Fig: Pin Diagram

Timing and Control Unit:

  • The timing and control unit accepts information from the instruction decoder and generates different control signal. This unit synchronizes all the microprocessor operation and generates control and status signal necessary for communication between the microprocessor and peripherals.


A8 - A15 (Output 3 State):

  • Address Bus: The most significant 8 bits of the memory address or the 8 bits of the I/0 address,3 stated during Hold and Halt modes.

AD0 - AD7 (Input / Output 3 state):

  • Multiplexed Address/Data Bus; Lower 8 bits of the memory address (or I/0 address) appear on the bus during the first clock cycle of a machine state.
  • It then becomes the data bus during the second and third clock cycles. 3 stated during Hold and Halt modes.

ALE (Output):

  • Address Latch Enable: It occurs during the first clock cycle of a machine state and enables the address to get latched into the on chip latch of peripherals.
  • The falling edge of ALE is set to guarantee setup and hold times for the address information. ALE can also be used to strobe the status information. ALE is never 3stated.

SO, S1 (Output):

  • Data Bus Status. Encoded status of the bus cycle: S1 S0
  • 0 0 HALT
  • 1 WRITE
  • 0 READ
  • 1 1 FETCH S1 can be used as an advanced R/W status.

RD (Output 3state):

  • READ: indicates the selected memory or I/0 device is to be read and that the Data Bus is available for the data transfer.

WR (Output 3state):

  • WRITE: indicates the data on the Data Bus is to be written into the selected memory or I/0location.
  • Data is set up at the trailing edge of WR. 3stated during Hold and Halt modes.

READY (Input)

  • If Ready is high during a read or write cycle, it indicates that the memory or peripheral is ready to send or receive data.
  • If Ready is low, the CPU will wait for Ready to go high before completing the read or write cycle.

HOLD (Input):

  • HOLD: indicates that another Master is requesting the use of the Address and Data Buses
  • The CPU, upon receiving the Hold request will relinquish the use of buses as soon as the completion of. the current machine cycle.
  • Internal processing can continue. The processor can regain the buses only after the Hold is removed. When the Hold is acknowledged, the Address, Data, RD, WR, and IO/M lines are 3stated.

HLDA (Output):

  • HOLD ACKNOWLEDGE: indicates that the CPU has received the Hold request and that iwill relinquishsh the buses in the next clock cycle.
  • HLDA goes low after the Hold request is removed. The CPU takes the buses one half clock cycle after HLDA goes low.

INTR (Input):

  • INTERRUPT REQUEST is used as a general purpose interrupt. It is sampled only during thenext to the last clock cycle of the instruction. If it is active, the Program Counter (PC) will be inhibited from incrementing and an INTA will be issued.
  • During this cycle a RESTART or CALL instruction can be inserted to jump to the interrupt service routine. The INTR is enabled and disabled by software. It is disabled by Reset and immediately after an interrupt is accepted.

INTA (Output):

  • INTERRUPT ACKNOWLEDGE: is used instead of (and has the same timing as) RD during the instruction cycle after an INTR is accepted.
  • It can be used to activate the 8259 Interrupt chip or some other interrupt port.


These three inputs have the same timing as INTR except they cause an internal RESTART to be automatically inserted.

  • RST 7.5 Highest Priority
  • RST 6.5
  • RST 5.5 Lowest Priority

TRAP (Input):

  • Trap interrupt is a non maskable restart interrupt. It is recognized at the same time as INTR. It is unaffected by any mask or Interrupt Enable. It has the highest priority of any interrupt.

RESET IN (Input):

  • Reset sets the Program Counter to zero and resets the Interrupt    Enable  and HLDA flipflops.
  • None of the other flags or registers (except the instruction register) are affected The CPU is held in the reset condition as long as Reset is applied.

RESET OUT (Output):

  • Indicates CPlJ is being reset. Can be used as a system RESET. The signal is synchronized to the processor clock.

X1, X2 (Input)

  • Crystal or R/C network connections to set the internal clock generator X1 can also be an external clock input instead of a crystal. The input frequency is divided by 2 to give the internal operating frequency.

CLK (Output):

  • Clock Output for use as a system clock when a crystal or R/ C network is used as an input to the CPU. The period of CLK is twice the X1, X2 input period.

IO/M (Output):

  • IO/M indicates whether the Read/Write is to memory or l/O Tristated during Hold and Halt modes.

SID (Input):

  • Serial input data line The data on this line is loaded into accumulator bit 7 whenever a RIM instruction is executed.

SOD (output):

  • Serial output data line. The output SOD is set or reset as specified by the SIM instruction.

Vcc: +5 volt supply.

Vss: Ground Reference.