mc68hc05l25 Freescale Semiconductor, Inc, mc68hc05l25 Datasheet - Page 81

• Manufacturer Part Number: mc68hc05l25
• Description: M68hc05 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

STUP — XOSC Time Up Flag
Bits 5 and 4 — Reserved
SYS1 and SYS0 — System Clock Select
FOSCE — Fast (Main) Oscillator Enable
OPTM — Option Map Select
Freescale Semiconductor
The power-on detection clears this bit. This bit is set after the time base has counted 16,264 clocks. A
reset does not affect this bit.
Read:
These bits are not used and always read as zero.
These two bits select the system clock source. On reset the SYS1 and SYS0 bits are initialized to zero
and zero, respectively.
The FOSCE bit controls the main oscillator activity. This bit should not be cleared by the CPU when
the main oscillator is selected as the system clock source. This bit is set on reset.
The OPTM bit selects one of two register maps at $0000–$000F. This bit is cleared on reset.
1 = XOSC clock available for the system clock
0 = XOSC is not stabilized or no signal on XOSC1 and XOSC2 pins
1 = When this bit is set:
0 = When this bit is cleared:
1 = Option map selected
0 = Main register map selected
1. The main oscillator starts again.
2. The FTUP flag is set by the POR counter overflow (8072 clocks) and OSC is ready to be
1. OSC is shut down.
2. The 7-bit dividers at the OSC input and POR counter are initialized to $0078.
3. The FTUP flag is cleared.
SY
S1
Do not switch the system clock to XOSC (SYS1–SYS0 = 11) when the
XOSC clock is not available. The XOSC clock is available when the STUP
flag is set.
Do not switch the system clock to OSC (SYS1–SYS0 = 00, 01, or 10) when
the OSC clock is not available. OSC clock is available when the FTUP flag
is set.
used as the system and time base clocks.
0
0
1
1
SY
S0
0
1
0
1
XOSC Divided by 2
OSC Divided by 64
OSC Divided by 2
OSC Divided by 4
Table 8-4. System Bus Frequency Selection
Divide Ratio
(Default)
MC68HC05L25 Data Sheet, Rev. 3.1
OSC = 4.0 M
62.5 k
2.0 M
1.0 M
NOTE
—
φ2 CPU Bus Frequency (Hz)
OSC = 4.1943 M
2.0972 M
1.0486 M
65.536 k
—
XOSC = 32.768 k
16.384 k
—
—
—
Miscellaneous Register
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