As the Master Transmits its Data > 자유게시판

The Serial Peripheral Interface, SPI, is a fast synchronous serial interface. If the programmer has enabled the local interrupt mask for the SPI, an interrupt is recognized at this point. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, respectively. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master. When it is low, the transceiver is in receive mode. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode.

InitRS485() configures Port D to ensure that bit 5 is an output. Pin 3 of PortA is the Serial2 input, and pin 4 of PortA is the Serial2 output. The arrows in the diagram point to pins configured as inputs, and originate from output pins. Table 9-6 shows the connection diagram for a standard 9-pin serial cable. Table 9-6 Serial Cable Connections. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. In some circumstances a one-way data flow may suffice. It is a half duplex protocol, meaning that only one party at a time may transmit data. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. SPIE is a local interrupt mask that allows an interrupt to be recognized when an SPI data transfer has completed, or if a write collision or mode fault is detected.

Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. Because the requirements of every multi-drop application are so unique, it is difficult to specify or design a software protocol that meets everyone’s needs. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. 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).

For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code! In fact, you have been using it all along as you worked through the examples in this document. If you have not yet compiled the GETSTART program and you want to do the exercises here, open GETSTART.C in your TextPad editor, click on the Make Tool, and after the compilation is done, enter Mosaic Terminal by clicking on the terminal icon and use the "Send File" menu item to send GETSTART.DLF to the QScreen Controller.

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