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Chapter: Microprocessor and Microcontroller : 8086 System Bus Structure

Signal Description of 80286

CLK: This is the system clock input pin. A23-A0: These are the physical address output lines used to address memory or I/O devices.

Signal Description of 80286

CLK: This is the system clock input pin. The clock frequency applied at this pin is divided by two internally and is used for deriving fundamental timings for basic operations of the circuit. The clock is generated using 8284 clock generator.


D15-D0: These are sixteen bidirectional data bus lines.

A23-A0: These are the physical address output lines used to address memory or I/O devices. The address lines A23 - A16 are zero during I/O transfers


BHE: This output signal, as in 8086, indicates that there is a transfer on the higher byte of the data bus (D15 – D8) .


S1 , S0: These are the active-low status output signals which indicate initiation of a bus cycle and with M/IO and COD/INTA, they define the type of the bus cycle.


M/ IO: This output line differentiates memory operations from I/O operations. If this signal is it “0” indicates that an I/O cycle or INTA cycle is in process and if it is “1” it indicates

that a memory or a HALT cycle is in progress.


COD/ INTA: This output signal, in combination with M/ IO signal and S1 , S0 distinguishes different memory, I/O and INTA cycles.


LOCK: This active-low output pin is used to prevent the other masters from gaining the control of the bus for the current and the following bus cycles. This pin is activated by a "LOCK" instruction prefix, or automatically by hardware during XCHG, interrupt acknowledge or descriptor table access

READY This active-low input pin is used to insert wait states in a bus cycle, for interfacing low speed peripherals. This signal is neglected during HLDA cycle.


HOLD and HLDA This pair of pins is used by external bus masters to request for the control of the system bus (HOLD) and to check whether the main processor has granted the control (HLDA) or not, in the same way as it was in 8086.


INTR: Through this active high input, an external device requests 80286 to suspend the current instruction execution and serve the interrupt request. Its function is exactly similar to that of INTR pin of 8086.


NMI: The Non-Maskable Interrupt request is an active-high, edge-triggered input that is equivalent to an INTR signal of type 2. No acknowledge cycles are needed to be carried out.


PEREG and PEACK (Processor Extension Request and Acknowledgement)


Processor extension refers to coprocessor (80287 in case of 80286 CPU). This pair of pins extends the memory management and protection capabilities of 80286 to the processor extension 80287. The PEREQ input requests the 80286 to perform a data operand transfer for a processor extension. The PEACK active-low output indicates to the processor extension that the requested operand is being transferred.


BUSY and ERROR: Processor extension BUSY and ERROR active-low input signals indicate the operating conditions of a processor extension to 80286. The BUSY goes low, indicating 80286 to suspend the execution and wait until the BUSY become inactive. In this duration, the processor extension is busy with its allotted job. Once the job is completed the processor extension drives the BUSY input high indicating 80286 to continue with the program execution. An active ERROR signal causes the 80286 to perform the processor extension interrupt while executing the WAIT and ESC instructions. The active ERROR signal indicates to 80286 that the processor extension has committed a mistake and hence it

is reactivating the processor extension interrupt.


CAP: A 0.047 μf, 12V capacitor must be connected between this input pin and ground to filter the output of the internal substrate bias generator. For correct operation of 80286 the capacitor must be charged to its operating voltage. Till this capacitor charges to its full capacity, the 80286 may be kept stuck to reset to avoid any spurious activity.


Vss: This pin is a system ground pin of 80286.

Vcc: This pin is used to apply +5V power supply voltage to the internal circuit of 80286. RESET The active-high RESET input clears the internal logic of 80286, and reinitializes it RESET The active-high reset input pulse width should be at least 16 clock cycles. The 80286 requires at least 38 clock cycles after the trailing edge of the RESET input signal, before it makes the first opcode fetch cycle.


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Microprocessor and Microcontroller : 8086 System Bus Structure : Signal Description of 80286 |

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