PCI7421 datasheet(94/298 Pages) TI

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4−4

4.4

Command Register

The PCI command register provides control over the PCI7x21/PCI7x11 interface to the PCI bus. All bit functions

adhere to the definitions in the PCI Local Bus Specification (see Table 4−3). None of the bit functions in this register

are shared among the PCI7x21/PCI7x11 PCI functions. Three command registers exist in the PCI7x21/PCI7x11

controller, one for each function. Software manipulates the PCI7x21/PCI7x11 functions as separate entities when

enabling functionality through the command register. The SERR_EN and PERR_EN enable bits in this register are

internally wired OR between the three functions, and these control bits appear to software to be separate for each

function.

Bit

Name

Command

Type

Default

Command

Offset:

04h

Type:

Read-only, Read/Write

Default:

0000h

Table 4−3. Command Register Description

BIT

SIGNAL

TYPE

FUNCTION

15−11

RSVD

Reserved. Bits 15−11 return 0s when read.

INT_DISABLE

INTx disable. When set to 1, this bit disables the function from asserting interrupts on the INTx signals.

0 = INTx assertion is enabled (default)

1 = INTx assertion is disabled

FBB_EN

Fast back-to-back enable. The PCI7x21/PCI7x11 controller does not generate fast back-to-back

transactions; therefore, this bit is read-only. This bit returns a 0 when read.

SERR_EN

System error (SERR) enable. This bit controls the enable for the SERR driver on the PCI interface. SERR

can be asserted after detecting an address parity error on the PCI bus. Both this bit and bit 6 must be set

for the PCI7x21/PCI7x11 controller to report address parity errors.

0 = Disables the SERR output driver (default)

1 = Enables the SERR output driver

RSVD

Reserved. Bit 7 returns 0 when read.

PERR_EN

Parity error response enable. This bit controls the PCI7x21/PCI7x11 response to parity errors through the

PERR signal. Data parity errors are indicated by asserting PERR, while address parity errors are indicated

by asserting SERR.

0 = PCI7x21/PCI7x11 controller ignores detected parity errors (default).

1 = PCI7x21/PCI7x11 controller responds to detected parity errors.

VGA_EN

VGA palette snoop. When set to 1, palette snooping is enabled (i.e., the PCI7x21/PCI7x11 controller does

not respond to palette register writes and snoops the data). When the bit is 0, the PCI7x21/PCI7x11

controller treats all palette accesses like all other accesses.

MWI_EN

Memory write-and-invalidate enable. This bit controls whether a PCI initiator device can generate memory

write-and-invalidate commands. The PCI7x21/PCI7x11 controller does not support memory

write-and-invalidate commands, it uses memory write commands instead; therefore, this bit is hardwired

to 0. This bit returns 0 when read. Writes to this bit have no effect.

SPECIAL

Special cycles. This bit controls whether or not a PCI device ignores PCI special cycles. The

PCI7x21/PCI7x11 controller does not respond to special cycle operations; therefore, this bit is hardwired

to 0. This bit returns 0 when read. Writes to this bit have no effect.

MAST_EN

Bus master control. This bit controls whether or not the PCI7x21/PCI7x11 controller can act as a PCI bus

initiator (master). The PCI7x21/PCI7x11 controller can take control of the PCI bus only when this bit is set.

0 = Disables the PCI7x21/PCI7x11 ability to generate PCI bus accesses (default)

1 = Enables the PCI7x21/PCI7x11 ability to generate PCI bus accesses

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