External Synchronization VIA Test

8086

As an alternative to the interrupts and general I/O capabilities, the 8086 provides a single software- testable input known as the TEST signal. At any time the program may execute a WAIT instruction. If at that time the TEST signal is inactive (HIGH), pro- gram execution becomes suspended while the proc- essor waits for TEST to become active. It must remain active for at least 5 CLK cycles. The WAIT instruction is re-executed repeatedly until that time. This activity does not consume bus cycles. The processor remains in an idle state while waiting. All 8086 drivers go to 3-state OFF if bus ‘‘Hold’’ is en- tered. If interrupts are enabled, they may occur while the processor is waiting. When this occurs the proc- essor fetches the WAIT instruction one extra time, processes the interrupt, and then re-fetches and re- executes the WAIT instruction upon returning from the interrupt.

Basic System Timing

Typical system configurations for the processor op- erating in minimum mode and in maximum mode are shown in Figures 4a and 4b, respectively. In mini- mum mode, the MN/MX pin is strapped to VCC and the processor emits bus control signals in a manner similar to the 8085. In maximum mode, the MN/MX pin is strapped to VSS and the processor emits cod- ed status information which the 8288 bus controller uses to generate MULTIBUS compatible bus control signals. Figure 5 illustrates the signal timing relation- ships.

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Figure 7. 8086 Register Model

SYSTEM TIMING—MINIMUM SYSTEM

The read cycle begins in T1 with the assertion of the Address Latch Enable (ALE) signal. The trailing (low- going) edge of this signal is used to latch the ad- dress information, which is valid on the local bus at this time, into the address latch. The BHE and A0 signals address the low, high, or both bytes. From T1 to T4 the M/IO signal indicates a memory or I/O operation. At T2 the address is removed from the local bus and the bus goes to a high impedance state. The read control signal is also asserted at T2. The read (RD) signal causes the addressed device to enable its data bus drivers to the local bus. Some time later valid data will be available on the bus and the addressed device will drive the READY line HIGH. When the processor returns the read signal to a HIGH level, the addressed device will again 3- state its bus drivers. If a transceiver is required to buffer the 8086 local bus, signals DT/R and DEN are provided by the 8086.

A write cycle also begins with the assertion of ALE and the emission of the address. The M/IO signal is again asserted to indicate a memory or I/O write operation. In the T2 immediately following the ad- dress emission the processor emits the data to be written into the addressed location. This data re- mains valid until the middle of T4. During T2, T3, and TW the processor asserts the write control signal. The write (WR) signal becomes active at the begin- ning of T2 as opposed to the read which is delayed somewhat into T2 to provide time for the bus to float.

The BHE and A0 signals are used to select the prop- er byte(s) of the memory/IO word to be read or writ- ten according to the following table:


BHE	A0	Characteristics

0	0	Whole word
0	1	Upper byte from/to
		odd address
1	0	Lower byte from/to
		even address
1	1	None

I/O ports are addressed in the same manner as memory location. Even addressed bytes are trans- ferred on the D7 – D0 bus lines and odd addressed bytes on D15 – D8.

The basic difference between the interrupt acknowl- edge cycle and a read cycle is that the interrupt ac- knowledge signal (INTA) is asserted in place of the read (RD) signal and the address bus is floated. (See Figure 6.) In the second of two successive INTA cycles, a byte of information is read from bus