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PIC16F627-04 Datasheet(PDF) 84 Page - Microchip Technology |
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PIC16F627-04 Datasheet(HTML) 84 Page - Microchip Technology |
84 / 160 page PIC16F62X DS40300B-page 84 Preliminary © 1999 Microchip Technology Inc. TABLE 12-1: REGISTERS ASSOCIATED WITH ASYNCHRONOUS RECEPTION Addr Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Value on POR Value on all other Resets 0Ch PIR1 EEIF CMIF RCIF TXIF — CCP1IF TMR2IF TMR1IF 0000 -000 0000 -000 18h RCSTA SPEN RX9 SREN CREN ADEN FERR OERR RX9D 0000 -00x 0000 -00x 1Ah RCREG RX7 RX6 RX5 RX4 RX3 RX2 RX1 RX0 0000 0000 0000 0000 8Ch PIE1 EEIE CMIE RCIE TXIE — CCP1IE TMR2IE TMR1IE 0000 -000 0000 -000 98h TXSTA CSRC TX9 TXEN SYNC —BRGH TRMT TX9D 0000 -010 0000 -010 99h SPBRG Baud Rate Generator Register 0000 0000 0000 0000 Legend: x = unknown, - = unimplemented locations read as '0'. Shaded cells are not used for Asynchronous Reception. 12.4 USART Synchronous Master Mode In Synchronous Master mode, the data is transmitted in a half-duplex manner, i.e. transmission and reception do not occur at the same time. When transmitting data, the reception is inhibited and vice versa. Synchronous mode is entered by setting bit SYNC (TXSTA<4>). In addition enable bit SPEN (RCSTA<7>) is set in order to configure the RB2/TX/CK and RB1/RX/DT I/O pins to CK (clock) and DT (data) lines respectively. The Master mode indicates that the processor transmits the master clock on the CK line. The Master mode is entered by setting bit CSRC (TXSTA<7>). 12.4.1 USART SYNCHRONOUS MASTER TRANSMISSION The USART transmitter block diagram is shown in Figure 12-5. The heart of the transmitter is the transmit (serial) shift register (TSR). The shift register obtains its data from the read/write transmit buffer register TXREG. The TXREG register is loaded with data in software. The TSR register is not loaded until the last bit has been transmitted from the previous load. As soon as the last bit is transmitted, the TSR is loaded with new data from the TXREG (if available). Once the TXREG register transfers the data to the TSR register (occurs in one Tcycle), the TXREG is empty and inter- rupt bit, TXIF (PIR1<4>) is set. The 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. TRMT is a read only bit which is set when the TSR is empty. No inter- rupt logic is tied to this bit, so the user has to poll this bit in order to determine if the TSR register is empty. The TSR is not mapped in data memory so it is not available to the user. 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. The first data bit will be shifted out on the next available rising edge of the clock on the CK line. Data out is sta- ble around the falling edge of the synchronous clock (Figure 12-12). The transmission can also be started by first loading the TXREG register and then setting bit TXEN (Figure 12-13). This is advantageous when slow baud rates are selected, since the BRG is kept in reset when bits TXEN, CREN, and SREN are clear. Setting enable bit TXEN will start the BRG, creating a shift clock immediately. Normally when transmission is first started, the TSR register is empty, so a transfer to the TXREG register will result in an immediate transfer to TSR resulting in an empty TXREG. Back-to-back trans- fers are possible. Clearing enable bit TXEN, during a transmission, will cause the transmission to be aborted and will reset the transmitter. The DT and CK pins will revert to hi-imped- ance. If either bit CREN or bit SREN is set, during a transmission, the transmission is aborted and the DT pin reverts to a hi-impedance state (for a reception). The CK pin will remain an output if bit CSRC is set (internal clock). The transmitter logic however is not reset although it is disconnected from the pins. In order to reset the transmitter, the user has to clear bit TXEN. If bit SREN is set (to interrupt an on-going transmission and receive a single word), then after the single word is received, bit SREN will be cleared and the serial port will revert back to transmitting since bit TXEN is still set. The DT line will immediately switch from hi-impedance receive mode to transmit and start driving. To avoid this, bit TXEN should be cleared. In order to select 9-bit transmission, the TX9 (TXSTA<6>) bit should be set and the ninth bit should be written to bit TX9D (TXSTA<0>). The ninth bit must be written before writing the 8-bit data to the TXREG register. This is because a data write to the TXREG can result in an immediate transfer of the data to the TSR register (if the TSR is empty). If the TSR was empty and the TXREG was written before writing the “new” TX9D, the “present” value of bit TX9D is loaded. |
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