AM486DX2 datasheet(16/67 Pages) AMD

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Am5X86 Microprocessor

AMD

PRELIMINARY

LOCK

Bus Lock (Active Low; Output)

A Low output on this pin indicates that the current bus

cycle is locked. The microprocessor ignores HOLD

when LOCK is asserted (although it does acknowledge

AHOLD and BOFF). LOCK goes active in the first clock

of the first locked bus cycle and goes inactive after the

last clock of the last locked bus cycle. The last locked

cycle ends when RDY is returned. LOCK is active Low

and is not driven during bus hold. Locked read cycles

are not transformed into cache fill cycles if KEN is active.

M/IO

Memory/Input-Output (Active High/Active Low;

Output)

A High output indicates a memory cycle. A Low output

indicates an I/O cycle.

NMI

Non-Maskable Interrupt (Active High; Input)

A High NMI input signal indicates that an external non-

maskable interrupt has occurred. NMI is rising-edge

sensitive. NMI must be held Low for at least four CLK

periods before this rising edge. The NMI input does not

have an internal pull-down resistor. The NMI input is

asynchronous, but must meet setup and hold times t20

and t21 for recognition in any specific clock.

PCD

Page Cache Disable (Active High; Output)

This pin reflects the state of the PCD bit in the page

table entry or page directory entry (programmable

through the PCD bit in CR3). If paging is disabled, the

CPU ignores the PCD bit and drives the PCD output

Low. PCD has the same timing as the cycle definition

pins (M/IO, D/C, and W/R). PCD is active High and is

not driven during bus hold. PCD is masked by the Cache

Disable bit (CD) in Control Register 0 (CR0).

PCHK

Parity Status (Active Low; Output)

Parity status is driven on the PCHK pin the clock after

RDY for read operations. The parity status reflects data

sampled at the end of the previous clock. A Low PCHK

indicates a parity error. Parity status is checked only for

enabled bytes as is indicated by the byte enable and

bus size signals. PCHK is valid only in the clock imme-

diately after read data is returned to the microprocessor;

at all other times PCHK is inactive High. PCHK is floated

only during Tri-state Test mode (see FLUSH).

PLOCK

Pseudo-Lock (Active Low; Output)

In Write-back mode, the processor forces the output

High and the signal is always read as inactive. In Write-

through mode, PLOCK operates normally. When

asserted, PLOCK indicates that the current bus

transaction requires more than one bus cycle. Examples

of such operations are segment table descriptor reads

(8 bytes) and cache line fills (16 bytes). The micropro-

cessor drives PLOCK active until the addresses for the

last bus cycle of the transaction have been driven,

whether or not RDY or BRDY is returned. PLOCK is a

function of the BS8, BS16, and KEN inputs. PLOCK

should be sampled on the clock when RDY is returned.

PLOCK is active Low and is not driven during bus hold.

PWT

Page Write-Through (Active High; Output)

This pin reflects the state of the PWT bit in the page

table entry or page directory entry (programmable

through the PWT bit in CR3). If paging is disabled, the

CPU ignores the PWT bit and drives the PWT output

Low. PWT has the same timing as the cycle definition

pins (M/IO, D/C, and W/R). PWT is active High and is

not driven during bus hold.

RESET

Reset (Active High; Input)

RESET forces the microprocessor to initialize. The mi-

croprocessor cannot begin execution of instructions un-

til at least 1 ms after VCC and CLK have reached their

proper DC and AC specifications. To ensure proper mi-

croprocessor operation, the RESET pin should remain

active during this time. RESET is active High. RESET

is asynchronous but must meet setup and hold times

t20 and t21 to ensure recognition on any specific clock.

RDY

Non-Burst Ready (Active Low; Input)

A Low input on this pin indicates that the current bus

cycle is complete, that is, either the external system has

presented valid data on the data pins in response to a

read, or the external system has accepted data from the

microprocessor in response to a write. RDY is ignored

when the bus is idle and at the end of the bus cycle’s

first clock. RDY is active during address hold. Data can

be returned to the processor while AHOLD is active.

RDY is active Low and does not have an internal pull-

up resistor. RDY must satisfy setup and hold times t16

and t17 for proper chip operation.

SMI

SMM Interrupt (Active Low; Input)

A Low signal on the SMI pin signals the processor to

enter System Management mode (SMM). SMI is the

highest level processor interrupt. The SMI signal is rec-

ognized on an instruction boundary, similar to the NMI

and INTR signals. SMI is sampled on every rising clock

edge. SMI is a falling-edge sensitive input. The SMI input

has an internal pull-up resister. Recognition of SMI is

guaranteed in a specific clock if it is asserted synchro-

nously and meets the setup and hold times. If SMI is

asserted asynchronously, it must go High for a minimum

of two clocks before going Low, and it must remain Low

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