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76 Pin Descriptions Chapter 11
AMD Athlon™ XP Processor Model 8 Data Sheet 25175H—March 2003
Preliminary Information
The FID[3:0] signals are open-drain processor outputs that are
pulled High on the motherboard and sampled by the chipset to
determine the SIP (Serialization Initialization Packet) that is
sent to the processor. The FID[3:0] signals are valid after
PWROK is asserted. The FID[3:0]signals must not be sampled
until they become valid. See the AMD Athlon™ System Bus
Specification, order# 21902 for more information about
Serialization Initialization Packets and SIP protocol.
The processor FID[3:0] outputs are open-drain and 2.5-V
tolerant. To prevent damage to the processor, if these signals
are pulled High to above 2.5 V, they must be electrically
isolated from the processor. For information about the FID[3:0]
isolation circuit, see the AMD Athlon™ Processor-Based
Motherboard Design Guide, order# 24363.
Table 26. FID[3:0] Clock Multiplier Encodings
FID[3:0]
2
Processor Clock to SYSCLK Frequency Ratio
0000 11
0001 11.5
0010 12
0011
≥ 12.5
1
0100 5
0101 5.5
0110 6
0111 6.5
1000 7
1001 7.5
1010 8
1011 8.5
1100 9
1101 9.5
1110 10
1111 10.5
Notes:
1. All ratios greater than or equal to 12.5x have the same FID[3:0] code of 0011b, which causes
the SIP configuration for all ratios of 12.5x or greater to be the same.
2. BIOS initializes the CLK_Ctl MSR during the POST routine. This CLK_Ctl setting is used with all
FID combinations and selects a Halt disconnect divisor and a Stop Grant disconnect divisor.
For more information, refer to the AMD Athlon™ and AMD Duron™ Processors BIOS,
Software, and Debug Developers Guide, order# 21656.