Details about interrupt of 8085

1. Prof. Nitin Ahire 1
Microprocessor & Microcontroller - I
T.E Sem V (Rev)
Prof. Nitin Ahire
XIE, Mahim

2. Prof. Nitin Ahire 2
Connection of I/O devices.
Polling method
Interrupt method

3. Prof. Nitin Ahire 3
Interrupt system of 8085
Definition: “It is a mechanism by which an
I/O device ( Hardware interrupt) or an
instruction (software interrupt) can suspend
the normal execution of the processor and
get it self serviced.”

4. Prof. Nitin Ahire 4
Types of interrupt
1) Hardware interrupt
2) Software interrupt

5. Prof. Nitin Ahire 5
Hardware interrupt
Interrupt : “It is an external asynchronous
input that inform the ‘up’ to complete the
instruction that it is currently executing and
fetch a new routine in order to offer a service
to that I/O devices. Once the I/O device is
serviced, the ‘up’ will continue with execution
of its normal program.”

6. Prof. Nitin Ahire 6
Hardware interrupt
8085 has ‘5’ hardware interrupt
1)Trap
2)RST 7.5
3)RST 6.5
4)RST 5.5
5)INTR

7. Prof. Nitin Ahire 7
Types of Hardware interrupt
NMI( non maskable)
1) It can’t be masked or
made pending
2) Highest priority
3) This interrupt disable
all maskable interrupts
4) Used for emergency
purpose like power
failure, smoke detector
e.g. TRAP
Maskable
1) It can be masked or made
pending
2) Lower priority
3) These interrupt dose not
disable non maskable
interrupt
4) Used to interface
peripherals.
e.g. RST 7.5

8. Prof. Nitin Ahire 8
Hardware Interrupt
Priority interrupt ISR address trigger
1 TRAP 0024h edge +level
2 RST 7.5 003Ch edge
3 RST 6.5 0034h level
4 RST 5.5 002Ch level
5 INTR No specific level
location

9. Prof. Nitin Ahire 9
Software interrupt
8085 has ‘8’ software interrupt
1)RST0
2)RST1
3)RST2
4)RST3
5)RST4
6)RST5
7)RST6
8)RST7

10. Prof. Nitin Ahire 10
Software interrupt
These instruction ( RST0-RST7) allow the
‘up’ to transfer the program control from main
program to the subroutine program (i.e. ISR)
ISR: interrupt service routing

11. Prof. Nitin Ahire 11
Software interrupt
Interrupt Restart locations
RST 0 0 X 8 = 0000h
RST 1 1 X 8 = 0008h
RST 2 2 x 8 = 0010h
RST 3 3 X 8 = 0018h
RST 4 4 X 8 = 0020h
RST 5 5 X 8 = 0028h
RST 6 6 X 8 = 0030h
RST 7 7 X 8 = 0038h

12. Prof. Nitin Ahire 12
Software interrupt / hardware interrupt
Software interrupt
1)It is as synchronous event
2)This interrupt is requested
by executing instruction
3)PC is incremented
4)The priority is highest
5)It can’t be ignored
6)It is not used to interface the
peripheral
Used in debugging
Hardware interrupt
1)It is an asynchronous event
2)This interrupt is requested
by external device
3)PC is not incremented
4)The priority is lower than
softer interrupt
5)Can be masked
6)It is used to interface
peripheral devices

13. Prof. Nitin Ahire 13
Interrupt related instructions
1) EI : it is used to enable the all maskable
interrupt. It required 1-byte, one MC (4T). It
does not affect on TRAP
2) DI : it is used to disable all maskable
interrupt. 1-byte (4T). It does not affect on
TRAP

14. Prof. Nitin Ahire 14
Interrupt related instructions
SIM : (set interrupt mask)
1-byte (4T) state.
Used to enable or disable RST 7.5, RST 6.5,
RST 5.5 interrupts.
It does not affect on TRAP & INTR .
It is used in serial data transmission
It also transfer serial data bit ‘D7’of ‘A’ to the
SOD pin
Hence the CWR format must be load in the ‘A’
before execution of SIM instruction.

15. Prof. Nitin Ahire 15
SIM (bit pattern)
SOD pin
D7= SOD
D6= serial data enable 1=enable, 0=disable
D5= Don’t care
D4= Reset R7.5 F/F, 1=Reset 0=no effect
D3=MSE Mask set enable 1=D2,D1,D0 bit are effective
0=D2,D1,D0 bit are ignored
D2= M’7.5 Mask RST 7.5 1= Mask or disable R7.5
0= Enable RST 7.5
D1=M’6.5 Mask RST 6.5 1= Mask or disable R6.5
0= Enable RST 6.5
D0=M’5.5 Mask RST 5.5 1= Mask or disable R5.5
0= Enable RST 5.5
SOD SDE M’ 6.5M’ 7.5MSER 7.5X M’ 5.5

16. Prof. Nitin Ahire 16
Interrupt related instructions
RIM : ( read interrupt mask)
1-byte (4T) state.
It gives the status of the pending maskable interrupt
(RST 7.5 – RST 5.5)
It does not affect on TRAP & INTR
It can also transfer the contents of the serial input data
on the SID pin into the accumulator (‘D7’ bit.)
Hence after execution of this instruction serial data get
load in to the accumulator

17. Prof. Nitin Ahire 17
RIM (bit pattern)
SID pin
D7= SID
D6=
D5= if 1 respective interrupt is pending
D4= 0 respective interrupt is not pending
D3=IE interrupt enable
D2=
D1= if 1 respective interrupt is Masked
D0= 0 respective interrupt is unmasked
SID I 7.5 M 6.5M 7.5IEI 5.5I 6.5 M 5.5