if((pwm>pgm_read_byte(&brightness[pwm_ch[0]]))|(pwm==0)){pd &= ~(1<<0);}//1
        else {pd |= (1<<0);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[1]]))|(pwm==0)){pd &= ~(1<<1);}//2    
        else {pd |= (1<<1);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[2]]))|(pwm==0)){pa &= ~(1<<1);}//3
        else {pa |= (1<<1);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[3]]))|(pwm==0)){pa &= ~(1<<0);}//4
        else {pa |= (1<<0);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[4]]))|(pwm==0)){pd &= ~(1<<2);}//5
        else {pd |= (1<<2);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[5]]))|(pwm==0)){pd &= ~(1<<3);}//6
        else {pd |= (1<<3);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[6]]))|(pwm==0)){pd &= ~(1<<4);}//7
        else {pd |= (1<<4);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[7]]))|(pwm==0)){pd &= ~(1<<5);}//8
        else {pd |= (1<<5);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[8]]))|(pwm==0)){pb &= ~(1<<4);}//9
        else {pb |= (1<<4);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[9]]))|(pwm==0)){pb &= ~(1<<3);}//10
        else {pb |= (1<<3);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[10]]))|(pwm==0)){pb &= ~(1<<2);}//11
        else {pb |= (1<<2);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[11]]))|(pwm==0)){pb &= ~(1<<1);}//12
        else {pb |= (1<<1);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[12]]))|(pwm==0)){pb &= ~(1<<0);}//13
        else {pb |= (1<<0);}
        if((pwm>pgm_read_byte(&brightness[pwm_ch[13]]))|(pwm==0)){pd &= ~(1<<6);}//14
        else {pd |= (1<<6);}

#проверяем и сбрасываем сразу при нуле
#два массива мапят порты (кто хочет можно массивом двумерных массивов , но по моему так проще)
#цикл проверки каналов и задания соответствующих значений (тут считается что автор пересчитал свой массивчик brightness до статических значений )
if ( pwm == 0 ) { pa |= 3; pb |= 31; pd |= 127; } else {
  int     log2phy[] = { pd, pd, pa, pa, pd, pd, pd, pd, pb, pb, pb, pb, pb, pd };
  char  log2val[] = {  1,  2,  2,  1,  4,  8, 16, 32, 16,  8,  4,  2,  1, 64 };
  for (int i = 0; i < 14; ++i )
   { log2phy[i] = (pwm > brightness[pwm_ch[i]]) ? (log2phy[i] & ~log2val[i]]) : (log2phy[i] | log2val[i]); }
}

#include <avr/io.h>
#include <avr/pgmspace.h>
PROGMEM char brightness[] = "888888888888888888888888888";

volatile unsigned char pwm;
volatile unsigned int pd;
volatile unsigned char pwm_ch[14];

int main(void)
{
if(pwm)
{
unsigned char *pwmch = &pwm_ch;
int d = 0;
for(char i=0; i<14; i++)
{
unsigned int msk = 1;
if(pwm <= pgm_read_byte(&brightness[*pwmch++])) pd |= msk;
msk <<= 1;
}
pd = d;
}
else
{
pd = 0;
}
return 0;
}

Timer_CompA_ISR()
{
    turn_off( CurrentChan++ );

    if (CurrentChan > MAX_CHAN_COUNT)
    {
        CurrentChan = 0;
        return;
    }

    OCRA = ChanGetLifeTime( CurrentChan );
}


Timer_OvrISR()
{
    // врубить все
    turn_on( AllChannels );

    // посчитать такт отключения каждого из каналов (время "жизни")
    UpdateLifeTime( ... );

    // отсортировать каналы по времени жизни по-возрастанию
    SortChannelsByTimeAscending( ... );
}

