Lecture-10
Intel 8085 Microprocessor
The 8085A is an 8- bit microprocessor suitable for a wide range of
application. It is a 40-pin DIP (Dual in package) chip, based on
NMOS technology. It contains approximately 6200 transistors on a
164 x 222 mil chip. The pin configuration is shown in fig.3.1.
X1 1 40 Vcc
X2 2 39 HOLD
RESET OUT 3 38 HLDA
SOD 4 37 CLK(OUT)
SID 5 36 RESET IN
TRAP 6 READY
35
RST7.5 7 34 IO/M
RST6.5 8 Intel 8085A 33 S1
RST5.5 9 Microprocessor 32 RD
INTR 10 31 WR
INTA 11 30 ALE
AD0 12 29 S0
AD1 13 28 A15
AD2 14 27 A14
AD3 15 26 A13
AD4 16 25 A12
AD5 17 24 A11
AD6 18 23 A10
AD7 19 22 A9
Vss 20 21 A8
Fig.3.1 Pin Configuration of Intel 8085A Microprocessor
It requires a single +5V supply between Vcc at pin no. 40 and Vss at
pin no. 20.
, Pin Configuration of Intel 8085A Microprocessor:
A15 – A8 at pin no. 28 to pin no. 21:
The microprocessor can address directly 216 memory locations
or 65536 memory locations or 64k memory locations using 16-
address lines (A15-A0). Pin no. 28 to pin no. 21 give us the higher
order 8-bits of the address (A15-A8). These address lines are uni-
directional, tri-state address lines. These address lines become tri-
stated under three conditions namely:
(a) During DMA (direct memory access) operation.
(b) When a HALT instruction is executed.
(c) When microprocessor is being RESET.
AD7–AD0 at pin no. 19 to pin no. 12:
Pin no. 19 to pin no.12 marked AD7–AD0 are used for dual
purpose. It is time multiplexed lower 8-bit address bus (A7-A0) and 8-
bit data bus (D7-D0). Because at the time when this chip was
developed, the practical limit on the numbers of pins was 40. The
only solution was to multiplex part of the address bus with the data
bus.
Before discussing, the functions of different pins, it is better to
know few more points about the processor. The microprocessor,
being a logic circuit, shall move from one state to the other state
during it operation. There are ten (10) different possible states for the
processor and the processor will be in one of these states as long as
the power is ON. These states are:
Intel 8085 Microprocessor
The 8085A is an 8- bit microprocessor suitable for a wide range of
application. It is a 40-pin DIP (Dual in package) chip, based on
NMOS technology. It contains approximately 6200 transistors on a
164 x 222 mil chip. The pin configuration is shown in fig.3.1.
X1 1 40 Vcc
X2 2 39 HOLD
RESET OUT 3 38 HLDA
SOD 4 37 CLK(OUT)
SID 5 36 RESET IN
TRAP 6 READY
35
RST7.5 7 34 IO/M
RST6.5 8 Intel 8085A 33 S1
RST5.5 9 Microprocessor 32 RD
INTR 10 31 WR
INTA 11 30 ALE
AD0 12 29 S0
AD1 13 28 A15
AD2 14 27 A14
AD3 15 26 A13
AD4 16 25 A12
AD5 17 24 A11
AD6 18 23 A10
AD7 19 22 A9
Vss 20 21 A8
Fig.3.1 Pin Configuration of Intel 8085A Microprocessor
It requires a single +5V supply between Vcc at pin no. 40 and Vss at
pin no. 20.
, Pin Configuration of Intel 8085A Microprocessor:
A15 – A8 at pin no. 28 to pin no. 21:
The microprocessor can address directly 216 memory locations
or 65536 memory locations or 64k memory locations using 16-
address lines (A15-A0). Pin no. 28 to pin no. 21 give us the higher
order 8-bits of the address (A15-A8). These address lines are uni-
directional, tri-state address lines. These address lines become tri-
stated under three conditions namely:
(a) During DMA (direct memory access) operation.
(b) When a HALT instruction is executed.
(c) When microprocessor is being RESET.
AD7–AD0 at pin no. 19 to pin no. 12:
Pin no. 19 to pin no.12 marked AD7–AD0 are used for dual
purpose. It is time multiplexed lower 8-bit address bus (A7-A0) and 8-
bit data bus (D7-D0). Because at the time when this chip was
developed, the practical limit on the numbers of pins was 40. The
only solution was to multiplex part of the address bus with the data
bus.
Before discussing, the functions of different pins, it is better to
know few more points about the processor. The microprocessor,
being a logic circuit, shall move from one state to the other state
during it operation. There are ten (10) different possible states for the
processor and the processor will be in one of these states as long as
the power is ON. These states are:
