82870P2P64H2 Datasheet, PDF (166/217 Pages) –

Functional Description
R
4.7.1.2 Architecture
The P64H2 SMBus register interface appears to an SMBus master as a stack. The first register in
the stack is at offset 00h, and is the Command Register; the second register is at offset 01h, and is
the bus number register, etc. as shown in Figure 10.
Figure 10. Stack View of SMBus Register Space
Register Index
Offset
111
110
101
100
011
010
001
000
Register
Data 3
Data 2
Data 1
Data 0
Register Num ber
Device / Function Num ber
Bus Number
Command / Status
smbus_reg_space
The internal stack has the following characteristics:
• Every write cycle must start with a write to the stack pointer, called “Register Index” in
Figure 10.
• The act of receiving a STOP indication on the SMBus from the master causes the P64H2 to
perform the command encoded in the Command Register to internal configuration register
pointed to by the Bus Number, Device/Function Number, and Register Number Registers. If
the action programmed in the Command Register is a write, then the data registers are also
used. The P64H2 should make no assumption that all bytes will be received. In other words,
on a word write, BIOS should not expect to receive the command, bus number,
device/function number, register number, and then two data bytes. The master could stop at
any byte or within any byte, and the P64H2 must run the cycle with the data it has currently
collected.
• Once a cycle has started the register index stack pointer is no longer considered valid
(i.e., if 4 bytes were written, it is not guaranteed that the pointer is now 4 bytes from its
previous location).
• When a read is attempted, the P64H2 expects 4 bytes to be returned. However, if it receives a
NACK from the master on one if its bytes, it will issue a STOP on the bus and end the
transfer. The first byte returned will start from the current register index.
• When the P64H2 is busy with a command (read or write), it will issue NACKs on the bus on
any subsequent command. It is expected that the master will come back.
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Intel^® 82870P2 P64H2 Datasheet