參數(shù)資料
型號: PIC16F77
廠商: Microchip Technology Inc.
英文描述: 40-Pin 8-Bit CMOS FLASH Microcontrollers(40腳、8位CMOS 閃速微控制器)
中文描述: 40引腳8位CMOS閃存微控制器(40腳,8位的CMOS閃速微控制器)
文件頁數(shù): 46/170頁
文件大?。?/td> 3150K
代理商: PIC16F77
PIC16F7X
DS30325A-page 46
Advance Information
2000 Microchip Technology Inc.
5.2
Using Timer0 with an External Clock
When no prescaler is used, the external clock input is
the same as the prescaler output. The synchronization
of T0CKI, with the internal phase clocks, is accom-
plished by sampling the prescaler output on the Q2 and
Q4 cycles of the internal phase clocks. Therefore, it is
necessary for T0CKI to be high for at least 2Tosc (and
a small RC delay of 20 ns) and low for at least 2Tosc
(and a small RC delay of 20 ns). Refer to the electrical
specification of the desired device.
5.3
Prescaler
There is only one prescaler available, which is mutually
exclusively shared between the Timer0 module and the
Watchdog Timer. A prescaler assignment for the
Timer0 module means that there is no prescaler for the
Watchdog Timer, and vice-versa. This prescaler is not
readable or writable (see Figure 5-1).
The PSA and PS2:PS0 bits (OPTION_REG<3:0>)
determine the prescaler assignment and prescale ratio.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g.
CLRF
1,
MOVWF
1,
BSF
1,x
....etc.) will clear the prescaler. When assigned
to WDT, a
CLRWDT
instruction will clear the prescaler
along with the Watchdog Timer. The prescaler is not
readable or writable.
REGISTER 5-1:
OPTION_REG REGISTER
Note:
Writing to TMR0 when the prescaler is
assigned to Timer0, will clear the prescaler
count but will not change the prescaler
assignment.
R/W-1
RBPU
bit 7
R/W-1
INTEDG
R/W-1
T0CS
R/W-1
T0SE
R/W-1
PSA
R/W-1
PS2
R/W-1
PS1
R/W-1
PS0
bit 0
bit 7
bit 6
bit 5
RBPU
INTEDG
T0CS:
TMR0 Clock Source Select bit
1
= Transition on T0CKI pin
0
= Internal instruction cycle clock (CLKOUT)
T0SE:
TMR0 Source Edge Select bit
1
= Increment on high-to-low transition on T0CKI pin
0
= Increment on low-to-high transition on T0CKI pin
PSA
: Prescaler Assignment bit
1
= Prescaler is assigned to the WDT
0
= Prescaler is assigned to the Timer0 module
PS2:PS0
: Prescaler Rate Select bits
bit 4
bit 3
bit 2-0
Legend:
R = Readable bit
- n = Value at POR reset
W = Writable bit
1
= Bit is set
U = Unimplemented bit, read as
0
0
= Bit is cleared
x = Bit is unknown
000
001
010
011
100
101
110
111
1 : 2
1 : 4
1 : 8
1 : 16
1 : 32
1 : 64
1 : 128
1 : 256
1 : 1
1 : 2
1 : 4
1 : 8
1 : 16
1 : 32
1 : 64
1 : 128
Bit Value
TMR0 Rate
WDT Rate
Note:
To avoid an unintended device RESET, the instruction sequence shown in the
PICmicro
Mid-Range MCU Family Reference Manual (DS33023) must be executed
when changing the prescaler assignment from Timer0 to the WDT. This sequence must
be followed even if the WDT is disabled.
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