ad locum , Let me talk about MSP430 Use of single chip microcomputer __delay_cycles The problem of delay .
IAR for MSP430 The compiler provides a compiler inline precise delay function ( Not really
function ) To provide users with accurate delay use , The prototype of this function is :
__intrinsic void __delay_cycles(unsigned long __cycles);
The inner function implements __cycles individual CPU Delay of cycle , But for the setting of this parameter , I want to state :
__cycles What we need to deliver is CPU The number of running cycles

The common usage on the Internet is :
#define CPU_CLOCK 8000000
#define delay_us(us) __delay_cycles(CPU_CLOCK/1000000*(us))
#define delay_ms(ms) __delay_cycles(CPU_CLOCK/1000*(ms))

stay CPU The master clock frequency is 8MHz when , It's really no problem , But this way of writing :
#define CPU_CLOCK 8000000
It's easy for people to think of , We can modify its value to realize the unification of different main frequency system parameters , Actually
This is not true ! For example, change it to #define CPU_CLOCK 32768 In order to realize the 32KHz The delay of the main frequency ...

Let's take a look at the calculation :
When the system master clock frequency CPU_CLOCK by 8MHz when :
frequency f = 8MHz = 8,000,000Hz
Machine cycle Tm = 1/f = 1/8MHz = 1/8us
in other words , One machine cycle (nop) The length of time is 1/8us, So the delay 1us namely 8*Tm, Same as above :
#define delay_us(us) __delay_cycles(8*(us))
#define delay_ms(ms) __delay_cycles(8000*(ms))

Follow the macro definition above , We put CPU_CLOCK Defined as 32768, that :
frequency f = 32KHz = 32,768Hz
Machine cycle Tm = 1/f = 1/32768Hz ~= 30.5us
As one can imagine ,CPU The shortest instruction execution cycle is 30.5us, At this time , Want to delay 1us, Is that possible? ?
therefore , Simply change the above definition to
#define CPU_CLOCK 32768
It's absolutely wrong .

Again , And some friends have realized 0.5us Time delay of , This is when f = 1MHz = 1000000Hz When they were
It's unrealistic , At this point, the machine cycle Tm = 1us. stay f = 8Mhz when , 4 One machine cycle is 0.5us fair .

therefore , In order to avoid causing wrong use or incorrect understanding , It's like defining a macro like this :
#if CPU_CLOCK == 8000000
#define delay_us(us) __delay_cycles(8*(us))
#define delay_ms(ms) __delay_cycles(8000*(ms))
#else
#pragma error "CPU_CLOCK is defined implicitly!"
#endif

in addition :
  __delay_cycles It's not really a function , Just provide compiler inline expansion , This function does not
Variable parameters are not supported , Its parameters can only be constants .

Yes MSP430 Single chip microcomputer __delay_cycles Description and correction of precise delay

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