Datasheet

Configuration Register (CONFIG)
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 Data Sheet, Rev. 2.1
48 Freescale Semiconductor
EXTCLKEN — External Clock Enable Bit
EXTCLKEN enables an external clock source or crystal/ceramic resonator to be used as a clock input.
Setting this bit enables PTB6/(OSC1) pin to be a clock input pin. Clearing this bit (default setting) allows
the PTB6/(OSC1) and PTB7/(OSC2)/RST
pins to function as a general-purpose input/output (I/O) pin.
Refer to Table 4-1 for configuration options for the external source. See Chapter 7 Internal Clock
Generator Module (ICG) for a more detailed description of the external clock operation.
1 = Allows PTB6/(OSC1) to be an external clock connection
0 = PTB6/(OSC1) and PTB7/(OSC2)/RST
function as I/O port pins (default).
EXTSLOW — Slow External Crystal Enable Bit
The EXTSLOW bit has two functions. It configures the ICG module for a fast (1 MHz to 8 MHz) or slow
(30 kHz to 100 kHz) speed crystal. The option also configures the clock monitor operation in the ICG
module to expect an external frequency higher (307.2 kHz to 32 MHz) or lower (60 Hz to 307.2 kHz)
than the base frequency of the internal oscillator. See Chapter 7 Internal Clock Generator Module
(ICG).
1 = ICG set for slow external crystal operation
0 = ICG set for fast external crystal operation
EXTXTALEN — External Crystal Enable Bit
EXTXTALEN enables the external oscillator circuits to be configured for a crystal configuration where
the PTB6/(OSC1) and PTB7/(OSC2)/RST
pins are the connections for an external crystal.
NOTE
This bit does not function without setting the EXTCLKEN bit also.
Address: $001F
Bit 7654321Bit 0
Read:
COPRS LVISTOP LVIRSTD LVIPWRD LVI5OR3
(1)
SSREC STOP COPD
Write:
Reset:00000000
Other Resets:0000U000
1. The LVI5OR3 bit is cleared only by a power-on reset (POR).
U = Unaffected
Figure 4-2. Configuration Register 1 (CONFIG1)
Table 4-1. External Clock Option Settings
External Clock
Configuration Bits
Pin Function
Description
EXTCLKEN EXTXTALEN
PTB6/(OSC1) PTB7/(OSC2)/RST
00
PTB6 PTB7
Default setting — external oscillator disabled
01
PTB6 PTB7
External oscillator disabled since EXTCLKEN not set
10
OSC1 PTB7
External oscillator configured for an external clock
source input (square wave) on OSC1
11
OSC1 OSC2
External oscillator configured for an external crystal
configuration on OSC1 and OSC2. System will also
operate with square-wave clock source in OSC1.