Alternatively, the if the SPI interrupts are enabled, the SPI interrupt handler determines what caused the interrupt by reading the SPSR register to see which of the three status bits is set. Then reading the data that was received (by reading the SPDR) or initiating a new data transfer (by writing to the SPDR) automatically clears the SPIF flag. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. Setting SPE (SPI enable) to 1 turns on the SPI system. The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port. Setting the MSTR bit initializes the QScreen as a master, and clearing the MSTR bit initializes it as a slave.

If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, what is rs485 cable the /SS line may be tied low between successive transfers. The clock’s polarity is controlled by a bit named CPOL (clock polarity) and its phase is controlled by CPHA (clock phase). Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B). A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged.

A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.8-7) exists on the network as explained above in connection with the /SS input. This function properly configures the directions of the SPI I/O pins, and configures the data transfer such that data is valid on the falling trailing edge of the clock, with the clock idling in the low state. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. There are three flag bits implemented in the SPSR (SPI status register). After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. The device that initiates a data transfer is the master, and all other devices on the network are slaves. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device.

This ability to exchange messages means that the SPI is capable of full duplex communication. This means that the unit cannot initiate a communication, it can only respond when addressed by the master computer. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. The configuration and specifications of RS485 make it faster and extend the range of data transmission. For the actual value, please refer to the wire specifications. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application. If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. The maximum sustainable baud rate on the secondary serial port is 4800 baud. You might also consider operating the secondary serial port at a lower baud rate to relax the timing constraints.