Assign delays for 1 ms or 2 ms in C? - c

Question

I'm using code to configure a simple robot. I'm using WinAVR, and the code used there is similar to C, but without stdio.h libraries and such, so code for simple stuff should be entered manually (for example, converting decimal numbers to hexadecimal numbers is a multiple-step procedure involving ASCII character manipulation).
Example of code used is (just to show you what I'm talking about :) )
.
.
.
DDRA = 0x00;
A = adc(0); // Right-hand sensor
u = A>>4;
l = A&0x0F;
TransmitByte(h[u]);
TransmitByte(h[l]);
TransmitByte(' ');
.
.
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For some circumstances, I must use WinAVR and cannot external libraries (such as stdio.h). ANYWAY, I want to apply a signal with pulse width of 1 ms or 2 ms via a servo motor. I know what port to set and such; all I need to do is apply a delay to keep that port set before clearing it.
Now I know how to set delays, we should create empty for loops such as:
int value= **??**
for(i = 0; i<value; i++)
;
What value am I supposed to put in "value" for a 1 ms loop ?

Answer 1

Chances are you'll have to calculate a reasonable value, then look at the signal that's generated (e.g., with an oscilloscope) and adjust your value until you hit the right time range. Given that you apparently have a 2:1 margin, you might hit it reasonably close the first time, but I wouldn't be much on it.
For your first approximation, generate an empty loop and count the instruction cycles for one loop, and multiply that by the time for one clock cycle. That should give at least a reasonable approximation of time taken by a single execution of the loop, so dividing the time you need by that should get you into the ballpark for the right number of iterations.
Edit: I should also note, however, that (at least most) AVRs have on-board timers, so you might be able to use them instead. This can 1) let you do other processing and/or 2) reduce power consumption for the duration.
If you do use delay loops, you might want to use AVR-libc's delay loop utilities to handle the details.

Answer 2

If my program is simple enough there is not a need of explicit timer programming, but it should be portable. One of my choices for a defined delay would be AVR Libc's delay function:
#include <delay.h>
_delay_ms (2) // Sleeps 2 ms

Answer 3

Is this going to go to a real robot? All you have is a CPU, no other integrated circuits that can give a measure of time?
If both answers are 'yes', well... if you know the exact timing for the operations, you can use the loop to create precise delays. Output your code to assembly code, and see the exact sequence of instructions used. Then, check the manual of the processor, it'll have that information.

Answer 4

If you need a more precise time value you should employ an interrupt service routine based on an internal timer. Remember a For loop is a blocking instruction, so while it is iterating the rest of your program is blocked. You could set up a timer based ISR with a global variable that counts up by 1 every time the ISR runs. You could then use that variable in an "if statement" to set the width time. Also that core probably supports PWM for use with the RC type servos. So that may be a better route.

