Interrupts
8051 provides 5 vectored interrupts. They are -
- ~INT0
- TF0
- ~INT1
- TF1
- RI/TI
Out of these, ~INT0 and ~INT1 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
EX0: INT0 interrupt (External) enable bit
ET0: Timer-0 interrupt enable bit
EX1: INT1 interrupt (External) enable bit
ET1: Timer-1 interrupt enable bit
ES: Serial port interrupt enable bit
ET2: Timer-2 interrupt enable bit
EA: Enable/Disable all
Setting '1': Enable the corresponding interrupt
Setting '0': Disable the corresponding interrupt
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 Priority register (IP)
'0' - low priority
'1' - high priority
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.
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.
Instruction Cycles -
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.
Fig 1. 8051 Interrupt Details
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
EX0: INT0 interrupt (External) enable bit
ET0: Timer-0 interrupt enable bit
EX1: INT1 interrupt (External) enable bit
ET1: Timer-1 interrupt enable bit
ES: Serial port interrupt enable bit
ET2: Timer-2 interrupt enable bit
EA: Enable/Disable all
Setting '1': Enable the corresponding interrupt
Setting '0': Disable the corresponding interrupt
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 Priority register (IP)
'1' - high priority
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.
- An interrupt of equal or higher priority level is already in progress.
- The current polling cycle is not the final cycle in the execution of the instruction in progress.
- The instruction in progress is RETI or any write to IE or IP registers.
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 -
Register
| Value(Hex) on Reset |
|---|---|
PC
|
0000H
|
DPTR
|
0000H
|
A
|
00H
|
B
|
00H
|
SP
|
07H
|
PSW
|
00H
|
Ports P0-3 Latches
|
FFH
|
IP
|
XXX 00000 b
|
IE
|
0 XX 00000 b
|
TCON
|
00H
|
TMOD
|
00H
|
TH0
|
00H
|
TL0
|
00H
|
TH1
|
00H
|
TL1
|
00H
|
SCON
|
00H
|
SBUF
|
XX H
|
PCON
|
0 XXXX XXX b
|
The schematic diagram of the detection and processing of interrupts is given as follows.
Instruction Cycles -
Fig 2. Interrupt Handling in 8051
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.
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