1. CSCI1600: Embedded and Real Time Software
Lecture 18: Advanced Programming with I/O
Steven Reiss, Fall 2017
Check the termination condition on pop for circular queue.

2. Tasks Need to Communicate
Simple communication is one-way
Task A decides what lights should be on
Task B just displays what it is told
Global variable
Written only by task A
Read by task B
This is safe as long as there is only one writer / variable
Why care about multiple writers?
Multiple writers: synchronization issues with multiple processes, synchronization issues with interrupts
Caching behaviors.
Might be different processors in the same system (variable might be communications link)
Lecture 17: Advanced Programming
11/11/2018

3. 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?
What are the problems?
FROM LAST TIME
Lecture 16: Input/Output II
11/11/2018

4. Making Communication Simple
Here we have a button press task that sets the currently pressed button
And a second task that checks for a correct series of buttons.
Problem: how to only commuicate one-way between these tasks
Lecture 16: Input/Output II
11/11/2018

5. 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
Handshakes are more common in communications
Arduino is single threaded – can this be done with a single variable?
Lecture 16: Input/Output II
11/11/2018

6. 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
Lecture 16: Input/Output II
11/11/2018

7. 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?
Lecture 16: Input/Output II
11/11/2018

8. 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
Lecture 17: Advanced Programming
11/11/2018

9. Queue-based Tasks
Lecture 17: Advanced Programming
11/11/2018

10. Queue Implementation Queues in embedded programs
Are generally fixed in size
Often sized so they never overflow
How to program a queue?
Lecture 17: Advanced Programming
11/11/2018

11. Simple Queue What’s wrong with this? 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?
What is wrong with this code?
First: both tasks are writing qp – would need to be synchronized
Second: lots of copying going on of elements in the queue
Third: both are reading/writing queue arbitrarily
Lecture 17: Advanced Programming
11/11/2018

12. Circular Buffer Queue Use a single array Maintain two pointers
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
Lecture 17: Advanced Programming
11/11/2018

13. Circular Buffer Pop: if read == write then exception
result = buf[read];
read = (read + 1) % size
Lecture 17: Advanced Programming
11/11/2018

14. Circular Buffer Push(data): If read == write then exception
buf[write] = data
write = (write + 1)%size
Lecture 17: Advanced Programming
11/11/2018

15. Circular Buffer Advantages Extensions Little data movement
Pointers are only changed by one task
Queue cells are safely read/written
Code is simpler
Extensions
Can read/write multiple things at once
Data stream or communications channel
Lecture 17: Advanced Programming
11/11/2018

16. Circular Buffers and Handshakes
What happens with a circular buffer of size one?
Lecture 17: Advanced Programming
11/11/2018

17. 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
Use diodes to prevent back-flow
Other logic circuits/devices: Open collector
Lecture 18: Advanced Programming w/ I/O
11/11/2018

18. 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
Lecture 18: Advanced Programming w/ I/O
11/11/2018

19. Serial Communication Send data from one machine to another
Using only 2 wires
Using clocked serial input
10 bits to send 8
Plus parity?
Lecture 18: Advanced Programming w/ I/O
11/11/2018

20. 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, …
Lecture 18: Advanced Programming w/ I/O
11/11/2018

21. UART Internals
Lecture 18: Advanced Programming w/ I/O
11/11/2018

22. 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)
Lecture 18: Advanced Programming w/ I/O
11/11/2018

23. Other Interrupts Analog interrupts Timer interrupts
Configure based on threshold
Timer interrupts
Based on clock counter overflowing
Direct interrupt lines into CPU
Lecture 18: Advanced Programming w/ I/O
11/11/2018

24. Interrupt Synchronization
Interrupts are asynchronous
What can go wrong?
Need to synchronize for them
noInterrupts() – disables interrupts
interrupts() – enables interrupts
Can have multiple levels of interrupts
Lecture 18: Advanced Programming w/ I/O
11/11/2018

25. NIM How could you use interrupts for nim? Lights? Switches? Timers?
Sound?
Lecture 18: Advanced Programming w/ I/O
11/11/2018

26. 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
Lecture 18: Advanced Programming w/ I/O
11/11/2018

27. Interrupt Problems What can go wrong with this code?
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! }
What can go wrong with this code?
Where can the interrupt occur?
Need to disable and enable interrupts around the reads of temperatures
This leads us to concurrent programming issues (next time).
Lecture 18: Advanced Programming w/ I/O
11/11/2018

28. Homework Read Chapters 10, 11 Lecture 18: Advanced Programming w/ I/O
11/11/2018

29. NIM Redux What were your tasks? What did your main loop look like?
What is done in the various tasks?
Did you model your tasks before coding them?
How often are the tasks run?
Do this for minutes unless there is material to cover from previous lecture.
Lecture 18: Advanced Programming w/ I/O
11/11/2018