CREATING LOOP-BASED TIME DELAYS IN MICROPROCESSOR SYSTEMS

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1 CREATING LOOP-BASED TIME DELAYS IN MICROPROCESSOR SYSTEMS Assoc. Prof. Dr. Dogan Ibrahim Department Of Computer Engineering, Near East University, Mersin 10, Turkey ABSTRACT Time delays are an important part of programming. A microprocessor operates very fast and it only takes a few microseconds or less to execute an instruction. There are many applications where it is required to slow down the processor so that an output can be observed. For example, flashing an LED on and off every second. This paper describes the methods that can be employed to introduce loop-based time delays into microprocessors. The Z80 microprocessor is taken as an example processor in the paper, but the techniques developed are easily applicable to other microprocessors. 1. INTRODUCTION There are many cases in microprocessor and microcontroller based designs where it is required to create time delays in order to slow down the execution time. For example, flashing an LED on and off every second, performing some operations at specified time intervals and so on. In microprocessors time delays can either be introduced by using external hardware (e.g. timer and counter chips), or program loops can be created to keep the microprocessor busy inside the loop. By adjusting the size of the loop it is possible to create various delay loops, ranging from milliseconds to seconds and even more. This paper describes how accurate time delays can be generated using program loops in microprocessors. The Z80 microprocessor is taken as an example processor in the paper, but the techniques described are general and should easily be applicable to other types of microprocessors. 2. CREATING TIME DELAYS WITH SIMPLE LOOPS The simplest form of creating time delay in a program is to keep the microprocessor busy in a loop for the required amount of time. This is usually achieved by loading a general purpose register with the appropriate count and then setting up a loop to decrement the count until it reaches to zero. The delay is then determined by the time it takes for the microprocessor to execute this loop. Therefore, the delay depends on the clock frequency of the microprocess the number of instructions inside the loop, and the number of times the loop is repeated. The higher the clock frequency, the less will be the delay. Similarly, the higher the number of times a loop is repeated, the more will be the delay. Figure 1 shows a flowchart of how time delay can be created using a simple program loop. The following Z80 microprocessor program code shows how time delay can be created using a simple program loop: LD LOOP: DEC JP B,0AH B NZ,LOOP In the above code, the loop is repeated 10 times. In order to calculate the amount of delay generated by this code, we have to know the time it takes to execute each instruction. For the case of the Z80 microprocessor this is given by the instruction T-states. One T-state is equivalent to one clock period of the microprocessor clock. For example, if a 2MHz clock is used, one clock period is equivalent to 0.5µs. Similarly, for a 4MHz clock, one clock period is 0.25µs. 23

2 If N tout is the total number of T-states outside the loop, and T c is the clock period of the microprocess then T out is given by: Load count in register T out = T c x N tout (2) Similarly, if N is the loop count and N tloop is the total T-states inside the loop, the total delay inside the loop can be calculated as: no Decrement count Count = 0? yes T loop = T c x N tloop x N (3) The total delay can then be calculated by using equations (1), (2), and (3) as: T D = T c x N tout + T c x N tloop x N (4) T D = T c (N tout + N tloop x N) (5) Equation (5) can be used to calculate the total delay created by a single delay loop. An example is given below to illustrate how equation (5) can be used. Fig. 1. Creating time delay using a simple loop The T-states of the instructions are given in the Z80 instruction set. The T-states of the above code are: LD B,0AH 7 LOOP: DEC B 4 JP NZ,LOOP 10 The LD B,0AH instruction is executed in 7 T- states. Using a 2 MHz clock frequency, this instruction will take 3.5µs. If the clock frequency is 4MHz, the same instruction will only take 1.750µs. As can be seen from the above code, the delay program is in two parts: the fixed delay outside the loop,a nd the delay inside the loop. The total delay is the sum of these two components and can be expressed as: Example 1 Calculate the total delay in the following program code. Assume a clock frequency of 2MHz. Solution 1 LD B, 100 LOOP: DEC B JP NZ, LOOP The clock period is, 1/f = 0.5µs. The T-states are: LD B, LOOP: DEC B 4 JP NZ, LOOP 10 In equation (5), where, T D = T out + T loop (1) T D is the total delay T out is the delay outside the loop T loop is the delay inside the loop T c = 0.5µs N tout = 7 N tloop = 14 N = 100 The delay is then calculated as: T D = T c (N tout + N tloop x N) 24

