Chapter: Microprocessor and Microcontroller

8051 interrupts

8051 provides 5 vectored interrupts. They are



8051 provides 5 vectored interrupts. They are 

are external interrupts whereas Timer and Serial port interrupts are generated internally. The external interrupts could be negative edge triggered or low level triggered. All these interrupt, when activated, set the corresponding interrupt flags. Except for serial interrupt, the interrupt flags are cleared when the processor branches to the Interrupt Service Routine (ISR). The external interrupt flags are cleared on branching to Interrupt Service Routine (ISR), provided the interrupt is negative edge triggered. For low level triggered external interrupt as well as for serial interrupt, the corresponding flags have to be cleared by software by the programmer.


The schematic representation of the interrupts is as follows -




Each of these interrupts can be individually enabled or disabled by 'setting' or 'clearing' the corresponding bit in the IE (Interrupt Enable Register) SFR. IE contains a global enable bit EA which enables/disables all interrupts at once.


Interrupt Enable register (IE): Address: A8H

Priority level structure:


Each interrupt source can be programmed to have one of the two priority levels by setting (high priority) or clearing (low priority) a bit in the IP (Interrupt Priority) Register . A low priority interrupt can itself be interrupted by a high priority interrupt, but not by another low priority interrupt. If two interrupts of different priority levels are received simultaneously, the request of higher priority level is served. If the requests of the same priority level are received simultaneously, an internal polling sequence determines which request is to be serviced. Thus, within each priority level, there is a second priority level determined by the polling sequence, as follows.

Interrupt handling:


The interrupt flags are sampled at P2 of S5 of every instruction cycle (Note that every instruction cycle has six states each consisting of P1 and P2 pulses). The samples are polled during the next machine cycle (or instruction cycle). If one of the flags was set at S5P2 of the preceding instruction cycle, the polling detects it and the interrupt process generates a long call (LCALL) to the appropriate vector location of the interrupt. The LCALL is generated provided this hardware generated LCALL is not blocked by any one of the following conditions.


1.     An interrupt of equal or higher priority level is already in progress.


2.     The current polling cycle is not the final cycle in the execution of the instruction in progress.


3.     The instruction in progress is RETI or any write to IE or IP registers.


When an interrupt comes and the program is directed to the interrupt vector address, the Program Counter (PC) value of the interrupted program is stored (pushed) on the stack. The required Interrupt Service Routine (ISR) is executed. At the end of the ISR, the instruction RETI returns the value of the PC from the stack and the originally interrupted program is resumed.


Reset is a non-maskable interrupt. A reset is accomplished by holding the RST pin high for at least two machine cycles. On resetting the program starts from 0000H and some flags are modified as follows –

The schematic diagram of the detection and processing of interrupts is given as follows.

It should be noted that the interrupt which is blocked due to the three conditions mentioned before is not remembered unless the flag that generated interrupt is not still active when the above blocking conditions are removed, i.e. ,every polling cycle is new.



Jump and Call Instructions


There are 3 types of jump instructions. They are:-


1.     Relative Jump

2.     Short Absolute Jump

3.     Long Absolute Jump


Relative Jump


Jump that replaces the PC (program counter) content with a new address that is greater than (the address following the jump instruction by 127 or less) or less than (the address following the jump by 128 or less) is called a relative jump. Schematically, the relative jump can be shown as follows: -

The advantages of the relative jump are as follows:-


Only 1 byte of jump address needs to be specified in the 2's complement form, ie. For jumping ahead, the range is 0 to 127 and for jumping back, the range is –

1 to -128.


2.     Specifying only one byte reduces the size of the instruction and speeds up program execution.


3.     The program with relative jumps can be relocated without reassembling to generate absolute jump addresses.


Disadvantages of the absolute jump: -


1.     Short jump range (-128 to 127 from the instruction following the jump instruction)


Instructions that use Relative Jump


SJMP <relative address>


(The remaining relative jumps are conditional jumps)


JC <relative address> JNC <relative address> JB bit, <relative address> JNB bit, <relative address> JBC bit, <relative address>


CJNE <destination byte>, <source byte>, <relative address> DJNZ <byte>, <relative address>


JZ <relative address> JNZ <relative address>


Short Absolute Jump


In this case only 11bits of the absolute jump address are needed. The absolute jump address is calculated in the following manner.


In 8051, 64 kbyte of program memory space is divided into 32 pages of 2 kbyte each. The hexadecimal addresses of the pages are given as follows:-

It can be seen that the upper 5bits of the program counter(PC) hold the page number and the lower 11bits of the PC hold the address within that page. Thus, an absolute address is formed by taking page numbers of the instruction (from the program counter) following the jump and attaching the specified 11bits to it to form the 16-bit address.


Advantage: The instruction length becomes 2 bytes.


However, difficulty is encountered when the next instruction following the jump instruction begins from a fresh page (at X000H or at X800H). This does not give any


problem for the forward jump, but results in an error for the backward jump. In such a case the assembler prompts the user to relocate the program suitably.


Example of short absolute jump: - ACALL <address 11>


AJMP <address 11>


Long Absolute Jump/Call


Applications that need to access the entire program memory from 0000H to FFFFH use long absolute jump. Since the absolute address has to be specified in the op-code, the instruction length is 3 bytes (except for JMP @ A+DPTR). This jump is not re-locatable.


Example: -


LCALL <address 16>


LJMP <address 16>


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