Pin Description (Continued)
The following pin function descriptions are for 80C86 systems in either minimum or maximum mode. The “Local Bus” in these description is
the direct multiplexed bus interface connection to the 80C86 (without regard to additional bus buffers).
I INTERRUPT REQUEST: is a level triggered input which is sampled during the last clock cycle
of each instruction to determine if the processor should enter into an interrupt acknowledge op-
eration. A subroutine is vectored to via an interrupt vector lookup table located in system mem-
ory. It can be internally masked by software resetting the interrupt enable bit.
lNTR is internally synchronized. This signal is active HIGH.
I TEST: input is examined by the “Wait” instruction. If the TEST input is LOW execution continues,
otherwise the processor waits in an “Idle” state. This input is synchronized internally during each
clock cycle on the leading edge of CLK.
I NON-MASKABLE INTERRUPT: is an edge triggered input which causes a type 2 interrupt. A
subroutine is vectored to via an interrupt vector lookup table located in system memory. NMI is
not maskable internally by software. A transition from LOW to HIGH initiates the interrupt at the
end of the current instruction. This input is internally synchronized.
I RESET: causes the processor to immediately terminate its present activity. The signal must tran-
sition LOW to HIGH and remain active HIGH for at least four clock cycles. It restarts execution,
as described in the Instruction Set description, when RESET returns LOW. RESET is internally
I CLOCK: provides the basic timing for the processor and bus controller. It is asymmetric with a
33% duty cycle to provide optimized internal timing.
VCC: +5V power supply pin. A 0.1µF capacitor between pins 20 and 40 is recommended for de-
GND: Ground. Note: both must be connected. A 0.1µF capacitor between pins 1 and 20 is rec-
ommended for decoupling.
I MINIMUM/MAXIMUM: Indicates what mode the processor is to operate in. The two modes are
discussed in the following sections.
Minimum Mode System
The following pin function descriptions are for the 80C86 in minimum mode (i.e., MN/MX = VCC). Only the pin functions which are unique to
minimum mode are described; all other pin functions are as described below.
O STATUS LINE: logically equivalent to S2 in the maximum mode. It is used to distinguish a mem-
ory access from an I/O access. M/lO becomes valid in the T4 preceding a bus cycle and remains
valid until the ﬁnal T4 of the cycle (M = HIGH, I/O = LOW). M/lO is held to a high impedance logic
one during local bus “hold acknowledge”.
WR 29 O WRITE: indicates that the processor is performing a write memory or write I/O cycle, depending
on the state of the M/IO signal. WR is active for T2, T3 and TW of any write cycle. It is active
LOW, and is held to high impedance logic one during local bus “hold acknowledge”.
O INTERRUPT ACKNOWLEDGE: is used as a read strobe for interrupt acknowledge cycles. It is
active LOW during T2, T3 and TW of each interrupt acknowledge cycle. Note that INTA is never
ADDRESS LATCH ENABLE: is provided by the processor to latch the address into the
82C82/82C83 address latch. It is a HIGH pulse active during clock LOW of T1 of any bus cycle.
Note that ALE is never ﬂoated.