3 = 0.5( x 100) T D = 703.5µs In many applications, the delay outside the loop is very small compared to the loop delay and it can be ignored. 3. CALCULATING THE LOOP COUNT FOR A REQUIRED TIME DELAY In practical applications we need to know the value of the loop count (N) to use so that a required delay can be generated by a delay loop. The value of N can be calculated from equation (5) as: N = [T D /T c - N tout ] / N tloop (6) An example is given below which illustrates how the loop count can be calculated to give a required delay. Example 2 It is required to generate a 400µs delay using a simple loop. Assuming the clock frequency is 2MHz, what will be the value of the loop count? Solution 2 The delay loop is given below: LD B, N 7 LOOP: DEC B 4 JP NZ, LOOP 10 Where it is required to calculate N. With a clock frequency of 2MHz, the period is, 1/f = 0.5µs. From equation (6): 4. TIME DELAY SUBROUTINE Delays are usually created as subroutines in separate parts of a program and they are called from the main program. Delay subroutines are developed so that they do not change any registers of the main program. This is usually achieved by saving the registers used on the stack (with PUSH instructions) at the beginning of the delay subroutines. These registers are restored (with POP instructions) at the end of the delay subroutine. An example subroutine based delay loop is given below. Main Program CALL DELAY Delay Subroutine DELAY: PUSH BC LD B, N LOOP: DEC B JP NZ, LOOP POP BC RET Using equation (5), the total delay can be calculated as: T D = T c (N tout + N tloop x N) The CALL instruction is executed in 17 T- states, the RET instruction is executed in 10 T- states, the PUSH and POP instructions are executed in 11 and 10 T states respectively. Thus, N tout = = 55 N tloop = 14 N = [T D /T c - N tout ] / N tloop = [400/0.5 7] / 14 N = 56.6 The total delay is then related to the loop count N as shown by equation (7): T D = T c ( N) (7) Since N is an integer, we can use N = 56. With a 2MHz clock the maximum delay is: 25

4 T D = 0.5( x 255) T D = µs and with a 4MHz clock the maximum delay is: T D = 0.250( x 255) T D = µs Figure 2 shows the change of delay as the loop count N is varied, for both 2MHz and 4MHz clock frequencies. microseconds Loop count, N 2MHz 4MHz Fig. 2 Change of delay with loop count, N 5. CREATING LARGER TIME DELAYS The maximum delay that can be generated with a simple loop and a register is in the region of a few milliseconds. There are some applications where much larger delays are required. This section gives two methods which can be used to create higher delays in programs. 5.1 Using A Register Pair The maximum value a register can hold is 255. As a result of this the maximum delay is limited to a few milliseconds as shown in previous section. It is possible to create higher delays by using a register pair. The maximum value a register pair can hold is The following delay subroutine shows how register pair BC can be used to create higher delay values. Note that when BC is decremented the flags do not change and as a result of this it is not possible to use a conditional jump instruction following a DEC BC instruction. The accumulator is set to zero at the beginning of the subroutine and B and C are compared with the accumulator. When both B and C are zero, the subroutine returns to the main program. Main Program CALL DELAY Delay Subroutine DELAY: PUSH BC PUSH AF LD BC, NN LD A,0 MORE:DEC BC CP B JP NZ, MORE CP C JP NZ, MORE POP AF POP BC RET The T-states of the instructions are written against each instruction as shown below: Main Program CALL DELAY 17 Delay Subroutine DELAY: PUSH BC 11 PUSH AF 11 LD BC, NN 10 LD A,0 7 MORE: DEC BC 6 CP B 4 JP NZ, MORE 10 CP C 4 26