Answer 5

This is a really neat little tasker that I use sometimes. It's for an AVR.
************************Header File***********************************
// Scheduler data structure for storing task data
typedef struct
{
// Pointer to task
void (* pTask)(void);
// Initial delay in ticks
unsigned int Delay;
// Periodic interval in ticks
unsigned int Period;
// Runme flag (indicating when the task is due to run)
unsigned char RunMe;
} sTask;
// Function prototypes
//-------------------------------------------------------------------
void SCH_Init_T1(void);
void SCH_Start(void);
// Core scheduler functions
void SCH_Dispatch_Tasks(void);
unsigned char SCH_Add_Task(void (*)(void), const unsigned int, const unsigned int);
unsigned char SCH_Delete_Task(const unsigned char);
// Maximum number of tasks
// MUST BE ADJUSTED FOR EACH NEW PROJECT
#define SCH_MAX_TASKS (1)
************************Header File***********************************
************************C File***********************************
#include "SCH_AVR.h"
#include <avr/io.h>
#include <avr/interrupt.h>
// The array of tasks
sTask SCH_tasks_G[SCH_MAX_TASKS];
/*------------------------------------------------------------------*-
SCH_Dispatch_Tasks()
This is the 'dispatcher' function. When a task (function)
is due to run, SCH_Dispatch_Tasks() will run it.
This function must be called (repeatedly) from the main loop.
-*------------------------------------------------------------------*/
void SCH_Dispatch_Tasks(void)
{
unsigned char Index;
// Dispatches (runs) the next task (if one is ready)
for(Index = 0; Index < SCH_MAX_TASKS; Index++)
{
if((SCH_tasks_G[Index].RunMe > 0) && (SCH_tasks_G[Index].pTask != 0))
{
(*SCH_tasks_G[Index].pTask)(); // Run the task
SCH_tasks_G[Index].RunMe -= 1; // Reset / reduce RunMe flag
// Periodic tasks will automatically run again
// - if this is a 'one shot' task, remove it from the array
if(SCH_tasks_G[Index].Period == 0)
{
SCH_Delete_Task(Index);
}
}
}
}
/*------------------------------------------------------------------*-
SCH_Add_Task()
Causes a task (function) to be executed at regular intervals
or after a user-defined delay
pFunction - The name of the function which is to be scheduled.
NOTE: All scheduled functions must be 'void, void' -
that is, they must take no parameters, and have
a void return type.
DELAY - The interval (TICKS) before the task is first executed
PERIOD - If 'PERIOD' is 0, the function is only called once,
at the time determined by 'DELAY'. If PERIOD is non-zero,
then the function is called repeatedly at an interval
determined by the value of PERIOD (see below for examples
which should help clarify this).
RETURN VALUE:
Returns the position in the task array at which the task has been
added. If the return value is SCH_MAX_TASKS then the task could
not be added to the array (there was insufficient space). If the
return value is < SCH_MAX_TASKS, then the task was added
successfully.
Note: this return value may be required, if a task is
to be subsequently deleted - see SCH_Delete_Task().
EXAMPLES:
Task_ID = SCH_Add_Task(Do_X,1000,0);
Causes the function Do_X() to be executed once after 1000 sch ticks.
Task_ID = SCH_Add_Task(Do_X,0,1000);
Causes the function Do_X() to be executed regularly, every 1000 sch ticks.
Task_ID = SCH_Add_Task(Do_X,300,1000);
Causes the function Do_X() to be executed regularly, every 1000 ticks.
Task will be first executed at T = 300 ticks, then 1300, 2300, etc.
-*------------------------------------------------------------------*/
unsigned char SCH_Add_Task(void (*pFunction)(), const unsigned int DELAY, const unsigned int PERIOD)
{
unsigned char Index = 0;
// First find a gap in the array (if there is one)
while((SCH_tasks_G[Index].pTask != 0) && (Index < SCH_MAX_TASKS))
{
Index++;
}
// Have we reached the end of the list?
if(Index == SCH_MAX_TASKS)
{
// Task list is full, return an error code
return SCH_MAX_TASKS;
}
// If we're here, there is a space in the task array
SCH_tasks_G[Index].pTask = pFunction;
SCH_tasks_G[Index].Delay =DELAY;
SCH_tasks_G[Index].Period = PERIOD;
SCH_tasks_G[Index].RunMe = 0;
// return position of task (to allow later deletion)
return Index;
}
/*------------------------------------------------------------------*-
SCH_Delete_Task()
Removes a task from the scheduler. Note that this does
*not* delete the associated function from memory:
it simply means that it is no longer called by the scheduler.
TASK_INDEX - The task index. Provided by SCH_Add_Task().
RETURN VALUE: RETURN_ERROR or RETURN_NORMAL
-*------------------------------------------------------------------*/
unsigned char SCH_Delete_Task(const unsigned char TASK_INDEX)
{
// Return_code can be used for error reporting, NOT USED HERE THOUGH!
unsigned char Return_code = 0;
SCH_tasks_G[TASK_INDEX].pTask = 0;
SCH_tasks_G[TASK_INDEX].Delay = 0;
SCH_tasks_G[TASK_INDEX].Period = 0;
SCH_tasks_G[TASK_INDEX].RunMe = 0;
return Return_code;
}
/*------------------------------------------------------------------*-
SCH_Init_T1()
Scheduler initialisation function. Prepares scheduler
data structures and sets up timer interrupts at required rate.
You must call this function before using the scheduler.
-*------------------------------------------------------------------*/
void SCH_Init_T1(void)
{
unsigned char i;
for(i = 0; i < SCH_MAX_TASKS; i++)
{
SCH_Delete_Task(i);
}
// Set up Timer 1
// Values for 1ms and 10ms ticks are provided for various crystals
OCR1A = 15000; // 10ms tick, Crystal 12 MHz
//OCR1A = 20000; // 10ms tick, Crystal 16 MHz
//OCR1A = 12500; // 10ms tick, Crystal 10 MHz
//OCR1A = 10000; // 10ms tick, Crystal 8 MHz
//OCR1A = 2000; // 1ms tick, Crystal 16 MHz
//OCR1A = 1500; // 1ms tick, Crystal 12 MHz
//OCR1A = 1250; // 1ms tick, Crystal 10 MHz
//OCR1A = 1000; // 1ms tick, Crystal 8 MHz
TCCR1B = (1 << CS11) | (1 << WGM12); // Timer clock = system clock/8
TIMSK |= 1 << OCIE1A; //Timer 1 Output Compare A Match Interrupt Enable
}
/*------------------------------------------------------------------*-
SCH_Start()
Starts the scheduler, by enabling interrupts.
NOTE: Usually called after all regular tasks are added,
to keep the tasks synchronised.
NOTE: ONLY THE SCHEDULER INTERRUPT SHOULD BE ENABLED!!!
-*------------------------------------------------------------------*/
void SCH_Start(void)
{
sei();
}
/*------------------------------------------------------------------*-
SCH_Update
This is the scheduler ISR. It is called at a rate
determined by the timer settings in SCH_Init_T1().
-*------------------------------------------------------------------*/
ISR(TIMER1_COMPA_vect)
{
unsigned char Index;
for(Index = 0; Index < SCH_MAX_TASKS; Index++)
{
// Check if there is a task at this location
if(SCH_tasks_G[Index].pTask)
{
if(SCH_tasks_G[Index].Delay == 0)
{
// The task is due to run, Inc. the 'RunMe' flag
SCH_tasks_G[Index].RunMe += 1;
if(SCH_tasks_G[Index].Period)
{
// Schedule periodic tasks to run again
SCH_tasks_G[Index].Delay = SCH_tasks_G[Index].Period;
SCH_tasks_G[Index].Delay -= 1;
}
}
else
{
// Not yet ready to run: just decrement the delay
SCH_tasks_G[Index].Delay -= 1;
}
}
}
}
// ------------------------------------------------------------------
************************C File***********************************

