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MC68F375
CONFIGURABLE TIMER MODULE (CTM9)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
13-11
bits to zero, thereby preventing a signal on the CTML pin from loading the counter reg-
ister until EDGEN and EDGEP have been initialized by the software. The modulus
load input pin CTML is Schmitt triggered and synchronized to the system clock (fSYS).
NOTE
The read-only IN1 bit of the MCSMSIC reflects the state of the input
pin CTML.
13.3.2.2 Using the MCSM as a Free-Running Counter
The MCSM is a modulus counter. However it can be made to behave like a free-run-
ning counter by loading the modulus register with the value 0x0000.
13.3.3 MCSM Clock Sources
The User can choose from eight software selectable counter clock sources:
Six prescaler outputs (PCLKx)
Input pin rising edge detection on the input pin CTMC
Input pin falling edge detection on the input pin CTMC
The clock source is selected by the CLK[2:0] bits in the MCSM status, interrupt and
Register). When the CLK[2:0] bits are being changed, internal circuitry ensures that
spurious edges occurring on the CTMC pin do not affect the MCSM. The clock input
pin CTMC is Schmitt triggered and is synchronized with the system clock (fSYS).
NOTE
The read-only IN2 bit of the MCSMSIC register reflects the state of
the input pin CTMC.
13.3.4 MCSM External Event Counting
When an external clock source (on the CTMC input pin) is selected, the MCSM is in
the event counter mode. The counter can simply count the number of events occurring
on the input pin. Alternatively, the MCSM can be programmed to generate an interrupt
when a predefined number of events have been counted; this is done by presetting the
counter with the two’s complement value of the desired number of events. When using
the external clock source, the maximum external guaranteed frequency is fSYS/4.
13.3.5 The MCSM Time Base Bus Driver
The DRVA and DRVB bits in the MCSMSIC register select the time base buses to be
13.3.6 MCSM interrupts
A valid MCSM interrupt can be generated when the COF bit in the MCSMSIC register
is set as a result of the counter overflowing. If the interrupt priority level of the MCSM
is non-zero, as defined by the three IL bits in the MCSMSIC register, a valid interrupt
request will occur on the IMB.
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Freescale Semiconductor, Inc.
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