24888 Rev 3.03 - July 12, 2004 AMD-8151
TM
AGP Tunnel Data Sheet
28
Link PHY Compensation Control Registers DevA:0x[E8, E4, E0]
The link PHY circuitry includes automatic compensation that is used to adjust the electrical characteristics for
the link transmitters and receivers on both sides of the tunnel. There is one compensation circuit for the receiv-
ers 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. The overrides and difference values may be dif-
ferent between sides A and B of the tunnel. 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 01h to 13h (19
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# does not alter any of the
fields.
Default: See below. Attribute: See below.
Bits Description
31 Must be low. Read-write. This bit is required to be low at all times; setting it high results in undefined
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 field is not predictable.
15 Reserved.
14:13 BCTL: link side B PHY control value. Read-write. These two bits combine to specify the PHY
compensation value that is applied to side B of the tunnel as follows:
BCTL
Description
00b Apply CALCCOMP directly as the compensation value.
01b Apply BDATA directly as the compensation value.
10b Apply the sum of CALCCOMP and BDATA 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 BDATA as the compensation value. If the
difference is less than 01h, then 01h is applied.
The default value of this field (from PWROK reset) is controlled by the CMPOVR signal. If
CMPOVR = 0, the default is 00b. If CMPOVR = 1, the default is 01b.
12:8 BDATA: link side B data value. Read-write. This value is applied to the side B of the tunnel PHY
compensation as described in BCTL. The default for DevA:0x[E4, E0] is 08h. The default for
DevA:0xE8 is 0Fh.
7 Reserved.
