All signals can be classified into six groups:
- Address Bus
- Data Bus
- Control & Status Signals
- Power Supply & Frequency signals
- Externally initiated signals
- Serial I/O Ports
1. Address Bus (pin 12 to 28)
- 16 signal lines are used as address bus.
- However these lines are split into two segments: A15 - A8 and AD7 - AD0
- A15 - A8 are unidirectional and are used to carry high-order address of 16-bit address.
- AD7 - AD0 are used for dual purpose.
2. Data Bus/ Multiplexed Address (pin 12 to 19)
- Signal lines AD7-AD0 are bidirectional and serve dual purpose.
- They are used as low-order address bus as well as data bus.
- The low order address bus can be separate from these signals by using a latch.
3. Control & Status Signals
- To identify nature of operation
- Two Control Signals
- 1) RD’ (Read-pin 32)
- This is a read control signal (active low)
- This signal indicates that the selected I/O or Memory device is to be read & data are available on data bus.
- 2) WR’ (Write-pin 31)
- This is a write control signal (active low)
- This signal indicates that the selected I/O or Memory device is to be write.
Three Status Signals
- (1) S1 (pin 33)
- (2) S0 (pin 29)
- S1 and S0 status signals can identify various operations, but they are rarely used in small systems.
- (3) IO/M’ (pin 34)
- This is a status signal used to differentiate I/O and memory operation
- When it is high, it indicates an I/O operation
- When it is low, it indicates a memory operation
- This signal is combined with RD’ and WR’ to generate I/O & memory control signals
To indicate beginning of operation
- One Special Signal called ALE (Address Latch Enable-Pin 30)
- This is positive going pulse generated every time the 8085 begins an operation (machine cycle)
- It indicates that the bits on AD7-AD0 are address bits
- This signal is used primarily to latch the low-address from multiplexed bus & generate a separate set of address lines A7-A0.
4. Power Supply & Frequency Signal
- Vcc → Pin no. 40, +5V Supply
- Vss → Pin no.20, Ground Reference
- X1, X2 → Pin no.1 & 2, Crystal Oscillator is connected at these two pins. The frequency is internally divided by two;
- Therefore, to operate a system at 3MHz, the crystal should have a frequency of 6MHz.
- CLK (OUT) → Clock output. Pin No.37: This signal can be used as the system clock for other devices.
5. Externally Initiated Signals including Interrupts
- INTR (Input) Interrupt Request. It is used as general purpose interrupt
- INTA’ (Output) Interrupt Acknowledge. It is used to acknowledge an interrupt.
- RST7.5, RST6.5, RST5.5 (Input) Restart Interrupts.
- These are vector interrupts that transfer the program control to specific memory locations.
- They have higher priorities than INTR interrupt.
- Among these 3 interrupts, the priority order is RST7.5, RST6.5, RST5.5
- TRAP (Input) → This is a non maskable interrupt & has the highest priority.
- HOLD (Input) → This signal indicates that a peripheral such as DMA Controller is requesting the use of address & data buses
- HLDA (Output) → Hold Acknowledge. This signal acknowledges the HOLD request
- READY (Input) → This signal is used to delay the microprocessor read or write cycles until as low- responding peripheral is ready to send or accept data. When the signal goes low, the microprocessor waits for an integral no. of clock cycles until it goes high.
- RESET IN’ (Input) → When the signal on this pin goes low, the Program Counter is set to zero, the buses are tri-stated & microprocessor is reset.
- RESET OUT (Output) → This signal indicates that microprocessor is being reset. The signal can be used to reset other devices.
6. Serial I/O Ports
- Two pins for serial transmission
- SID (Serial Input Data-pin 5)
- SOD (Serial Output Data-pin 4)
- In serial transmission, data bits are sent over a single line, one bit at a time.