP^® F08xA Series
Product Specification
165
Crystal Oscillator
Overview
Note:
The products in the Z8 Encore! XP^® F08xA Series contain an on-chip crystal oscillator for
use with external crystals with 32KHz to 20MHz frequencies. In addition, the oscillator
supports external RC networks with oscillation frequencies up to 4MHz or ceramic reso-
nators with frequencies up to 8MHz. The on-chip crystal oscillator can be used to generate
the primary system clock for the internal eZ8 CPU and the majority of the on-chip periph-
erals. 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 uncon-
nected. The Z8 Encore! XP^® F08xA Series products do not contain an internal clock
divider. The frequency of the signal on the X_IN input pin determines the frequency of the
system clock.
Although the XIN pin can be used as an input for an external clock generator, the CLKIN
pin is better suited for such use (See “System Clock Selection” on page 160.)
Operating Modes
The Z8 Encore! XP^® F08xA Series products support four oscillator modes:
• Minimum power for use with very low frequency crystals (32KHz–1MHz)
• Medium power for use with medium frequency crystals or ceramic resonators
(0.5MHz to 8MHz)
• Maximum power for use with high frequency crystals (8MHz to 20MHz)
• On-chip oscillator configured for use with external RC networks (<4MHz)
The oscillator mode is selected using user-programmable Flash Option Bits. Please refer
to the chapter “Flash Option Bits” on page 138 for information.
Crystal Oscillator Operation
The Flash Option bit XTLDIS controls whether the crystal oscillator is enabled during
reset. The crystal may later be disabled after reset if a new oscillator has been selected as
the system clock. If the crystal is manually enabled after reset through the OSCCTL regis-
ter, the user code must wait at least 1000 crystal oscillator cycles for the crystal to stabi-
lize. After this, the crystal oscillator may be selected as the system clock.
Figure 25 illustrates a recommended configuration for connection with an external funda-
mental-mode, parallel-resonant crystal operating at 20MHz. Recommended 20MHz crys-
PS024705-0405
PRELIMINARY
Crystal Oscillator