Answer 6

Most ATmega AVR chips, which are commonly used to make simple robots, have a feature known as pulse-width modulation (PWM) that can be used to control servos. This blog post might serve as a quick introduction to controlling servos using PWM. If you were to look at the Arduino platform's servo control library, you would find that it also uses PWM.
This might be a better choice than relying on running a loop a constant number of times as changes to compiler optimization flags and the chip's clock speed could potentially break such a simple delay function.

Answer 7

You should almost certainly have an interrupt configured to run code at a predictable interval. If you look in the example programs supplied with your CPU, you'll probably find an example of such.
Typically, one will use a word/longword of memory to hold a timer, which will be incremented each interrupt. If your timer interrupt runs 10,000 times/second and increments "interrupt_counter" by one each time, a 'wait 1 ms' routine could look like:
extern volatile unsigned long interrupt_counter;
unsigned long temp_value = interrupt_counter;
do {} while(10 > (interrupt_counter - temp_value));
/* Would reverse operands above and use less-than if this weren't HTML. */
Note that as written the code will wait between 900 µs and 1000 µs. If one changed the comparison to greater-or-equal, it would wait between 1000 and 1100. If one needs to do something five times at 1 ms intervals, waiting some arbitrary time up to 1 ms for the first time, one could write the code as:
extern volatile unsigned long interrupt_counter;
unsigned long temp_value = interrupt_counter;
for (int i=0; 5>i; i++)
{
do {} while(!((temp_value - interrupt_counter) & 0x80000000)); /* Wait for underflow */
temp_value += 10;
do_action_thing();
}
This should run the do_something()'s at precise intervals even if they take several hundred microseconds to complete. If they sometimes take over 1 ms, the system will try to run each one at the "proper" time (so if one call takes 1.3 ms and the next one finishes instantly, the following one will happen 700 µs later).