
148
Registers
Chapter 4
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
HyperTransport
Link PHY Compensation Control Registers
DevA:0x[E8, E4, E0]
The HyperTransport PHY circuitry includes automatic compensation that is used to adjust the
electrical characteristics for the HyperTransport transmitters and receivers. There is one
compensation circuit for the receivers and one for each polarity of the transmitters. These registers
provide visibility into the calculated output of the compensation circuits, the ability to override the
calculated value with software-controlled values, and the ability to offset the calculated values with a
fixed difference. These registers specify the compensation parameters as follows:
DevA:0xE0: transmitter rising edge (P) drive strength compensation.
DevA:0xE4: transmitter falling edge (N) drive strength compensation.
DevA:0xE8: receiver impedance compensation.
For DevA:0x[E4, E0], higher values represent higher drive strength; the values range from 00h to 13h
(20 steps). For DevA:0xE8, higher values represent lower impedance; the values range from 00h to
1Fh (32 steps).
Note:
The default state of these registers is set by PWROK reset; assertion of RESET_L does not
alter any of the fields.
Default:
See below.
Attribute:
See below.
Bits
31
Description
Must be Low. Read-write. This bit is required to be Low at all times; setting it High results in unde
fi
ned
behavior.
30:21 Reserved.
20:16
CALCCOMP. Calculated compensation value.
Read-only. This provides the calculated value from
the auto compensation circuitry. The default value of this
fi
eld is not predictable.
15:7
Reserved.
6:5
CTL. HyperTransport
PHY control value.
Read-write. These two bits combine to specify the PHY
compensation value as follows:
CTL
Description
00b
Apply CALCCOMP directly as the compensation value.
01b
Apply DATA directly as the compensation value.
10b
Apply the sum of CALCCOMP and DATA as the compensation value. In DevA:0x[E4,
E0], if the sum exceeds 13h, then 13h is applied. In DevA:0x[E8], if the sum exceeds 1Fh, then 1Fh is
applied.
11b
Apply the difference of CALCCOMP minus DATA as the compensation value. If the
difference is less than 00h, then 00h is applied.
The default value of this
fi
eld (from PWROK reset) is controlled by DevB:3x48[CMPOVR]. If
CMPOVR = 0, the default is 00b. If CMPOVR = 1, the default is 01b.
4:0
DATA. HyperTransport PHY data value.
Read-write. This value is applied to the HyperTransport
PHY compensation as described in CTL. The default for DevA:0x[E4, E0] is 08h. The default for
DevA:0xE8 is 0Fh.