参数资料
型号: PIC16F870-E/SP
厂商: Microchip Technology
文件页数: 87/143页
文件大小: 0K
描述: IC MCU CMOS 20MHZ 2K FLASH 28DIP
产品培训模块: Asynchronous Stimulus
标准包装: 15
系列: PIC® 16F
核心处理器: PIC
芯体尺寸: 8-位
速度: 20MHz
连通性: UART/USART
外围设备: 欠压检测/复位,POR,PWM,WDT
输入/输出数: 22
程序存储器容量: 3.5KB(2K x 14)
程序存储器类型: 闪存
EEPROM 大小: 64 x 8
RAM 容量: 128 x 8
电压 - 电源 (Vcc/Vdd): 4 V ~ 5.5 V
数据转换器: A/D 5x10b
振荡器型: 外部
工作温度: -40°C ~ 125°C
封装/外壳: 28-DIP(0.300",7.62mm)
包装: 管件
配用: I3-DB16F871-ND - BOARD DAUGHTER ICEPIC3
PIC16F870/871
DS30569B-page 46
2003 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 2 TOSC (and
a small RC delay of 20 ns) and low for at least 2 TOSC
(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., 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.
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
INTEDG
T0CS
T0SE
PSA
PS2
PS1
PS0
bit 7
bit 0
bit 7
RBPU
bit 6
INTEDG
bit 5
T0CS: TMR0 Clock Source Select bit
1
= Transition on T0CKI pin
0
= Internal instruction cycle clock (CLKO)
bit 4
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
bit 3
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
Legend:
R = Readable bit
W = Writable bit
U = Unimplemented bit, read as ‘0’
- n = Value at POR
‘1’ = Bit is set
‘0’ = Bit is cleared
x = Bit is unknown
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.
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
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