Pulse oximetry using MSP430FG437

Other Parts Discussed in Thread: MSP430FG437

Hi , I want to use the code and schematic that TI offers to make a  Pulse Oximetry by MSP430FG437.

Now I can see the visible red light on the sensor and see the right IR Heart signal.

But I have two questions now.

1. The value of "heart_rate_LSB" and " SaO2_LSB" is always 0X00 , which I think it would change every 2ms with IR heart signal. 

        TXBUF0 = 0x00;                   // Byte 1 - 0x00 (synchronization byte)
           while (!(IFG1 & UTXIFG0)); 
           TXBUF0 = 0xFF;                   // Byte 2 - 0xFF (synchronization byte)
           while (!(IFG1 & UTXIFG0)); 
           TXBUF0 = i;                      // Byte 3 - IR Heart signal (AC only)
           while (!(IFG1 & UTXIFG0));
           TXBUF0 = heart_rate_LSB;         // Byte 4 - Heart rate data
           while (!(IFG1 & UTXIFG0));
          TXBUF0 = SaO2_LSB;               // Byte 5 - %SaO2 data
  

2. If I include the "mul.s43" to implement " int32_t mul16(register int16_t x, register int16_t y);" , the signal would be different 

when I use

long mul16(short x, short y)
{
return ((long)x) * y;
}

to define this function.However, the output signal of later one is right.

I want to know why and should I use the "mul.s43" to define " int32_t mul16"?

Hope for your reply for these two questions.

Best regards,

Steve.