How to generate an accurate delay time?

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a_ziliu
Posts: 207
Joined: 20 Mar 2017, 10:52

31 May 2021, 16:24

If user wants to generate an accurate delay time, it is recommended to use the __nop() to get the accurate one CPU clock cycle delay. However, because the speed of Flash is lower than CPU, some optimization and cache technology will make the __nop() delay not accurate. Hence, it is recommended to put the source code of delay function on SRAM to avoid inaccurate delay time of __nop().

Example: to generate a 2 us delay time accurately.
(1) CPU clock= 32MHz: 1 CPU clock cycle takes 1/32000000 sec = 31.25 ns
(2) 2 us delay time = 2000 ns / 31.25 ns = 64 times of CPU clock cycle

1. Open a new .c file in KEIL project.
2. Specify the file destination on SRAM.


3. Set up Linker.


4. Write the delay function.

Since it takes 5 CPU clock cycles to execute one for loop, user can use the following function code to achieve 2 us delay.

(1) Execute one for loop: 5 CPU clock cycles
(2) Execute one __NOP(): 1 CPU clock cycle
(3) 64 CPU Clock Cycles = 8 ( 5(for loop) + 3 * 1( __NOP() ) )

void Delay_Test_Function(void)
{
for(i = 0; i < 8 ; i++) /* Delay for 2 micro seconds. */
{
__NOP();
__NOP();
__NOP();
}
}

5. Test
User can take the following function code to test the delay timing. It is recommended to use an oscilloscope to measure the I/O toggle delay time. However, the command executing time (PA0 = 0) needs to be added to the delay time.

Execute one PA = 0: 11 CPU clock cycles

Hence, it will take 64+11 clock cycles. It means that it will take the time:
(64+11) * 31.25 ns = 2343.75 ns.

void Delay_Test_Function(void)
{
uint32_t i, DelayCNTofCPUClock = 8;
PA0 = 1;
for(i = 0; i < DelayCNTofCPUClock ; i++) /* Delay for 2 micro seconds. */
{
__NOP();
__NOP();
__NOP();
}
PA0 = 0;
}

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