Different Signals in 8085 & their significance

In this post, we will study Different Signals in 8085 & their significance. Here, we have answered a set of frequently asked questions to study this.


Describe in brief the function of timing and control circuitry of 8085 microprocessor

The timing and control unit section is a part of the CPU and generates timing and control signals for the execution of instructions. This section includes Clock signals, Control signals, Status signals, DMA signals, and also the Reset signals. This section controls fetching and decoding operations. It also generates appropriate control signals for instruction execution and also the signals required to interface external devices.

Mention the following types of signals in 8085 microprocessor

  1. Control and Status signals
  2. Interrupt signals
  3. Serial I/O signals
  4. DMA signals
  5. Reset signals.

Control and status signals are ALE, RD, WR, IO/M’, S0, S1, and READY.

Interrupt signals are TRAP, RST 7.5, RST 6.5, RST 5.5, INTR.

INTA is an interrupt acknowledgment signal indicating that the processor has acknowledged an INTR interrupt.

Serial I/O signals are SID and SOD

DMA signals are HOLD and HLDA

Reset signals are RESET IN and RESET OUT.

Explain the function of the two DMA signals HOLD and HLDA in 8085

DMA mode of data transfer is the fastest and pins 39 and 38 (HOLD and HLDA) become active only in this mode. When DMA is required, the DMA controller IC (8257) sends a 1 to pin 39 of 8085.

At the end of the current instruction cycle of the microprocessor, it issues a1 to pin 38 of the controller. After this, the bus control is totally taken over by the controller.

When 8085 is active and 8257 is idle, then the former is MASTER and the latter is SLAVE, while the roles of 8085 and 8257 are reversed when 8085 is idle and 8257 becomes active.

Discuss the function of three signals IO/ M’, S0 and S1 in 8085

IO/ M’ signal indicates whether I/O or memory operation is being carried out.

A high on this signal indicates I/O operation while a low indicates memory operation.

S0 and S1 indicate the type of machine cycle in progress.

What happens when RESET IN signal goes low in 8085?

RESET IN is an input signal which is active when its status is low. When this pin is low, the following occurs: The program counter is set to zero (0000H).

Interrupt enable and HLDA F/Fs are reset. All the buses are tri-stated. Internal registers of 8085 are affected in a random manner.

Is there any minimum time required for the effective RESET IN signal?

For proper resetting to take place, the reset signal RESET IN must be held low for at least 3 clock cycles.

Indicate the function of RESET OUT signal in 8085

When this signal is high, the processor is being reset. This signal is synchronized to the processor clock and is used to reset other devices which need resetting.

Related Posts

Here are links to the articles we published on the Architecture of the 8085 microprocessor.

Functional block diagram of Intel 8085 microprocessor and the functional units

General-purpose registers of 8085 microprocessor

Special-purpose registers of 8085 microprocessor – FAQs

Different Signals used in 8085 microprocessor & their significance – FAQs

Author of this post

This post is co-authored by Professor Saraswati Saha, who is an assistant professor at RCCIIT, a renowned degree engineering college in India. Professor Saha teaches subjects related to digital electronics & microprocessors.

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