Clock Interrupt in MCU Programming

This article takes the AT89C51 system with a 6MHz clock as an example to illustrate the application of clock interrupts:

Timer initial value and interrupt cycle The clock interrupt does not need to be too frequent, generally 20mS (50Hz) is sufficient. If a time base signal of one hundredth of a second is required, 10mS (100Hz) can be used. Here we take 20mS, and use timer T0 to work in 16-bit timer mode (mode 1). The working mode of T0 is: it automatically adds 1 after each machine cycle. When the count reaches 0FFFFh and is about to overflow, an interrupt will be generated, and the hardware will set the corresponding flag bit for software query. That is, when the interrupt occurs, N+1 machine cycles have passed compared to when it was started. Therefore, we only need to pre-store a number that is N less than the full value 0FFFFh in T0, and then start the timer, and an interrupt will be generated after N machine cycles. This value is the so-called "initial value". The following is the calculation of the initial value we need: the clock is 6MHz, 12 clock cycles are one machine cycle, and there are 10,000 machine cycles in 20mS. (10000)10=(2710)16, then 0FFFFh-2710h+1=0D8F0h. Since it takes 7~8 machine cycles to respond to interrupt, protect the context and reload the initial value, add 7 to this value, that is, the initial value that should be loaded into T0 is 0D8F7h. After each interrupt is entered, the values ​​of A and W are pushed into the stack first, and then 0D8F7h is loaded into T0.

Set a unit, add 1 every time an interruption We can take a unit in the internal RAM and name it INCPI (Increase Per Interrupt). In the interruption, after loading the initial value of T0, use the INC INCPI instruction to add 1. From this unit, both the interrupt program and the main program can obtain any integer multiple of 20mS between 1 and 256. For example: there is a program that sends display to the digital tube, which needs to be executed every 0.5 seconds to refresh the display. You can set a unit (called a waiting unit) W_DI, use the /MOV A,INCPI/ADD A,#25/MOV W_DI ,A/ statement to make it 25 greater than the current INCPI value, and then check whether it is equal to the INCPI value in each interruption. If it is equal, it means that 25 interrupt cycles have passed, then the display sending program will be executed, and W_DI will be added with 25, waiting for the next 0.5 seconds. We can set multiple waiting units to extract multiple different time base signals. Let the interrupt program query each waiting unit in turn to see if it is equal to INCPI at each interrupt. If they are equal, execute the corresponding processing and reset the value of the waiting unit, otherwise skip it. For example: refresh or flash the display with a 0.5 second signal, generate a real-time clock with a 1 second signal, or output a square wave of a certain frequency, query the input device at a certain interval, etc.

Reading keys in interrupts Usually, we read the keyboard in the main program, the steps are: scan the keyboard, if a key is pressed, delay for tens of milliseconds to debounce, confirm again that the key is indeed pressed, and then process the work corresponding to the key, and repeat the above steps after completion. But there are two shortcomings: 1. The key input cannot be latched when processing the corresponding work, that is, the key may be missed. 2. The CPU cannot do other things during the delay debounce, which is not efficient. If the key reading is put into the clock interrupt, the above shortcomings can be avoided. The method is: if the same key is read as pressed in two adjacent interrupts, the key is valid (the debounce purpose is achieved), and it is latched into the first-in-first-out (queue) keyboard buffer, waiting for the main program to process. In this way, the main program can still respond to keyboard input while processing the key. The buffer depth can usually be set to 8 levels. If the number of latched keys is more than 8, the new key is ignored, and an alarm is issued to remind the user that the new key will be invalid. If the keyboard buffer queue stagnates for much longer than the maximum time required for the main program to process a key press, it means that the main program has made an error or run away. In this case, you can use an instruction to reset the system during an interrupt, thus fulfilling the purpose of a watchdog.

The delay in the main program has a normally open clock interrupt, so when the main program needs a shorter delay with higher precision, the clock interrupt should be temporarily closed. If the program needs a longer delay with lower precision, the following writing method can be used to avoid multi-layer nested loop delays.

Example: Output a high level pulse for 1 second on P1.1

MOV A,INCPI

INC A

CJNE A,INCPI$ ；Wait for an interrupt to complete

SET P1.1; Set P1.1 to H, pulse starts

ADD A,#50; 50 20mS equals 1 second

CJNE A, INCPI, $ ; Wait for the interruption to increase INCPI by 50 times

CLR P1.1; Set P1.1 to L, the pulse ends

Conclusion: From the above, we can see that we need to flexibly apply clock interrupts, reasonably allocate tasks to interrupts and main programs, and the two should have clear division of labor and simple interfaces.

Note: The execution time of the interrupt handler should be shortened as much as possible, and should not be longer than 20mS.