24888 Rev 3.03 - July 12, 2004 AMD-8151
TM
AGP Tunnel Data Sheet
20
AGP PHY Skew Control Register DevA:0x58
DSKEW and SSKEW are designed such that when they are both programmed to the same value, the AGP out-
put strobes transition near the center of the data eye. To move the strobe to a later point in the data eye, the
value of SSKEW is increased. To move the strobe to an earlier point in the data eye, DSKEW is increased.
These values translate into skew approximately as follows:
For values 0h to 8h, the skew is about: [D, S]SKEW x 80 picoseconds.
For values 9h to Fh, the skew is about: 800 + ([D, S]SKEW - 8) x 400 picoseconds.
However, these values vary with process, temperature, and voltage. Note that the lower values provide fine res-
olution and the upper values provide coarse resolution.
Default: 0000 0000h. Attribute: Read-write.
15:14 PCTL: AGP PHY P (rising edge) compensation control. Read-write. These two bits combine to
specify the PHY rising edge compensation value that is applied to AGP signals as follows:
PCTL
Description
00b Apply PCOMP directly as the compensation value.
01b Apply PDATA directly as the compensation value.
10b Apply the sum of PCOMP and PDATA as the compensation value. If the sum exceeds
1Fh, then 1Fh is applied.
11b Apply the difference of PCOMP minus PDATA as the compensation value. If the dif-
ference is less than 00h, then 00h is applied.
13:12 Reserved.
11 RW: read-write bit. Read-write. This controls no logic.
10:6 PDATA: AGP rising edge drive strength control. Read-write. This value is applied to the rising-
edge (P transistor) PHY compensation as described in PCTL.
5 Reserved.
4:0 PCOMP: AGP rising edge drive strength. Read only. This provides the calculated value of the ris-
ing-edge (P transistor) drive strength of the AGP signals. The default for this field varies. This field is
updated by the hardware approximately every 8 microseconds.
Bits Description
31:8 Reserved.
7:4 DSKEW: AGP data skew. Read-write. This specifies the alignment of the AGP data signal outputs,
A_AD[31:0], A_CBE_L[3:0], and A_DBI[H, L], relative internal clocks. 0h=The strobe transitions
earliest. Fh=The strobe transitions latest.
3:0 SSKEW: AGP strobe skew. Read-write. This specifies the alignment of the AGP strobe signal out-
puts, A_ADSTB[1:0], relative internal clocks. 0h=The strobe transitions earliest. Fh=The strobe tran-
sitions latest.
