Ch.12 Communication Systems Valvano’s text

12.1 Introduction SCI can be used to create a network. Figure 12.1 gives a general visualization. –At lowest level frames are transferred between I/O ports, along the physical link. –At the next level the operating system of one computer sends a message or “packet” to the OS of another computer. –The message protocol will specify types and formats of the messages.

12. 1 (cont.) Messages typically have –Addresses –Synchronization or handshake field –Data field –Error detection and correction field

12.1 (cont.) Highest level—communications between users, or high level software tasks.

12.2 Reentrant Programming and Critical Sections If two threads access the same global memory, and one is a write, then there is a causal dependency between the threads. –Shared global variables are important because they are required to pass data between threads. –Hard to find bugs.

12.2 (cont.) A program segment is reentrant if it can be concurrently executed by two or more threads. –Implementation of reentrant program segments avoids storing in global memory. –Also, variables are placed in registers or on the stack.

12.2 (cont.) A nonreentrant subroutine will have a section of code called a vulnerable window or critical section. Errors –(1)One thread calls the non-reentrant subroutine –(2) It is executing in the critical section when interrupted by a second thread. –(3) The second thread calls the same subroutine.

12.2 (cont.) Possibilities: –The second thread is allowed to complete the execution of the subroutine, then the first thread finishes the subroutine. –The second thread executes part of the subroutine, is interrupted, and then re-entered by a third thread; the third thread finishes, the second thread finishes, then control is passed to the first thread and it finishes.

12.2 (cont.) Program 12.1 illustrates a non-reentrant function, since num is a global variable used by the functions (in assembly and C). –(In C) short num; – short Ave (short first, short second){ – num = first; – return (num+second)/2; –This is non-reentrant because of the access to a global—num.

12.3 Interthread Communication and Synchronization A simple inter-thread scheme is a mailbox. Figure 12.2 illustrates an input device interfaced using interrupt synchronization.

Producer Consumer Problem First In First Out (FIFO) circular Queue and double buffer. Data Structures that can be used to link a source process (producer) with a sink process (consumer). Fig (page 440) Table 12.1 (examples of producers/consumers)

(cont.) A first in first out circular queue (FIFO) is useful for the implementation of a buffered I/O interface. After initialization, the queue “puts” and “gets” data. This allows the foreground thread to run while a second thread runs in the background.

A data flow graph is shown in Figure –There is buffered input and buffered output illustrated. –Valvano’s text—FIFO’s are implemented using statically allocated global structures. –An advantage of the FIFO is that the producer and consumer threads can be decoupled – this will improve system performance. –Without the FIFO, data will have to be processed one at a time.

Timing Let t p be the time (in sec.) between calls to Put, and r p be the arrival rate (in bytes/sec) into the system. Let t g be the time (in sec.) between calls to Get and r g be the service rate (in bytes/sec) out ot the system.

Timing (cont.) Then r g = 1/t g and r p = 1/t p. If min t p ≥ max t g, then a FIFO is not needed and a mailbox can be used. If the average input rate is larger than the average ouput rate then a FIFO will eventually overflow, no matter how large the FIFO is. But if r p is temporarily high or r g is temporarily low, (and the FIFO becomes full) then this problem can be solved by increasing the FIFO size.

Checkpoint 12.5 If the FIFO becomes full, can the situation be solved by increasing the size? Answer: –No, if the average producer rate exceeds the average consumer rate. –Yes, if the temporary producer rate exceeds the temporary consumer rate.

Figure 12.6 This shows FIFO queues that are used to pass data between threads. –OutChar is called by the main program when it wishes to output data. –InChar is called by the main program when it wishes to input data.

FIFO Queue Implementation Figure 12.7 FIFO implementation with infinite memory. Program 12.3 illustrates some code fragments. Figures 12.8,12.9 showing Put and Get operations with a pointer wrap.

12.3.4Double Buffer Figure shows the storing of data in one buffer while data is read from a second buffer and processed. A double buffer is two buffers of fixed size. –Example: Disk (see figure.) –Disk has fixed size blocks—buffers are the size as the blocks.

12.4 Serial Port Interface Using Interrupt Synchronization Example: connect to a PC serial port. –Baud rate must match. –FIFO’s are used. –Initially just the receiver is armed. –Transmitter will be armed when data is available within the SCI_OutChar routine. –Interrupt occurs when new data arrives in the receiver data register, and an interrupt occurs when the transmit data register is empty.

Program 12.5 Page 448, implementations of an interrupting SCI interface.

12.5 Distributed Systems Three simple communication systems using the SCI port. –If distances are short, half-duplex can be implemented with simple open collector or open-drain TTL-level logic.

System 1 Master-slave configuration –Master transmit output is connected to all slave receive inputs (Fig , page 449) –Master broadcasts commands. –Slaves can respond, one at a time. –When Master transmits, it is to all slaves. –If slaves only transmit after being triggered by the master, there will be no collisions.

System 2: Ring Network Transmitter and Receiver lines are connected in a circle. Figure 12.13—three microcomputers connected using the SCI in each. Simple to build but slow performance (response time and bandwidth). It is also difficult to add/subtract nodes.

System 3: Multi-drop Figure 12.14, page 451 –Software activates the SP483 Driver and outputs the frame. –Half-duplex—the frame can be echoed back. –Errors can also be discovered using parity checks or checksums.

Checkpoint How can the transmitter detect a collision had corrupted its output? ANSWER: The frame sent by a transmitter is echoed to its own receiver. If the data does not match, or if thre are any framing or noise errors then a collision has occurred.

Checkpoint 12.12: How can the receiver detect a collision had corrupted its input? ANSWER: Parity could be used to detect collisions. Also the message could have a checksum added. Framing or noise errors can also indicate a collision.

12.6 Design an Implementation of a Controller Area Network (CAN) CAN –A serial data communications bus –Used for real-time applications. –Operate at speeds up to 1 Mbps. –Error detection capabilities. –Developed by Robert Bosch for use in autos. –Now used in industrial automation and control applications.

12.6 CAN (cont.) Has been standardized—ISO Figure 12.15—CAN base system using 9S12’s. Can have up to 112 nodes. Four Components –(1) Bus—2-wires;CANH and CANL; termination is a 120ohm resistor. –(2) Transceiver—can transmit and receive on the same channel. –(3) CAN Controller –(4) Software; used to transmit and process data.

12.7 Integrated Circuit (I 2 C) Interface Historically, users want microcontrollers to require less power and space for embedded systems application. Two Ways –Integrate functionality into the microcontroller. –Reduce the number of I/O pins. I 2 C—proposed by Phillips in the 1980’s; originally use to interface devices using 2-wires v1 became an industry standard.

12.7 (cont.) Figure 12.19—shows network made of masters and slaves. –Both lines are bidirectional. –SCL—serial clock line. –SDA—serial data line.

12.8 Wireless Communications Design concerns –Bandwidth –Distance –Topology –Security Figure –Short Range Device –SPI interface is illustrated