MSP430 Delay Subroutine

Question

I am writing a delay subroutine in assembly language for the MSP430. I want to know how to calculate the number that have to put in Register R10 in order to achieve a 20ms delay. Thanks

Delay mov #Number,R10 ;20 ms delay
L1 dec R10
 jnz L1
 ret

• Passerby: Depends on your clock speed. What did you try?
• EMPV: The clock speed is at 1Mhz. I am using the MSP430 g2553. I found a formula for the number: Number = (time/4*period) - 1, but i dont know if that is correct.

Answer 1

Have a look into section 3.4.4 (Instruction Cycles and Lengths) of the 2xx Family User's Guide (SLAU144):

CALL Delay 5 cycles
MOV #x, R10 2 cycles
DEC R10 x*1 cycles
JNZ L1 x*2 cycles
RET 3 cycles (Format-I instruction: MOV @SP+,PC)

For 20 ms at 1 MHz, you want 20,000 cycles. Do the math.

Or just let the compiler do the work:

__delay_cycles(20000);

• Passerby: Is mixing a c style intrinsic with assembly good practice,or even valid? +1 though
• CL.: The assembler wouldn't know about C intrinsics. That's why I wrote "compiler".