Timer1 interrupt debuging

hi all.. have problem code here. try set interrupt trigger every 200us, when checking show 400us instead of 200us timer1 setting.. can me wrong setting.

here code:

code: [select]

volatile int val;
volatile int isnewval;
volatile unsigned long time;
unsigned long timer;


void setup() {
 
serial.begin(115200); //setup serial comm.
delay(500);

nointerrupts(); //disable interrupt first
tccr1a =0;    // clear timer setting
tccr1b =0;    // ~~
tcnt1=0;       //~~
ocr1a= 400;         // 16m/8/5000 = 400. target freq 5khz
tccr1b |= (1<<wgm12);     //enable ctc mode
tccr1b |= (1<<cs11);      //setup prescaler 8.
timsk1 |= (1<< ocie1a);       //enable timer interrupts
interrupts();                  //enable interrupt
}

isr(timer1_compa_vect){
// val = analogread(a0);

  isnewval=1;
  } 
 
 
void loop() {
    if (isnewval == 1){
  timer = micros()-time;
  time = micros();
  serial.println(timer);
    isnewval =0;
   
   
  }
}

here result serial monitor

printing 4 characters (three digits plus newline) @ 115200 baud take 347µs.

the timer setup looks ok me, though. technically ocr1a should 399, 0-399 400 counts.

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