Q3Microprocessor and Interfaces

A microprocessor does not execute instructions instantaneously. Every single hardware operation is strictly governed by a microscopic internal oscillator, divided into clock cycles called T-states. An instruction is mathematically composed of multiple Machine Cycles (Opcode Fetch, Memory Read, Memory Write). A Timing Diagram is a highly complex, physical graphical representation of the absolute voltage levels (High/Low) on every single pin of the CPU during these T-states, guaranteeing perfect hardware synchronization.

The Memory Read Machine Cycle (3 T-states)

This cycle mathematically extracts 1 byte of data from RAM.

• T1 State: The CPU places the upper 8 bits of the address on $A_8-A_{15}$. It violently places the lower 8 bits on the multiplexed $A{D}_0-A{D}_7$ bus. Crucially, the ALE (Address Latch Enable) pin fires a massive positive voltage pulse. This mathematically commands the external 74LS373 latch to physically capture the lower address from the AD bus before it disappears. The $IO/M^{\prime }$ pin goes LOW (indicating a memory operation).
• T2 State: The ALE pulse drops. The CPU violently releases the $A{D}_0-A{D}_7$ bus, preparing it to receive data. The $R{D}^{\prime }$ (Read) control pin drops to an active-low 0V. This electrical signal strikes the RAM chip's Output Enable pin, mathematically forcing the RAM to blast the stored 8-bit data onto the $A{D}_0-A{D}_7$ bus.
• T3 State: The data mathematically stabilizes on the bus. The CPU aggressively pulls this binary data off the bus and loads it into an internal hardware register. The $R{D}^{\prime }$ pin violently returns to high (inactive), terminating the cycle.

The Memory Write Machine Cycle (3 T-states)

This cycle mathematically injects 1 byte of data into RAM.

• T1 State: Identical to the Read cycle. The 16-bit address is blasted out, ALE fires, and $IO/M^{\prime }$ goes LOW.
• T2 State: The CPU aggressively places the 8-bit mathematical data (usually from the Accumulator) directly onto the $A{D}_0-A{D}_7$ bus. The $W{R}^{\prime }$ (Write) control pin violently drops to active-low 0V. This signal hits the RAM chip's Write Enable pin.
• T3 State: The data is physically burned into the RAM's silicon memory matrix. The $W{R}^{\prime }$ pin returns to high, successfully terminating the mathematical write sequence.

The 8085 operates at a blistering 3 MHz, meaning one T-state is exactly 333 nanoseconds long. In a Memory Read cycle, the RAM chip is mathematically given less than one T-state (during T2) to locate the data and place it on the bus. If the motherboard utilizes cheap, archaic RAM with a catastrophic access time of 500 nanoseconds, the CPU will attempt to read the data in T3 before the RAM has physically output it, causing a fatal system crash.

To solve this hardware crisis, engineers utilize the physical READY pin on the 8085.

• Execution: If the slow RAM chip knows it cannot output data in time, its hardware controller aggressively pulls the CPU's READY pin to a LOW voltage during the T2 state.
• The Wait State ($T_W$): The CPU samples the READY pin. Because it is low, the CPU mathematically suspends the transition from T2 to T3. It violently inserts an artificial, blank clock cycle known as a Wait State ($T_W$). It holds the Address and the $R{D}^{\prime }$ signal perfectly stable, simply waiting.
• Resolution: The CPU continues injecting $T_W$ states infinitely until the slow RAM finally pulls the READY pin HIGH. The CPU then instantly transitions to T3, reads the data, and successfully completes the cycle without crashing.