Compiler/MSP430F5310: Pwm input

What sort of signal is FAN_SPEED_R? Is it a once-per-rev tick, or a high-frequency PWM drive?

A once-per-rev will be pretty slow (a few-100 Hz at most) so your best bet would be to use timer Capture (TA2.1=timer TA2, register CCR1) and subtract the capture values from consecutive edges. [Ref User Guide (SLAU208Q) Sec 17.2.4.1]

I don't see any Capture examples in Resource Explorer, but there are some timer-A skeletons you might be able to use:

https://dev.ti.com/tirex/explore/node?node=AEC6YEnWUPrjclYUKg9ntw__IOGqZri__LATEST

TA2CCTL1 = CM_1 + CCIS_0 + SCS + CAP; //capture on rising mode, CCI2A input, synchronous capture, capture mode
TA2CTL = TASSEL_2 + MC_0; //SMCLK, timer halted


#pragma vector=TIMER2_A1_VECTOR
__interrupt void TIMER2_A1_ISR(void)
{
	switch(__even_in_range(TA2IV,14)){
		case  0: 
			break;                          // No interrupt
		case  2:
			new_cap = TA2CCR1;
			cap_diff = new_cap - old_cap;
			old_cap = new_cap;
			if(max_cap < cap_diff){
				max_cap = cap_diff;
			}
			if(min_cap > cap_diff){
				min_cap = cap_diff;
			}
			TA2CTL = TASSEL_2 + MC_0;
			TA2CCTL1 &= ~CCIE;
			__no_operation();
			break;                          // CCR1
		case  4:
			break;                          // CCR2
		case  6: 
			break;                          // reserved
		case  8: 
			break;                          // reserved
		case 10: 
			break;                          // reserved
		case 12: 
			break;                          // reserved
		case 14: 
			break;                          // overflow
		default:
			break;

	}
	
}

I enable TA2CCTL1 every 2 secs

In debuging process, the value of cap_diff is 200 ~ 218

Could someone tell me how to transfer the value into fan rpm??