Software delay

Necessary knowledge: [HW] Controller module, [AVR] Architecture, [LIB] Pins, [LIB] Delay

There is often a need to create delays in programs of microcontrollers, it is necessary to time the actions or wait them to end. By its concept one of the easiest methods for creating a break in the work of a microcontroller is overloading its processor with some alternative action – for example order it to count big numbers. From the stroke frequency of the processor can be calculated to which number it should count in order to get a certain time delay. Counting a number from zero up to the value of the stroke frequency of the processor in hertz-s, should theoretically create a delay for one second. For number of reasons this is not as simple in practice.

If the processor of the microcontroller calculates using numbers which’s binary form is as wide as its inner bus (AVR has 8 bits), then it takes one firing stroke of the processor to perform one arithmetical operation for example adding 1 to any value. In order to operate with thousands or millions the number has to be 16 or 32 bit and for an 8 bit processor it takes more than one firing stroke to calculate them. Hence when dealing with large numbers, one has to be familiar with the inside of the processor – exactly its commands.

When programming in advanced languages (C-language for example), the programs are not written directly on the basis of the command, in order to create a software delay, one needs to know also its compiler which converts the program to the machine code. From this depends how many instructions (and phases therefore) it takes to perform one arithmetical operation. Complexity is added by compilers ability to convert the program into the machine code in several ways – for example, by making the machine code as memory saving as possible or easily executable. Such compiler’s actions are called optimization. With different modes of optimization the software delay’s machine codes and their duration come out different.

The following is an example of generating software delay with AVR microcontroller. A part of a program in C-language is written, which counts the variable x of for-cycle from 0 to 100. Inside each cycle a no-action empty instruction is completed. It is needed, because if the content of the cycle is empty, the compiler optimizes it out of the program as it considers this to be useless.

unsigned char x;
 
// Cycle until x is 100
for (x = 0; x < 100; x++)
{
	// With empty instruction nop
	asm volatile ("nop");
}

This is the same part of a program after compiling. 2 hexadecimal numbers on the left is machine code and on the right is the command with operand(s) in assembler language. The machine code and the assembler language are conformal; the assembler is just for presenting the machine code for humans in a readable form. In compiling, the optimization of the length of the program is used (compiler’s parameter –Os).

80 e0     ldi  r24, 0x00 ; r24 loading number 0 to the index
00 00     nop            ; Empty operation
8f 5f     subi r24, 0xFF ; subtracting 255 form the r24 index, that means adding +1
84 36     cpi  r24, 0x64 ; comparing r24 index with number 100
e1 f7     brne .-8       ; If the comparison was wrong, then transfer 8-baits back

In the compiled form can be seen, what is actually happening with the cycle of the C-language and it can be used to calculate how many clock sycles is needed to complete a cycle of one period. The information about the effect of the instructions and operating time can be found from AVR’s instructions datasheet. In the given example, it takes 4 clock cycles to complete 4 instructions in one cycle period, because all instructions demand one clock rate. In addition, one clock rate is used before the cycle for loading instruction and afterwards one extra clock rate for exiting the cycle. By assuming, that the working clock rate of the controller is 14,7456 MHz, the whole delay produced by the program can be calculated.

(1 + 100 ⋅ 4 + 1) / 14745600 = 27,26 μs

The delay produced in the example, is in microseconds and the variable used is 8-bit so the machine code is fairly simple. In order to produce a break in millisecond, we need to count much larger numbers and thus the machine code gets longer. Cycles working inside each other may also be used, but with this method the delay is not in linear relation with the number of the cycles because with each level of cycle a small extra delay occurs.

The goal of this exercise is not creating precise software delay on the level of machine code, because it is quite an accurate work and we already have the functions necessary to produce delays in the avr-libc and in the library of the HomeLab. Those are used also in the following examples.

When dealing with the software delay it is important to know, that regardless its basic simplicity; it is extremely inefficient method from the power consumption point of view. During all of these clock rates when the microcontroller is counting the useless, energy is consumed. So if using applications operating on batteries, it is not wise to write long software delays. Wiser is to use hardware timers, which are working independently and wake the processor from hibernating when it is time to continue the work.

The following code of a program is about software delay function sw_delay_ms , which makes a given delay in milliseconds using the parameter count. The function uses avr-libc library’s function _delay_ms which is partly written in assembler language. The reason why the _delay_ms is not used in this exercise immediately is that when using the _delay_ms problems may occur when the delays are long. The sw_delay_ms enables creating 65535 ms long delays without any complications.

//
// Software delay in milliseconds.
//
void sw_delay_ms(unsigned short count)
{
	// Counting the variable of the delay to 0
	while (count-- > 0)
	{
		// 1ms delay with a special function.
		_delay_ms(1);
	}
}

The following program is for using the given function, this creates two delays in the endless loop: 100 ms and 900 ms. During the shorter delay LED is lit and during the longer it is switched off, the result – the LED is blinking periodically

//
// The demonstration program of the software delay of the HomeLab.
// The program is blinking a LED for a moment after ~1 second.
//
#include <homelab/pin.h>
#include <homelab/delay.h>
 
//
// Determining the pin of the test LED
//
pin debug_led = PIN(B, 7);
 
//
// Main program
//
int main(void)
{
	// Setting the pin of the LED as output.
	pin_setup_output(debug_led);
 
	// Endless loop	
	while (true)
	{
		// Lighting the LED
		pin_clear(debug_led);
 
		// Software delay for 100 ms
		sw_delay_ms(100);
 
		// Switching off the LED
		pin_set(debug_led);
 
		// Software delay for 900 milliseconds.
		sw_delay_ms(900);
	}
}

Although it seems that the LED blinks in every 1 second, the time is actually a little bit longer, because the callouts of LED’s and delay functions are taking a couple of clock rates of the microcontroller