24888 Rev 3.03 - July 12, 2004 AMD-8151
TM
AGP Tunnel Data Sheet
19
AGP PHY Control Register DevA:0x[54, 50]
These registers apply to the compensation values of AGP clock-forwarded data and strobe signals as follows:
• DevA:0x50: data signals A_AD[31:0], A_CBE_L[3:0], A_DBI[H, L], and A_SBA[7:0].
• DevA:0x54: strobe signals A_ADSTB[1:0]_[P, N] and A_SBSTB_[P, N].
NCTL, NDATA, and NCOMP are related to (1) the falling edge drive strength of the signals as outputs and (2)
the impedance of the signals as inputs. PCTL, PDATA, and PCOMP are related to the rising edge drive
strength of the signals as outputs only. For the [N, P]DATA and [N, P]COMP fields of these registers, 00h cor-
responds to the weakest drive strength and the highest receive impedance. For the [N, P]DATA and [N,
P]COMP fields of these registers, the highest values corresponds to the strongest drive strength and lowest
receive impedance.
External compensation resistors are used by the IC to determine the proper drive strength values. The resistors
correlate the calculated values as follows:
• A_CALD is used to calculate DevA:0x50[PCOMP] (data signal rising edge drive strength).
• A_CALD# is used to calculate DevA:0x50[NCOMP] (data signal falling edge drive strength and receive
impedance).
• A_CALS is used to calculate DevA:0x54[PCOMP] (strobe rising edge drive strength).
• A_CALS# is used to calculate DevA:0x54[NCOMP] (strobe falling edge drive strength and receive imped-
ance).
Note: when new values are written to these registers, new compensation values are not updated to the AGP
PHY automatically; the periodic calibration cycle specified by DevA:0xA8[PCALCYC] must pass in order for
the AGP PHY calibration values to take effect.
Default: 000? 000?h Attribute: See below.
Bits Description
31:30 NCTL: AGP PHY N (falling edge) compensation control. Read-write. These two bits combine to
specify the PHY falling edge compensation value that is applied to AGP signals as follows:
NCTL
Description
00b Apply NCOMP directly as the compensation value.
01b Apply NDATA directly as the compensation value.
10b Apply the sum of NCOMP and NDATA as the compensation value. If the sum
exceeds 3Fh, then 3Fh is applied.
11b Apply the difference of NCOMP minus NDATA as the compensation value. If the dif-
ference is less than 00h, then 00h is applied.
29:28 Reserved.
27:22 NDATA: AGP falling edge drive strength control. Read-write. This value is applied to the falling-
edge (N transistor) PHY compensation as described in NCTL.
21:16 NCOMP: AGP falling edge drive strength. Read only. This provides the calculated value of the
falling-edge (N transistor) drive strength of the AGP signals. The default for this field varies. This
field is updated by the hardware approximately every 8 microseconds.
