Last updated on May 10th, 2023 at 09:07 pm
In this post, we will study the Pin functions of the 8085 microprocessor. In this study, different signals in 8085 & their significance will be discussed as well.
- Timing and control circuitry of 8085 microprocessor
- Types of signals in 8085 microprocessor & examples of each type
- Function of HOLD and HLDA in 8085
- Function of IO/ M’, S0, and S1 in 8085
- Impact of RESET IN signal going low in 8085
- Minimum time required for the effective RESET IN signal
- Function of RESET OUT signal in 8085
- More posts (8085 microprocessor based)
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.
Types of signals in 8085 microprocessor & examples of each type
- Control and Status Signals
- Interrupt signals
- Serial I/O signals
- DMA signals
- 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.
Function of 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.
Function of 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.
Impact of RESET IN signal going 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.
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.
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.
Here are links to the articles we published on the Architecture of the 8085 microprocessor.
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.
More posts (8085 microprocessor based)
FAQs on addressing modes of instructions | execution of an instruction in 8085 microprocessor
Arithmetic Instructions in 8085 Microprocessor for Arithmetic operations: Revision notes
8085 Microprocessor – tutorials (hub)
Microprocessor class notes – PDF download