參數(shù)資料
型號(hào): MC68HC05CL48
廠商: FREESCALE SEMICONDUCTOR INC
元件分類: 微控制器/微處理器
英文描述: 8-BIT, MROM, 1.802 MHz, MICROCONTROLLER, PQFP112
封裝: TQFP-112
文件頁數(shù): 98/145頁
文件大?。?/td> 673K
代理商: MC68HC05CL48
GENERAL RELEASE SPECIFICATION
June 11, 1997
MOTOROLA
TIMER
MC68HC05CL48
8-4
REV 2.0
counter or counter alternate register rst addresses the most signicant byte
(MSB) ($19, $1B), the LSB ($1A, $1C) is transferred to a buffer. This buffer value
remains xed after the rst MSB read, even if the user reads the MSB several
times. This buffer is accessed when reading the free-running counter or counter
alternate register LSB ($1A or $1C) and, thus, completes a read sequence of the
total counter value. In reading either the free-running counter or counter alternate
register, if the MSB is read, the LSB must also be read to complete the sequence.
The counter alternate register differs from the counter register in one respect: a
read of the counter register MSB can clear the timer overow ag (TOF).
Therefore, the counter alternate register can be read at any time without the
possibility of missing timer overow interrupts due to clearing of the TOF.
Figure 8-2. Timer State Diagram For Timer Overow
The free-running counter is congured to $FFFC during reset and is always a
read-only register. During a power-on reset, the counter is also preset to $FFFC
and begins running after the oscillator start-up delay. Because the free-running
counter is 16 bits preceded by a xed divide-by-four prescaler, the value in the
free-running counter repeats every 262,144 internal bus clock cycles. When the
counter rolls over from $FFFF to $0000, the TOF bit is set. An interrupt can also
be enabled when counter roll over occurs by setting its interrupt enable bit (TOIE).
In some particular timing control applications it may be desirable to reset the 16-
bit free running counter under software control. When the low byte of the counter
($1A or $1C) is written to, the counter is congured to its reset value ($FFFC).
Internal Processor Clock
Internal
Timer
Clocks
T00
T01
T10
T11
Counter (16 bit)
Timer Overow Flag (TOF)
$FFFE
$FFFF
$0000
$0001
NOTE: The TOF bit is set at timer state T11 (transition of counter from $FFFF to $0000). It is
cleared by read of the Timer Status Register during the internal processor clock time
followed by a read of the counter low register.
$0002
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