control led brightness of microcontroller rtos/bios - embedded

Question

i'm trying to control my led in 256 (0-255) different levels of brightness. my controller is set to 80mhz and running on rtos. i'm setting the clock module to interrupt every 5 microseconds and the brightness e.g. to 150. the led is dimming, but i'm not sure if it is done right to really have 256 different levels
int counter = 1;
int brightness = 0;
void SetUp(void)
{
SysCtlClockSet(SYSCTL_SYSDIV_2_5|SYSCTL_USE_PLL|SYSCTL_OSC_MAIN|SYSCTL_XTAL_16MHZ);
GPIOPinTypeGPIOOutput(PORT_4, PIN_1);
Clock_Params clockParams;
Clock_Handle myClock;
Error_Block eb;
Error_init(&eb);
Clock_Params_init(&clockParams);
clockParams.period = 400; // every 5 microseconds
clockParams.startFlag = TRUE;
myClock = Clock_create(myHandler1, 400, &clockParams, &eb);
if (myClock == NULL) {
System_abort("Clock create failed");
}
}
void myHandler1 (){
brightness = 150;
while(1){
counter = (++counter) % 256;
if (counter < brightness){
GPIOPinWrite(PORT_4, PIN_1, PIN_1);
}else{
GPIOPinWrite(PORT_4, PIN_1, 0);
}
}
}

Answer 1

A 5 microsecond interrupt is a tall ask for an 80 MHz processor, and will leave little time for other work, and if you are not doing other work, you need not use interrupts at all - you could simply poll the clock counter; then it would still be a lot of processor to throw at a rather trivial task - and the RTOS is overkill too.
A better way to perform your task is to use the timer's PWM (Pulse Width Modulation) feature. You will then be able to accurately control the brightness with zero software overhead; leaving your processor to do more interesting things.
Using a PWM you could manage with a far lower performance processor if LED control is all it will do.
If you must use an interrupt/GPIO (for example your timer does not support PWM generation or the LED is not connected to a PWM capable pin), then it would be more efficient to set the timer incrementally. So for example for a mark:space of 150:105, you would set the timer for 150*5us (9.6ms), on the interrupt toggle the GPIO, then set the timer to 105*5us (6.72ms).
A major problem with your solution is the interrupt handler does not return - interrupts must run to completion and be as short as possible and preferably deterministic in execution time.
Without using hardware PWM, the following based on your code fragment is probably closer to what you need:
#define PWM_QUANTA = 400 ; // 5us
static volatile uint8_t brightness = 150 ;
static Clock_Handle myClock ;
void setBrightness( uint8_t br )
{
brightness = br ;
}
void SetUp(void)
{
SysCtlClockSet(SYSCTL_SYSDIV_2_5|SYSCTL_USE_PLL|SYSCTL_OSC_MAIN|SYSCTL_XTAL_16MHZ);
GPIOPinTypeGPIOOutput(PORT_4, PIN_1);
Clock_Params clockParams;
Error_Block eb;
Error_init(&eb);
Clock_Params_init(&clockParams);
clockParams.period = brightness * PWM_QUANTA ;
clockParams.startFlag = TRUE;
myClock = Clock_create(myHandler1, 400, &clockParams, &eb);
if (myClock == NULL)
{
System_abort("Clock create failed");
}
}
void myHandler1(void)
{
static int pin_state = 1 ;
// Toggle pin state and timer period
if( pin_state == 0 )
{
pin_sate = 1 ;
Clock_setPeriod( myClock, brightness * PWM_QUANTA ) ;
}
else
{
pin_sate = 0 ;
Clock_setPeriod( myClock, (255 - brightness) * PWM_QUANTA ) ;
}
// Set pin state
GPIOPinWrite(PORT_4, PIN_1, pin_state) ;
}

Answer 2

On the urging of Clifford I am elaborating on an alternate strategy for reducing the load of software dimming as as servicing interrupts every 400 clock cycles may prove difficult. The preferred solution should of course be to use hardware pulse-width modulation whenever available.
One option is to set interrupts only at the PWM flanks. Unfortunately this strategy tends to introduce races and drift as time elapses while adjustments are taking place and scales poorly to multiple channels.
Alternative we may switch from pulse-width to delta-sigma modulation. There is a fair bit of theory behind the concept but in this context it boils down to toggling the pin on and off as quickly as possible while maintaining an average on-time proportional to the dimming level. As a consequence the interrupt frequency may be reduced without bringing the overall switching frequency down to visible levels.
Below is an example implementation:
// Brightness to display. More than 8-bits are required to handle full 257-step range.
// The resolution also course be increased if desired.
volatile unsigned int brightness = 150;
void Interrupt(void) {
// Increment the accumulator with the desired brightness
static uint8_t accum;
unsigned int addend = brightness;
accum += addend;
// Light the LED pin on overflow, relying on native integer wraparound.
// Consequently higher brightness values translate to keeping the LED lit more often
GPIOPinWrite(PORT_4, PIN_1, accum < addend);
}
A limitation is that the switching frequency decreases with the distance from 50% brightness. Thus the final N steps may need to be clamped to 0 or 256 to prevent visible flicker.
Oh, and take care if switching losses are a concern in your application.