The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. 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 you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). 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. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. Bus Probes can be configured and tested using our web-based test app (you will need to connect the probes to your PC or Android smartphone via our Web485 board). When the keyword name is received by the Silence() routine running in the slave, the slave QScreen Controller executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment).
These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. 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. 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). 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. You decide that the water is not filling fast enough to keep up with demand. Let’s say you were monitoring a water pump filling a vessel. All of this allows you to speed up the VFD that controls the speed of that water pump. When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave". Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device.
A data transfer is initiated by a master device when it stores a message byte into its SPDR register. If the /SS pin of the master is an input and if a low input level is detected, the processor sets the MODF bit in the SPI status register a "mode fault" condition. The AVR1284-3U is a development board for 40 pin AVR microcontrollers. There are many possible configurations of master/slave networks. All headers are on a 0.1" grid to allow accessory proto-boards to span multiple connectors if needed. The power input connector can be a coaxial barrel connector, a 2-pin terminal block, or a MTA-100 connector or 0.1" spacing header. 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. 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. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate. Once the bytes have been exchanged, the master may write a new byte to initiate another byte exchange.
Although data byte transfers are easily executed once the network has been wired and configured properly, a carefully executed software protocol may be required to ensure data integrity. The BOM (bill of materials) and schematic diagram are available below to construct the circuit. The arrows in the diagram point to pins configured as inputs, and originate from output pins. Thus, the master has only one input, MISO, which is the slave’s only output. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. Note that the data is changed by the transmitting device one half clock cycle before it is valid. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal.
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