Intel 80C186XL 3.5BUS CYCLES, 3.5.1Read Bus Cycles, Bus Interface Unit, 2. Read Bus Cycle Types

BUS INTERFACE UNIT

An idle bus state may or may not drive the bus. An idle bus state following a bus read cycle con- tinues to float the bus. An idle bus state following a bus write cycle continues to drive the bus. The BIU drives no control strobes active in an idle state except to indicate the start of another bus cycle.

3.5BUS CYCLES

There are four basic types of bus cycles: read, write, interrupt acknowledge and halt. Interrupt acknowledge and halt bus cycles define special bus operations and require separate discussions. Read bus cycles include memory, I/O and instruction prefetch bus operations. Write bus cycles include memory and I/O bus operations. All read and write bus cycles have the same basic format.

The following sections present timing equations containing symbols found in the data sheet. The timing equations provide information necessary to start a worst-case design analysis.

3.5.1Read Bus Cycles

Figure 3-19 illustrates a typical read cycle. Table 3-2 lists the three types of read bus cycles.

				Table 3-2. Read Bus Cycle Types
	Status Bit			Bus Cycle Type
S2		S1	S0
0		0	1	Read I/O — Initiated by the Execution Unit for IN, OUT, INS, OUTS instructions
				or by the DMA Unit. A19:16 are driven to zero (see Chapter 10, “Direct Memory
				Access Unit”).
1		0	0	Instruction Prefetch — Initiated by the BIU. Data read from the bus fills the
				prefetch queue.
1		0	1	Read Memory — A19:0 select the desired byte or word memory location.

Figure 3-20 illustrates a typical 16-bit interface connection to a read-only device interface. The same example applies to an 8-bit bus system, except that no devices connect to an upper bus. Four parameters (Table 3-3) must be evaluated when determining the compatibility of a memory (or I/O) device. TADLTCH defines the delay through the address latch.

Table 3-3. Read Cycle Critical Timing Parameters

Memory Device		Description				Equation
Parameter
TOE	Output enable				low) to data valid	2TCLCL – TCLRL – TDVCL
			(RD
TACC	Address valid to data valid					3TCLCL – TCLAV –TADLTCH – TDVCL
TCE	Chip enable				to data valid	3TCLCL – TCLOV2 – TCLIS
		(UCS)
TDF	Output disable				high) to output float	TRHAV
				(RD

3-20