Project Details

The operating system I will describe, called TinyRealTime (TRT) was originally written by Dan Henriksson and Anton Cervin (technical report). (See also Resource−Constrained Embedded Control and Computing Systems, Dan Henriksson, 2006). I extended it, by adding a realtime trace ability, flexible soft timers, a mutex construct, a system dump and a simple command shell. The context switch time is a few hundred cycles on the 8-bit Atmel Mega1284, compiled using GCC. Full documentation is at http://people.ece.cornell.edu/land/courses/ece4760/TinyRealTime/index.html.

A realtime trace facility was added to TRT to enable following which tasks are executing and which semaphores are signaled. There are also to user defined events available. The trace facility uses one port of the MCU to dump data to an oscilloscope. A simple 3-bit DAC is used to convert task number and semaphore number to two separate voltages for display. Each task number adds about 125 mV to the output, so that when task 2, for example, is executing the voltage output is 250 mV. The semaphore number is specified when you initialize a semaphore. The task number starts at zero for the null task, then each task has a number defined by the order in which the tasks were created. Either of the events may be used as a scope trigger or to display. The image shown is from the realtime trace, which gives the task number as a voltage on the top trace and the semaphore number on the bottom trace.

With 4 tasks running, it takes about 700 cycles to perfrom a context switch by signalling a semaphore (Code to test this). The actual time spent in the scheduler ISR (timer1 compare-match) is 390 cycles, plus 70 cycles to store state, plus 70 cycles to restore state equals 530 cycles to service a timer tick. The scheduling scheme is Earliest Deadline First (EDF). Our student have used the kernel for motor controllers, wireless pedometer (http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/f2013/esc73_jsw267/esc73_jsw267/esc...), ultrasonic nagigators (http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/f2013/xs46_ebl43/xs46_ebl43/xs46_eb...) and more.

The API supplied with TRT includes core real time routines:

It is convienent to be able to define an arbitrary number of timers. An additional module allows you to define periodic or one-shot times of 0.1 millisecond to several second durations, and to signal a TRT semaphore when the timer completes. The timer API is:

A mutex is a binary semaphore with states LOCKED and UNLOCKED. A task can gain control of a resource by locking a mutex. The same task must unlock the mutex (unlike a semaphore which can be signaled by any task). A basic mutex facility was built on top of the existing TRT semaphores. Since a mutex is a special semaphore (and uses the semaphore signaling mechanism) , each mutex must be numbered not to conflict with another semaphore. The mutex API is:

The trace port is chosen in trtSettings.h. Defining a trace port turns on the trace facility. Commenting out the definition disables trace. You must also define the data direction register for the trace port as shown below.

// trace options
// use one port for realtime trace
// IF TRACE_PORT is undefined, then trace is turned off
#define TRACE_PORT PORTA
#define TRACE_DDR  DDRA

You can also choose to have a semaphore number held for only a few microceconds (INSTANT_SEM) or to be held through a context switch. This means that semaphore signals which force a context switch will result in longer lasting waveforms and that you can tell when a context switch occurs as a result of a signal. Comment out one of:

//#define EXTEND_SEM
  #define INSTANT_SEM 

There are four macros which can be inserted anywhere in your code. If you disable the trace facility you must remove the macros you inserted in your code.

A dump facility was added to TRT to enable debugging of timing errors and stack overflows. A set of low level routines read out the status of any task, semaphore or mutex. A set of printing routines uses the low level routines to print status information to the serial port. The task number parameter in the table below refers to the lexical order in which tasks are created in the program being tested. The first user task is labeled 1.

Simple command shell:

A simple command shell was written as a task which can be signaled (entered) from any other task and which supplies a command-line interface to task values, semaphore values, memory contents, and i/o registers. The example assumes at buttons are connected to PINB, LEDs to PORTC, and the D0 and D1 to the uart. The trtkernel_1284_trace_dump.c kernel and trtQuery.c are required as above. Pushing button 2 or 3 will enter the shell with different IDs.

Shell commands: