CSCI1600: Embedded and Real Time Software Lecture 16: Advanced Programming with I/O Steven Reiss, Fall 2015

Problem to Consider  Keypad for entering alarm code  Task to read the keypad  Determine when a button has been pushed  Want to check a sequence of button presses against known code(s)  Task that does the checking  How do these communicate?

Making Communication Simple

Handshake Communication  The previous depended on timing (or sequential tasks)  What should we do when tasks need to synchronize  Task A sends a request to Task B  Task A needs to know when Task B is done  In order to send another request  Ensure no new button is pressed until current one is processed  Can use 2 variables rather than one  Task A owns variable REQ  Task B owns variable ACK

Handshake Communication  Code  A: set Data, set REQ, wait for ACK, clear REQ, wait for ~ACK  B: wait for REQ, read Data, set ACK, wait for ~REQ, clear ACK  Sequence  A: set Data, set REQ  B: wait for REQ, read Data, set ACK  A: wait for ACK, clear REQ, wait for ~ACK  B: wait for ~REQ, clear ACK

Communicating Arduinos  Suppose you want to have two Arduinos talk to each other  Just simple message back and forth (sequence of bits)  How would you do this?

Communication: Queues  More complex communication involves multiple requests  Model railroad: multiple switches can be triggered  But only one can be activated at a time  This is generally implemented as a queue  Allows the two tasks to be asynchronous

Queue-based Tasks

Queue Implementation  Queues in embedded programs  Are generally fixed in size  Often sized so they never overflow  Or data is discarded if they would  How to program a queue?

Simple Queue byte queue[10]; int qp = 0; void push(byte x) [ TASK A ] if qp < 10, then queue[qp++] = x else exception byte pop() [ TASK B ] if (qp == 0) exception rslt = queue[0]; for (i = 1; i < qp; ++i) queue[i-1] = queue[i] qp = qp-1 return rslt  What’s wrong with this?

Circular Buffer Queue  Use a single array  Maintain a start and end pointer  Treat the array as a circular buffer  Element beyond the last is the first, etc.  Maintain two pointers  Head (read): pointer to the first element to extract  Tail (write): pointer to the next place to insert

Circular Buffer  Pop:  if read == write then exception  result = buf[read];  read = (read + 1) % size

Circular Buffer  Push(data):  If read == write then exception  buf[write] = data  write = (write + 1)%size

Circular Buffer  Advantages  Little data movement  Pointers are only changed by one task  Queue cells are safely read/written (why?)  Code is simpler  Extensions  Can read/write multiple things at once  Data stream or communications channel

Circular Buffers and Handshakes  What happens with a circular buffer of size one?

Multiple Inputs on a Port  How to multiplex a port with different inputs  Generally unsafe to connect multiple sensors at once  If sensor is at ground, another sensor at high might ground thru the first, not through the input port  Then the input would always be low  To get around this  Switches: only provide power/ground to enabled ones  Other logic circuits/devices: Open collector

Open Collector Circuit  Add a transistor to circuit  Acts as a switch  Effectively unconnected if off  External pull up makes it high  Grounded if on  Result is low  Can connect multiple  Effectively an OR gate  Many ICs have this built in  Hence negated outputs

Serial Communication  Send data from one machine to another  Using only 2 wires  Using clocked serial input  10 bits to send 8  Plus parity?

UART: Hardware for Serial I/O  Accepts bytes in parallel from the CPU  Sends these serially at fixed rate  Reads bytes from the serial line  Tells CPU when a byte is ready to read  Includes lines to ensure synchronization  Ready to transmit, …

UART Internals

Interrupts  Interrupts allow occasional inputs that aren’t polled  Limited set of pins support interrupts  Varies based on arduino model, for example  attachInterrupt(pin,routine,mode)  routine – pointer to function (function name in C)  mode: LOW, CHANGE, RISING, FALLING, HIGH  High not always supported on Arduino  detachInterrupt(pin)

Other Interrupts  Analog interrupts  Configure based on threshold  Timer interrupts  Based on clock counter overflowing  Direct interrupt lines into CPU

Interrupt Synchronization  Interrupts are asynchronous  Need to synchronize for them  noInterrupts() – disables interrupts  interrupts() – enables interrupts  Can have multiple levels of interrupts

Tic Tac Toe  How could you use interrupts for tic-tac-toe?  Lights?  Switches?

Interrupt Coding  Interrupt routines should do a minimum amount of work  Time added to time of executing task  Asynchronous -> race conditions can occur  Typically these routines just set a flag  Possibly read/write value and set flag  Possibly read/write from/to circular buffer

Interrupt Problems static int iTemperatures[2]; void interrupt vReadTemperatures (void) { iTemperatures[0] = read Dev 1 iTemperatures[1] = read Dev 2 } void main(void) { int iTemp0, iTemp1; // Setup code iTemperatures[0] = 0; iTemperatures[1] = 0; while(TRUE) { iTemp0 = iTemperatures[0]; iTemp1 = iTemperatures[1]; if ( iTemp0 != iTemp1 ) { // Set off alarm! }

Homework  Read Chapters 10, 11