5 JP NZ, MORE 10 POP AF 10 POP BC 10 RET 10 Using equation (5), the total delay can be calculated as: where, 10 = 86 Thus, T D = T c (N tout + N tloop x NN) N tout = N tloop = 34 and NN varies between 0 and T D = T c ( NN) (8) The maximum delay that can be generated with a 2MHz clock frequency is: T D = 0.5( x 65535) T D = seconds and the maximum delay that can be generated with a 4MHz clock frequency is: T D = 0.250( x 65535) T D = seconds Figure 3 shows the change of the delay with the loop count, NN for both 2MHz and 4MHz clock frequency. Example 3 It is required to generate an approximately one second delay loop in a program. Assuming that the clock frequency is 2MHz, what will be the approximate value of the loop count (NN) if the delay is to be generated using a subroutine as described in section 5.1. Use Figure 3 to find the value of NN. Solution 3 Using Figure 3, the required value of the loop count is approximately, NN = (i.e. the hexadecimal value EA60 should be loaded into register pair BC). milliseconds Loop count, NN Fig. 3 Variation of delay with loop count NN when a register pair is used 5.2 Using Multiple Delay Loops It is also possible to use nested loops to increase the delay time. This technique is usually used when large delays are required. In nested loop technique two loops are used and different registers are decremented in each loop. The following code shows how nested delay loops can be used with registers: LD C, N 1 LOOP1: LD B, N 2 LOOP2: DEC B JP NZ, LOOP2 DEC C JP NZ, LOOP1 When this code is used as a delay subroutine we have to save the registers modified by the code at the beginning. Also, the registers should be restored at the end of the subroutine. The following example code shows how a nested loop delay subroutine can be created. The T-states of each instruction are listed next to the instructions: DELAY:PUSH BC 11 LD C, N 1 7 LOOP1: LD B, N 2 7 2MHz 4MHz 27

6 LOOP2: DEC B 4 JP NZ, LOOP2 10 DEC C 4 JP NZ, LOOP1 10 POP BC 10 RET 10 The total delay can be calculated as follows: The delay in the inner loop (LOOP2) is given by: x N 2 The outer loop is repeated N 1 times and this loop includes the inner loop and in addition the instructions DEC C and JP NZ, LOOP1 which add 14 T-states every iteration. The total delay generated by the outer and the inner loops is therefore: N 1 ( x N ) The delays caused by the instructions outside the outer loop are: = 38 T-states programs described are for the Z80 microprocessor but the techniques discussed should easily be applicable to other types of microprocessors. REFERENCES [1] D. Ibrahim Dynamically Programmable Monostable/astable, Electronic Product Design, Design Ideas, January [2] R. Zaks Programming the Z80, Sybex publications, [3] R. Zaks and A. Lesea Microprocessor Interfacing Techniques, Sybex publications. [4] S.E. Derenzo Interfacing: A Laboratory approach using the microcomputer for instrumentation, data analysis and, control, Prentice-Hall Inc., Adding to this the delay caused by calling the subroutine (CALL DELAY instruction), the total fixed delays are = 55 T-states. The overall delay is then given by: T D = T c x [N 1 ( x N ) + 55] T D = T c x [N 1 (14N ) + 55] (9) Using equation (9), the maximum delay with a 2MHz clock frequency is: T D = 0.5 x [255(14 x )+55] T D = 457.8ms with a 4MHz clock frequency the maximum delay is 229ms. To achieve even higher delays, one can use register pairs in nested loops. 6. CONCLUSIONS This paper has described how loop-based time delays can be generated in microprocessor based systems using simple program loops. The 28

E3940 Microprocessor Systems Laboratory. Introduction to the Z80

E3940 Microprocessor Systems Laboratory Introduction to the Z80 Andrew T. Campbell comet.columbia.edu/~campbell campbell@comet.columbia.edu E3940 Microprocessor Systems Laboratory Page 1 Z80 Laboratory