参数资料
型号: MC68HC705JB2DW
厂商: FREESCALE SEMICONDUCTOR INC
元件分类: 微控制器/微处理器
英文描述: 8-BIT, OTPROM, 3 MHz, MICROCONTROLLER, PDSO20
封装: SOIC-20
文件页数: 87/128页
文件大小: 2493K
代理商: MC68HC705JB2DW
August 28, 1998
GENERAL RELEASE SPECIFICATION
PROGRAMMABLE TIMER
REV 1.1
9-3
The timer registers (TMRH, TMRL) shown in Figure 9-3 are read-only locations
which contain the current high and low bytes of the 16-bit free-running counter.
Writing to the timer registers has no effect. Reset of the device presets the timer
counter to $FFFC.
The TMRL latch is a transparent read of the LSB until the a read of the TMRH
takes place. A read of the TMRH latches the LSB into the TMRL location until the
TMRL is again read. The latched value remains xed even if multiple reads of the
TMRH take place before the next read of the TMRL. Therefore, when reading the
MSB of the timer at TMRH the LSB of the timer at TMRL must also be read to
complete the read sequence.
During power-on-reset (POR), the counter is initialized to $FFFC and begins
counting after the oscillator start-up delay. Because the counter is sixteen bits and
preceded by a xed divide-by-four prescaler, the value in the counter repeats
every 262, 144 internal bus clock cycles (524, 288 oscillator cycles).
When the free-running counter rolls over from $FFFF to $0000, the timer overow
ag bit (TOF) is set in the TSR. When the TOF is set, it can generate an interrupt if
the timer overow interrupt enable bit (TOIE) is also set in the TCR. The TOF ag
bit can only be reset by reading the TMRL after reading the TSR.
Other than clearing any possible TOF ags, reading the TMRH and TMRL in any
order or any number of times does not have any effect on the 16-bit free-running
counter.
NOTE
To prevent interrupts from occurring between readings of the TMRH
and TMRL, set the I bit in the condition code register (CCR) before
reading TMRH and clear the I bit after reading TMRL.
9.2
ALTERNATE COUNTER REGISTERS (ACRH, ACRL)
The functional block diagram of the 16-bit free-running timer counter and alternate
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
TMRH
R
TMRH7
TMRH6
TMRH5
TMRH4
TMRH3
TMRH2
TMRH1
TMRH0
$0018
W
reset:
11111111
TMRL
R
TMRL7
TMRL6
TMRL5
TMRL4
TMRL3
TMRL2
TMRL1
TMRL0
$0019
W
reset:
11111100
Figure 9-3. Programmable Timer Counter Registers (TMRH, TMRL)
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For More Information On This Product,
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相关PDF资料
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