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5.1.4 Application Notes
5.1.4.1 REQ/GNT
These signals are used only by bus-mastering PCI devices. Most SOM-Express
modules do not have enough REQ/GNT pairs available to support a bus-mastering
device at every slot position. A PCI arbiter design is recommended when extra
REQ/GNT pairs are required. Figure 5-5 shows an example design for PCI arbiter.
FRAME#
STOP#
PCIREQ#1
PCIGNT#1
MS1PREQ#1
MS1PGNT#1
MS1PREQ#2
MS1PGNT#2
MS1PREQ#3
MS1PGNT#3
If there are less than four REQ/GNT pairs available for external devices, they will be
assigned starting with the REQ0#/GNT0# pair. Therefore, external bus-mastering
devices should be placed in the lowest numbered slot positions and non-bus
mastering devices should be placed in the highest-numbered slot positions. Refer to
Chapter 2.3.1 REQ/GNT for details.
5.1.4.2 PC104-Plus Connector
If a PC104-Plus connector is used, the same signals are attached to the connector
but the pin numbers differ because of the different connector type. See the PC/104-
Plus Specification Version 1.0, February 1997, PC/104 Consortium (www.pc104.org)
for details.
46
Advantech SOM-Express Design Guide
1
28
FRAME#
AVCC
2
21
STOP#
VCC
3
9
SY SREQ#
VCC
4
SY SGNT#
5
27
PCIREQ1#
PCICLKI
7
26
PCIGNT1#
RESET#
8
23
PCIREQ2#
PCICLK0
10
22
PCIGNT2#
PCICLK1
11
20
PCIREQ3#
PCICLK2
12
19
PCIGNT3#
PCICLK3
18
PCICLK4
13
VC3A
16
25
VC3B
AVSS
24
VSS
14
17
VC5A
VSS
15
6
VC5B
VSS
MS-1
Figure 5-5 Design Example: PCI Arbiter
Chapter 5 Carrier Board Design Guidelines
MS1PCICLK
PCIRST#
33
PCICLK1
33
PCICLK2
33
PCICLK3
33
33
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