Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
61
where T
CYC
is a single common clock period and T
VAL
, T
FLT
, T
SU
, and T
SKEW
are as defined in Table 31
below. If a very short bus connects the driver and receiver, it is then necessary to make sure that there
is not a hold time violation where the timing relationship, which must be met, is the following:
T
VAL(min)
+ T
FLT(min)
- T
SKEW
> T
H
Equation 2
where T
VAL
is the sum of the skews between an agent’s input clock, the clock on the driver’s output
latch, and finally through the output driver. T
VAL
is composed of the following components:
• Input delay from the system clock through the package ball/pin/pad onto the die
• Clock skew in the internal clock distribution tree
• Clock phase error (for PLL operation only)
• Clock jitter (for PLL operation only)
• Data rise/fall time delay
T
SKEW
is the sum of all sources of skew between the two agents and includes the following
components:
• Clock jitter difference between the separate outputs on the clock generator
• Clock skew between separate outputs on the clock generator
• Trace skew for the two clock traces to the agents due to length or impedance mismatches
and crosstalk
• Skew from input loading differences between the two agents
For common clock operations, the sum of the factors affecting clock skew can easily be kept to less
than 10% of the total timing budget of 15.0 ns. Table 31 lists the common clock timing budget.
Table 31: Common Clock Timing Budget
Symbol Parameter Min Max Units
T
CYC
CLK Cycle time 15 30 ns
T
Skew
CLK Skew between
AGP3.0 devices
- 1.0 ns
T
Val
CLK to command valid 1.0 5.5 ns
T
Flt
Flight time to load 0 2.5 ns
T
SU
Setup to CLK 6.0 ns
T
H
Hold from CLK 0 ns
3.3.2 Source Synchronous Operation
In source synchronous operation, the signal timing relationships are defined in two parts: first and
foremost is the “inner loop” timing requirements that must be met. Within this loop, common clock
parameters such as buffer T
VAL
and flight time are mostly cancelled out by the source synchronous
methodology. The timing specifications of primary concern are those affecting the matching of data to
strobes. These include the following effects: