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MC908QC8CDRE Datasheet(PDF) 99 Page - Freescale Semiconductor, Inc
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MC908QC8CDRE Datasheet(HTML) 99 Page - Freescale Semiconductor, Inc
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MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 5
10.3.1.2 XTAL Oscillator Clock (XTALCLK)
XTALCLK is the XTAL oscillator output signal. It runs at the full speed of the crystal (f
) and comes
directly from the crystal oscillator circuit. Figure 10-2 shows only the logical relation of XTALCLK to OSC1
and OSC2 and may not represent the actual circuitry. The duty cycle of XTALCLK is unknown and may
depend on the crystal and other external factors. The frequency of XTALCLK can be unstable at start up.
10.3.1.3 RC Oscillator Clock (RCCLK)
RCCLK is the RC oscillator output signal. Its frequency is directly proportional to the time constant of the
external R (R
) and internal C. Figure 10-3 shows only the logical relation of RCCLK to OSC1 and may
not represent the actual circuitry.
10.3.1.4 Internal Oscillator Clock (INTCLK)
INTCLK is the internal oscillator output signal. INTCLK is software selectable to be nominally 25.6 MHz,
12.8 MHz, 8.0 MHz, or 4.0 MHz. INTCLK can be digitally adjusted using the oscillator trimming feature of
the OSCTRIM register (see 10.3.2.1 Internal Oscillator Trimming).
10.3.1.5 Bus Clock Times 4 (BUSCLKX4)
BUSCLKX4 is the same frequency as the input clock (XTALCLK, RCCLK, or INTCLK). This signal is
driven to the SIM module and is used during recovery from reset and stop and is the clock source for the
10.3.1.6 Bus Clock Times 2 (BUSCLKX2)
The frequency of this signal is equal to half of the BUSCLKX4. This signal is driven to the SIM for
generation of the bus clocks used by the CPU and other modules on the MCU. BUSCLKX2 will be divided
by two in the SIM. The internal bus frequency is one fourth of the XTALCLK, RCCLK, or INTCLK
10.3.2 Internal Oscillator
The internal oscillator circuit is designed for use with no external components to provide a clock source
with a tolerance of less than 25%untrimmed. An 8-bit register (OSCTRIM) allows the digital adjustment
to a tolerance of ACC
. See the oscillator characteristics in the Electrical section of this data sheet.
The internal oscillator is capable of generating clocks of 25.6 MHz, 12.8 MHz, 8.0 MHz, or 4.0 MHz
(INTCLK) resulting in a bus frequency (INTCLK divided by 4) of 6.4 MHz, 3.2 MHz, 2.0 MHz, or 1.0 MHz
respectively. The bus clock is software selectable and defaults to the 1.0-MHz bus out of reset. Users can
increase the bus frequency based on the voltage range of their application.
Figure 10-3 shows how BUSCLKX4 is derived from INTCLK and OSC2 can output BUSCLKX4 by setting
10.3.2.1 Internal Oscillator Trimming
OSCTRIM allows a clock period adjustment of +127 and –128 steps. Increasing the OSCTRIM value
increases the clock period, which decreases the clock frequency. Trimming allows the internal clock
frequency to be fine tuned to the target frequency.
All devices are factory programmed with a trim value that is stored in FLASH memory at location $FFC0.
This trim value is not automatically loaded into OSCTRIM register. User software must copy the trim value
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