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參數(shù)資料
| 型號： | COP8SCR9 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 8-Bit CMOS Flash Based Microcontroller with 32k Memory, Virtual EEPROM and Brownout(8位基于CMOS 閃速存儲器的帶32K存儲器，虛擬EEPROM和電壓過低復(fù)位的微控制器) |
| 中文描述： | 8位CMOS閃存為基礎(chǔ)的32K的內(nèi)存，虛擬EEPROM和欠壓（8位基于的CMOS閃速存儲器的帶32K的存儲器，微控制器虛擬的EEPROM和電壓過低復(fù)位的微控制器） |
| 文件頁數(shù)： | 26/68頁 |
| 文件大?。?/td> | 714K |
| 代理商： | COP8SCR9 |
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6.0 Timers
(Continued)
Timer interrupts prior to changing the value of the ITSEL bits
of the ITMR Register and then clear the T0PND bit before
attempting to synchronize operation to the IDLE Timer.
6.2 TIMER T1, TIMER T2, AND TIMER T3
The device has a set of three powerful timer/counter blocks,
T1, T2, and T3. Since T1, T2 and T3 are identical, except for
the high speed operation of T2 and T3, all comments are
equally applicable to any of the three timer blocks which will
be referred to as Tx. Differences between the timers will be
specifically noted.
Each timer block consists of a 16-bit timer, Tx, and two
supporting 16-bit autoreload/capture registers, RxA and
RxB. Each timer block has two pins associated with it, TxA
and TxB. The pin TxA supports I/O required by the timer
block, while the pin TxB is an input to the timer block. The
timer block has three operating modes: Processor Indepen-
dent PWM mode, External Event Counter mode, and Input
Capture mode.
The control bits TxC3, TxC2, and TxC1 allow selection of the
different modes of operation.
6.2.1 Timer Operating Speeds
Each of the Tx timers, except T1, have the ability to operate
at either the instruction cycle frequency (low speed) or the
internal clock frequency (MCLK). For 10 MHz CKI, the in-
struction cycle frequency is 2 MHz and the internal clock
frequency is 20 MHz. This feature is controlled by the High
Speed Timer Control Register, HSTCR. Its format is shown
below. To place a timer, Tx, in high speed mode, set the
appropriate TxHS bit to 1. For low speed operation, clear the
appropriate TxHS bit to 0. This register is cleared to 00 on
Reset.
HSTCR
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit
1
T3HS
Bit
0
T2HS
0
0
0
0
0
0
6.2.2 Mode 1. Processor Independent PWM Mode
One of the timer’s operating modes is the Processor Inde-
pendent PWM mode. In this mode, the timers generate a
“Processor Independent” PWM signal because once the
timer is set up, no more action is required from the CPU
which translates to less software overhead and greater
throughput. The user software services the timer block only
when the PWM parameters require updating. This capability
is provided by the fact that the timer has two separate 16-bit
reload registers. One of the reload registers contains the
“ON” time while the other holds the “OFF” time. By contrast,
a microcontroller that has only a single reload register re-
quires an additional software to update the reload value
(alternate between the on-time/off-time).
The timer can generate the PWM output with the width and
duty cycle controlled by the values stored in the reload
registers. The reload registers control the countdown values
and the reload values are automatically written into the timer
when it counts down through 0, generating interrupt on each
reload. Under software control and with minimal overhead,
the PWM outputs are useful in controlling motors, triacs, the
intensity of displays, and in providing inputs for data acqui-
sition and sine wave generators.
In this mode, the timer Tx counts down at a fixed rate of t
C
(T2 and T3 may be selected to operate from MCLK). Upon
every underflow the timer is alternately reloaded with the
contents of supporting registers, RxA and RxB. The very first
underflow of the timer causes the timer to reload from the
register RxA. Subsequent underflows cause the timer to be
reloaded from the registers alternately beginning with the
register RxB.
Figure 15shows a block diagram of the timer in PWM mode.
The underflows can be programmed to toggle the TxAoutput
pin. The underflows can also be programmed to generate
interrupts.
Underflows from the timer are alternately latched into two
pending flags, TxPNDA and TxPNDB. The user must reset
these pending flags under software control. Two control
enable flags, TxENA and TxENB, allow the interrupts from
the timer underflow to be enabled or disabled. Setting the
timer enable flag TxENA will cause an interrupt when a timer
underflow causes the RxA register to be reloaded into the
timer. Setting the timer enable flag TxENB will cause an
interrupt when a timer underflow causes the RxB register to
be reloaded into the timer. Resetting the timer enable flags
will disable the associated interrupts.
Either or both of the timer underflow interrupts may be
enabled. This gives the user the flexibility of interrupting
once per PWM period on either the rising or falling edge of
the PWM output. Alternatively, the user may choose to inter-
rupt on both edges of the PWM output.
6.2.3 Mode 2. External Event Counter Mode
This mode is quite similar to the processor independent
PWM mode described above. The main difference is that the
timer, Tx, is clocked by the input signal from the TxApin after
synchronization to the appropriate internal clock (t
or
MCLK). The Tx timer control bits, TxC3, TxC2 and TxC1
allow the timer to be clocked either on a positive or negative
edge from the TxApin. Underflows from the timer are latched
into the TxPNDApending flag. Setting the TxENAcontrol flag
will cause an interrupt when the timer underflows.
In this mode the input pin TxB can be used as an indepen-
dent positive edge sensitive interrupt input if the TxENB
control flag is set. The occurrence of a positive edge on the
TxB input pin is latched into the TxPNDB flag.
Figure 16 shows a block diagram of the timer in External
Event Counter mode.
Note: The PWM output is not available in this mode since the
TxA pin is being used as the counter input clock.
DS101389-19
FIGURE 15. Timer in PWM Mode
C
www.national.com
26
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