Spartan-3E FPGA Family: DC and Switching Characteristics
DS312 (v4.1) July 19, 2013
Product Specification
133
The capacitive load (CL) is connected between the output
and GND. The Output timing for all standards, as published
in the speed files and the data sheet, is always based on a
CL value of zero. High-impedance probes (less than 1 pF)
are used for all measurements. Any delay that the test
fixture might contribute to test measurements is subtracted
from those measurements to produce the final timing
numbers as published in the speed files and data sheet.
Using IBIS Models to Simulate Load
Conditions in Application
IBIS models permit the most accurate prediction of timing
delays for a given application. The parameters found in the
IBIS model (VREF, RREF, and VMEAS) correspond directly
with the parameters used in Table 95 (VT, RT, and VM). Do not confuse VREF (the termination voltage) from the IBIS
model with VREF (the input-switching threshold) from the
table. A fourth parameter, CREF, is always zero. The four
parameters describe all relevant output test conditions. IBIS
models are found in the Xilinx development software as well
as at the following link:
Delays for a given application are simulated according to its
specific load conditions as follows:
1.
Simulate the desired signal standard with the output
driver connected to the test setup shown in
Figure 72.Use parameter values VT, RT, and VM from Table 95. CREF is zero.
2.
Record the time to VM.
3.
Simulate the same signal standard with the output
driver connected to the PCB trace with load. Use the
appropriate IBIS model (including VREF, RREF, CREF,
and VMEAS values) or capacitive value to represent the
load.
4.
Record the time to VMEAS.
5.
Compare the results of steps 2 and 4. Add (or subtract)
the increase (or decrease) in delay to (or from) the
appropriate Output standard adjustment (
Table 94) to
yield the worst-case delay of the PCB trace.
DIFF_HSTL_I_18
-
VREF – 0.5
VREF + 0.5
50
0.9
VICM
DIFF_HSTL_III_18
-
VREF – 0.5
VREF + 0.5
50
1.8
VICM
DIFF_SSTL18_I
-
VREF – 0.5
VREF + 0.5
50
0.9
VICM
DIFF_SSTL2_I
-
VREF – 0.5
VREF + 0.5
50
1.25
VICM
Notes:
1.
Descriptions of the relevant symbols are as follows:
VREF – The reference voltage for setting the input switching threshold
VICM – The common mode input voltage
VM – Voltage of measurement point on signal transition
VL – Low-level test voltage at Input pin
VH – High-level test voltage at Input pin
RT – Effective termination resistance, which takes on a value of 1MΩ when no parallel termination is required
VT – Termination voltage
2.
The load capacitance (CL) at the Output pin is 0 pF for all signal standards.
3.
According to the PCI specification.
Table 95: Test Methods for Timing Measurement at I/Os (Cont’d)
Signal Standard
(IOSTANDARD)
Inputs
Outputs
Inputs and
Outputs
VREF (V)
VL (V)
VH (V)
RT (Ω)VT (V)
VM (V)