Timer_CompA_ISR()
{
    turn_off( CurrentChan++ );

    if (CurrentChan > MAX_CHAN_COUNT)
    {
        CurrentChan = 0;
        return;
    }

    if (OCRA == ChanGetLifeTime( CurrentChannel ) )
        Timer_CompA_ISR();  // запустить сл. канал прямо сейчас (лучше всего rjmp'ом)
    else
        OCRA = ChanGetLifeTime( CurrentChan );  // проапдейтить таймер на прерывание в нужное время
}

// Timer 0 overflow interrupt service routine
interrupt [TIM0_OVF] void timer0_ovf_isr(void)
{
// Reinitialize Timer 0 value
TCNT0=256-39+4;
//PWM-----------------------
pwr_cnt++;
if (red_a>pwr_cnt)
    { PORTD.7=1;}
else
    {PORTD.7=0;};
    
if (green_a>pwr_cnt)
    {PORTD.6=1;}
else
    {PORTD.6=0;};
    
if (blue_a>pwr_cnt)
    {PORTD.5=1;}
else
    {PORTD.5=0;};
}

Оттранслировалось в:
_timer0_ovf_isr:
    RCALL __SAVEISR
;      49 // Reinitialize Timer 0 value
;      50 TCNT0=256-39+4;
    LDI  R30,LOW(221)
    OUT  0x32,R30
;      51 //PWM-----------------------
;      52 pwr_cnt++;
    LDS  R30,_pwr_cnt
    SUBI R30,-LOW(1)
    STS  _pwr_cnt,R30
;      53 if (red_a>pwr_cnt) {PORTD.7=1;}
    LDS  R26,_red_a
    CP   R30,R26
    BRSH _0x2B
    SBI  0x12,7
;      54 else {PORTD.7=0;};
    RJMP _0x2C
_0x2B:
    CBI  0x12,7
_0x2C:
;      55 if (green_a>pwr_cnt) {PORTD.6=1;}
    LDS  R30,_pwr_cnt
    LDS  R26,_green_a
    CP   R30,R26
    BRSH _0x2D
    SBI  0x12,6
;      56 else {PORTD.6=0;};
    RJMP _0x2E
_0x2D:
    CBI  0x12,6
_0x2E:
;      57 if (blue_a>pwr_cnt) {PORTD.5=1;}
    LDS  R30,_pwr_cnt
    LDS  R26,_blue_a
    CP   R30,R26
    BRSH _0x2F
    SBI  0x12,5
;      58 else {PORTD.5=0;};
    RJMP _0x30
_0x2F:
    CBI  0x12,5
_0x30:
;      59 }
    RCALL __LOADISR
    RETI

for(unsigned char i=0;i<14;i++)
        {
            if((a & (1<<i))&(b & (1<<i))) {pwm_ch[i]=255;}//1и1
            if((a & (1<<i))&(!(b & (1<<i)))) {pwm_ch[i]=255;}//1и0
            if((!(a & (1<<i)))&(b & (1<<i))) {pwm_ch[i]=0;}//0и1
            if((!(a & (1<<i)))&(!(b & (1<<i)))) {pwm_ch[i]=0;}//0и0
        }

// Timer 0 overflow interrupt service routine
interrupt [TIM0_OVF] void timer0_ovf_isr(void)
{
char _tmp;
// Reinitialize Timer 0 value
TCNT0=256-39+4;
//PWM-----------------------
_tmp = ++pwr_cnt;
if (red_a > _tmp)
    { PORTD.7=1;}
else
    {PORTD.7=0;};
    
if (green_a > _tmp)
    {PORTD.6=1;}
else
    {PORTD.6=0;};
    
if (blue_a > _tmp)
    {PORTD.5=1;}
else
    {PORTD.5=0;};
}

for(unsigned char i=0, unsigned int tmp = 1; i<14; i++)
{
   if(a & tmp) pwm_ch[i]=255; //1и1, 1и0
   else pwm_ch[i]=0; //0и1, 0и0
   tmp <<= 1;
}

uint8_t *ptr = &pwm_ch[13];
for(uint16_t mask = (1<<13); mask; mask >>= 1)
{
    if(a & mask)
        *ptr-- = 255;
    else
        *ptr-- = 0;
}