The following pin function descriptions are for 80C88 systems in either minimum or maximum mode. The “local bus” in these
descriptions is the direct multiplexed bus interface connection to the 80C88 (without regard to additional bus buffers).
ADDRESS DATA BUS: These lines constitute the time multiplexed memory/IO address (T1) and
data (T2,T3,Tw and T4) bus. These lines are active HIGH and are held at high impedance to the last
valid level during interrupt acknowledge and local bus “hold acknowledge” or “grant sequence”
ADDRESS BUS: These lines provide address bits 8 through 15 for the entire bus cycle (T1-T4).
These lines do not have to be latched by ALE to remain valid. A15-A8 are active HIGH and are held
at high impedance to the last valid logic level during interrupt acknowledge and local bus “hold
acknowledge” or “grant sequence”.
ADDRESS/STATUS: During T1, these are the four most
signiﬁcant address lines for memory operations. During
I/O operations, these lines are LOW. During memory and
I/O operations, status information is available on these
lines during T2, T3, TW and T4. S6 is always LOW. The
status of the interrupt enable ﬂag bit (S5) is updated at the
beginning of each clock cycle. S4 and S3 are encoded as
This information indicates which segment register is
presently being used for data accessing.
These lines are held at high impedance to the last valid
logic level during local bus “hold acknowledge” or “grant
S4 S3 CHARACTERISTICS
0 0 Alternate Data
0 1 Stack
1 0 Code or None
1 1 Data
READ: Read strobe indicates that the processor is performing a memory or I/O read cycle, depend-
ing on the state of the IO/M pin or S2. This signal is used to read devices which reside on the 80C88
local bus. RD is active LOW during T2, T3, Tw of any read cycle, and is guaranteed to remain HIGH
in T2 until the 80C88 local bus has ﬂoated.
This line is held at a high impedance logic one state during “hold acknowledge” or “grant sequence”.
READY: is the acknowledgment from the address memory or I/O device that it will complete the data
transfer. The RDY signal from memory or I/O is synchronized by the 82C84A clock generator to from
READY. This signal is active HIGH. The 80C88 READY input is not synchronized. Correct operation
is not guaranteed if the set up and hold times are not met.
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 operation.
A subroutine is vectored to via an interrupt vector lookup table located in system memory. It can be
internally masked by software resetting the interrupt enable bit. INTR is internally synchronized. This
signal is active HIGH.
TEST: input is examined by the “wait for test” instruction. If the TEST input is LOW, execution con-
tinues, otherwise the processor waits in an “idle” state. This input is synchronized internally during
each clock cycle on the leading edge of CLK.
NONMASKABLE INTERRUPT: is an edge triggered input which causes a type 2 interrupt. A sub-
routine is vectored to via an interrupt vector lookup table located in system memory. NMI is not
maskable internally by software. A transition from a LOW to HIGH initiates the interrupt at the end
of the current instruction. This input is internally synchronized.
RESET: cases the processor to immediately terminate its present activity. The signal must transition
LOW to HIGH and remain active HIGH for at least four clock cycles. It restarts execution, as de-
scribed in the instruction set description, when RESET returns LOW. RESET is internally synchro-
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: is the +5V power supply pin. A 0.1µF capacitor between pins 20 and 40 recommended for de-
GND: are the ground pins (both pins must be connected to system ground). A 0.1µF capacitor be-
tween pins 1 and 20 is recommended for decoupling.
MINIMUM/MAXIMUM: indicates the mode in which the processor is to operate. The two modes are
discussed in the following sections.