MC68HC908QT2CDW Freescale Semiconductor, MC68HC908QT2CDW Datasheet - Page 122



Manufacturer Part Number
Freescale Semiconductor

Specifications of MC68HC908QT2CDW

Core Processor
Core Size
Number Of I /o
Program Memory Size
1.5KB (1.5K x 8)
Program Memory Type
Ram Size
128 x 8
Voltage - Supply (vcc/vdd)
2.7 V ~ 5.5 V
Oscillator Type
Operating Temperature
-40°C ~ 85°C
Package / Case
8-SOIC (5.3mm Width), 8-SOP, 8-SOEIAJ
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Eeprom Size
Data Converters
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Timer Interface Module (TIM)
14.4.1 TIM Counter Prescaler
The TIM clock source is one of the seven prescaler outputs or the TIM clock pin, TCLK. The prescaler
generates seven clock rates from the internal bus clock. The prescaler select bits, PS[2:0], in the TIM
status and control register (TSC) select the TIM clock source.
14.4.2 Input Capture
With the input capture function, the TIM can capture the time at which an external event occurs. When an
active edge occurs on the pin of an input capture channel, the TIM latches the contents of the TIM counter
into the TIM channel registers, TCHxH:TCHxL. The polarity of the active edge is programmable. Input
captures can generate TIM central processor unit (CPU) interrupt requests.
14.4.3 Output Compare
With the output compare function, the TIM can generate a periodic pulse with a programmable polarity,
duration, and frequency. When the counter reaches the value in the registers of an output compare
channel, the TIM can set, clear, or toggle the channel pin. Output compares can generate TIM CPU
interrupt requests. Unbuffered Output Compare
Any output compare channel can generate unbuffered output compare pulses as described in
the new value over the old value currently in the TIM channel registers.
An unsynchronized write to the TIM channel registers to change an output compare value could cause
incorrect operation for up to two counter overflow periods. For example, writing a new value before the
counter reaches the old value but after the counter reaches the new value prevents any compare during
that counter overflow period. Also, using a TIM overflow interrupt routine to write a new, smaller output
compare value may cause the compare to be missed. The TIM may pass the new value before it is written.
Use the following methods to synchronize unbuffered changes in the output compare value on channel x: Buffered Output Compare
Channels 0 and 1 can be linked to form a buffered output compare channel whose output appears on the
TCH0 pin. The TIM channel registers of the linked pair alternately control the output.
Setting the MS0B bit in TIM channel 0 status and control register (TSC0) links channel 0 and channel 1.
The output compare value in the TIM channel 0 registers initially controls the output on the TCH0 pin.
Writing to the TIM channel 1 registers enables the TIM channel 1 registers to synchronously control the
output after the TIM overflows. At each subsequent overflow, the TIM channel registers (0 or 1) that
When changing to a smaller value, enable channel x output compare interrupts and write the new
value in the output compare interrupt routine. The output compare interrupt occurs at the end of
the current output compare pulse. The interrupt routine has until the end of the counter overflow
period to write the new value.
When changing to a larger output compare value, enable TIM overflow interrupts and write the new
value in the TIM overflow interrupt routine. The TIM overflow interrupt occurs at the end of the
current counter overflow period. Writing a larger value in an output compare interrupt routine (at
the end of the current pulse) could cause two output compares to occur in the same counter
overflow period.
Compare. The pulses are unbuffered because changing the output compare value requires writing
MC68HC908QY/QT Family Data Sheet, Rev. 6
Freescale Semiconductor

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