參數(shù)資料
型號(hào): PIC16F876-20I/SP
廠商: Microchip Technology
文件頁數(shù): 4/218頁
文件大小: 0K
描述: IC MCU FLASH 8KX14 EE 28DIP
產(chǎn)品培訓(xùn)模塊: Asynchronous Stimulus
8-bit PIC® Microcontroller Portfolio
標(biāo)準(zhǔn)包裝: 15
系列: PIC® 16F
核心處理器: PIC
芯體尺寸: 8-位
速度: 20MHz
連通性: I²C,SPI,UART/USART
外圍設(shè)備: 欠壓檢測(cè)/復(fù)位,POR,PWM,WDT
輸入/輸出數(shù): 22
程序存儲(chǔ)器容量: 14KB(8K x 14)
程序存儲(chǔ)器類型: 閃存
EEPROM 大?。?/td> 256 x 8
RAM 容量: 368 x 8
電壓 - 電源 (Vcc/Vdd): 4 V ~ 5.5 V
數(shù)據(jù)轉(zhuǎn)換器: A/D 5x10b
振蕩器型: 外部
工作溫度: -40°C ~ 85°C
封裝/外殼: 28-DIP(0.300",7.62mm)
包裝: 管件
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2001 Microchip Technology Inc.
DS30292C-page 99
PIC16F87X
10.2
USART Asynchronous Mode
In this mode, the USART uses standard non-return-to-
zero (NRZ) format (one START bit, eight or nine data
bits, and one STOP bit). The most common data format
is 8-bits. An on-chip, dedicated, 8-bit baud rate gener-
ator can be used to derive standard baud rate frequen-
cies from the oscillator. The USART transmits and
receives the LSb first. The transmitter and receiver are
functionally independent, but use the same data format
and baud rate. The baud rate generator produces a
clock, either x16 or x64 of the bit shift rate, depending
on bit BRGH (TXSTA<2>). Parity is not supported by
the hardware, but can be implemented in software (and
stored as the ninth data bit). Asynchronous mode is
stopped during SLEEP.
Asynchronous mode is selected by clearing bit SYNC
(TXSTA<4>).
The USART Asynchronous module consists of the fol-
lowing important elements:
Baud Rate Generator
Sampling Circuit
Asynchronous Transmitter
Asynchronous Receiver
10.2.1
USART ASYNCHRONOUS
TRANSMITTER
The USART transmitter block diagram is shown in
Figure 10-1. The heart of the transmitter is the transmit
(serial) shift register (TSR). The shift register obtains its
data from the read/write transmit buffer, TXREG. The
TXREG register is loaded with data in software. The
TSR register is not loaded until the STOP bit has been
transmitted from the previous load. As soon as the
STOP bit is transmitted, the TSR is loaded with new
data from the TXREG register (if available). Once the
TXREG register transfers the data to the TSR register
(occurs in one TCY), the TXREG register is empty and
flag bit TXIF (PIR1<4>) is set. This interrupt can be
enabled/disabled by setting/clearing enable bit TXIE
( PIE1<4>). Flag bit TXIF will be set, regardless of the
state of enable bit TXIE and cannot be cleared in soft-
ware. It will reset only when new data is loaded into the
TXREG register. While flag bit TXIF indicates the status
of the TXREG register, another bit TRMT (TXSTA<1>)
shows the status of the TSR register. Status bit TRMT
is a read only bit, which is set when the TSR register is
empty. No interrupt logic is tied to this bit, so the user
has to poll this bit in order to determine if the TSR reg-
ister is empty.
Transmission is enabled by setting enable bit TXEN
(TXSTA<5>). The actual transmission will not occur
until the TXREG register has been loaded with data
and the baud rate generator (BRG) has produced a
shift clock (Figure 10-2). The transmission can also be
started by first loading the TXREG register and then
setting enable bit TXEN. Normally, when transmission
is first started, the TSR register is empty. At that point,
transfer to the TXREG register will result in an immedi-
ate transfer to TSR, resulting in an empty TXREG. A
back-to-back transfer is thus possible (Figure 10-3).
Clearing enable bit TXEN during a transmission will
cause the transmission to be aborted and will reset the
transmitter. As a result, the RC6/TX/CK pin will revert
to hi-impedance.
In order to select 9-bit transmission, transmit bit TX9
(TXSTA<6>) should be set and the ninth bit should be
written to TX9D (TXSTA<0>). The ninth bit must be
written before writing the 8-bit data to the TXREG reg-
ister. This is because a data write to the TXREG regis-
ter can result in an immediate transfer of the data to the
TSR register (if the TSR is empty). In such a case, an
incorrect ninth data bit may be loaded in the TSR
register.
FIGURE 10-1:
USART TRANSMIT BLOCK DIAGRAM
Note 1: The TSR register is not mapped in data
memory, so it is not available to the user.
2: Flag bit TXIF is set when enable bit TXEN
is set. TXIF is cleared by loading TXREG.
TXIF
TXIE
Interrupt
TXEN
Baud Rate CLK
SPBRG
Baud Rate Generator
TX9D
MSb
LSb
Data Bus
TXREG Register
TSR Register
(8)
0
TX9
TRMT
SPEN
RC6/TX/CK pin
Pin Buffer
and Control
8
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