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PIC16F877T-20PQ 参数 Datasheet PDF下载

PIC16F877T-20PQ图片预览
型号: PIC16F877T-20PQ
PDF下载: 下载PDF文件 查看货源
内容描述: 8位微控制器\n [8-Bit Microcontroller ]
分类和应用: 微控制器
文件页数/大小: 200 页 / 3338 K
品牌: ETC [ ETC ]
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PIC16F87X  
module means that there is no prescaler for the watch-  
dog timer, and vice-versa. This prescaler is not readable  
or writable (see Figure 5-1).  
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.  
The PSA and PS2:PS0 bits (OPTION_REG<3:0>) deter-  
mine the prescaler assignment and prescale ratio.  
When assigned to the Timer0 module, all instructions  
writing to the TMR0 register (e.g. CLRF1, MOVWF1,  
BSF1,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.  
5.3  
Prescaler  
Note: Writing to TMR0, when the prescaler is  
assigned to Timer0, will clear the prescaler  
count, but will not change the prescaler  
assignment.  
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  
REGISTER 5-1: OPTION_REG REGISTER  
R/W-1  
RBPU  
R/W-1  
R/W-1  
T0CS  
R/W-1  
T0SE  
R/W-1 R/W-1 R/W-1  
PSA PS2 PS1  
R/W-1  
PS0  
R
= Readable bit  
INTEDG  
W = Writable bit  
U
bit 7  
bit 0  
= Unimplemented bit,  
read as ‘0’  
- n = Value at POR reset  
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)  
bit 4:  
bit 3:  
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  
bit 2-0: PS2:PS0: Prescaler Rate Select bits  
Bit Value  
TMR0 Rate WDT Rate  
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  
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.  
DS30292B-page 48  
1999 Microchip Technology Inc.  
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