PC87200 datasheet(15/33 Pages) NSC

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6.0 Configuration

a. Connect the NOGO signal of the south bridge to Prohibit

pin of PC87200, SERIRQ to SERIRQ, REQ[A] to

PCPCIREQ#, GNT[A] to PCPCIGNT# (other PC/PCI REQ,

GNT pair may also be used). If BPD# is not being used, it

should be pulled high.

b. On power-up reset (i.e., cold boot), the NOGO signal is

high on reset and therefore disables the subtractive decod-

ing capability of PC87200. The rationale for doing this is as

follows: on the PCI bus, only one subtractive decoding

agent can exist. Since the south bridge will normally power

up as the subtractive decoding agent, the PC87200 must

be “prohibited” from doing so. Any other GPO pin which is

high on reset can also be used.

c. After PCI bus enumeration, when the device ID and ven-

dor ID of PC87200 is found, do the following:

d. Enable serial IRQ in continuous mode, 21 frames and 4

clock start frame by setting register 64h of south bridge to

d0h. (In the old south bridge, the serial IRQ pin is multi-

plexed with a GPIO and need to be selected as serial IRQ.

In the latest industry standard south bridge, however, the

serial IRQ is a dedicated pin and does not require initializa-

tion.)

e. Enable PC/PCI DMA by setting the PC/PCI request and

grant pins to PC/PCI DMA mode, instead of using them as

GPIOs. Any PC/PCI request and grant pair can be used,

but on the south bridge, GNT[B] is multiplexed with GNT[5],

so PC/PCI request and grant pair A is more preferable to

use.

Set all channels of DMA to PC/PCI DMA except for chan-

nels 4 & 2 (channel 4 is for cascading the DMAC and chan-

nel 2 is for FDC). The value to be put in register 91:90 is

5475 for the latest south bridge and 5455 for older south

bridges. The difference is due to the use of “11” for LPC

DMA, and the value of 00 (ISA DMA) being put into

reserved instead.

f. Enable positive decode for the devices under the new

south bridge: program the value of 3C0F into reg. E7:E6

will enable positive decode for most devices except sound

and gameport. Changing it to enable/disable other devices

will be straightforward for the new south bridge. (For old

south bridges, it will be harder, and requires one to ﬁnd all

the bits to set/reset. )

g. Enable the new south bridge to do positive decode by

setting bit 1 of reg. B0h to 1.

h. Set the NOGO signal to low.

i. Program register 42h of PC87200 to 20h. This is a

required initialization step.

j. Program a value of 03 into register D3h of device 1Fh in

function 0 of the new south bridge. This will prevent the

PC87200 from responding to conﬁguration accesses after-

wards. The rationale for doing this is to prevent Windows

from detecting two ISA bridges, the other one being the

PCI-to-LPC bridge, and thus creating a conﬂict. NOTE that

you must connect AD22 to IDSEL of PC87200 for this to

work.

After this, proceed in normal BIOS ﬂow. Note that step d to

j must be performed before ISA bus enumeration, and

therefore before ISA plug and play initialization and option

ROM checking. This is needed for plug and play cards and

cards with option ROM (e.g.,VGA cards, SCSI cards) to

work properly.

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