®
Altera Corporation 899
Understanding
FLEX 8000 Timing
May 1999, ver. 3 Application Note 76
A-AN-076-03
Introduction
Altera
®
devices provide predictable performance that is consistent from
simulation to application. Before configuring a device, you can determine
the worst-case timing delays for any design. You can calculate
propagation delays either with the MAX+PLUS
®
II Timing Analyzer or
with the timing models given in this application note and the timing
parameters listed in the
FLEX 8000 Programmable Logic Device Family Data
Sheet
in this data book.
1
For the most precise timing results, you should use the
MAX+PLUS II Timing Analyzer, which accounts for the effects
of secondary factors such as placement and fan-out.
This application note defines device internal and external timing
parameters, and illustrates the timing model for the FLEX
®
8000
(including FLEX 8000A) device family.
Familiarity with the FLEX 8000 architecture and characteristics is
assumed. Refer to the
FLEX 8000 Programmable Logic Device Family Data
Sheet
for a complete description of the FLEX 8000 architecture and for
specific timing parameter values.
Internal Timing
Parameters
The timing delays contributed by individual FLEX 8000 architectural
elements are called internal timing parameters, which cannot be
measured explicitly. All internal timing parameters are shown in italic
type. The following list defines the internal timing parameters for the
FLEX 8000 device family.
I/O Element Timing Parameters
The following list defines the I/O element (IOE) timing parameters for the
FLEX 8000 device family.
t
IN
I/O input pad and buffer delay. The time required for a
signal on an I/O pin, used as an input, to reach a row or
column channel of the FastTrack
®
Interconnect.
t
IOD
Output data delay. The delay incurred by a signal routed
from the FastTrack Interconnect to an IOE.
t
IOC
IOE control delay. The delay for a signal used to control the
I/O register’s clock or clear inputs.
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t
IOE
IOE output enable delay. The delay for a signal used to
control the output enable of the IOE’s tri-state buffer.
t
IOCO
I/O register clock-to-output delay. The delay from the rising
edge of the I/O register’s clock to the time the data appears
at the register output.
t
IOCOMB
I/O register bypass delay. The delay for a combinatorial
signal to bypass the I/O register.
t
IOSU
I/O register setup time for data and enable signals before
clock. The time required for a signal to be stable at the I/O
register’s data and enable inputs before the register clock’s
rising edge. This parameter ensures that the register
correctly stores the input data.
t
IOH
I/O register hold time for data and enable signals after
clock. The time required for a signal to be stable at the I/O
register’s data and enable inputs after the register clock’s
rising edge. This parameter ensures that the register
correctly stores the input data.
t
IOCLR
I/O register clear delay. The delay from the time the I/O
register’s asynchronous clear input is asserted to the time
the register output stabilizes at a logic low.
t
OD1
Output buffer and pad delay with the slow slew rate logic
option turned off and V
CCIO
= 5.0 V.
t
OD2
Output buffer and pad delay with the slow slew rate logic
option turned off and V
CCIO
= 3.3 V.
t
OD3
Output buffer and pad delay with the slow slew rate logic
option turned on and V
CCIO
= 5.0 V or 3.3 V.
t
XZ
Output buffer disable delay. The delay required for high
impedance to appear at the output pin after the tri-state
buffer’s enable control is disabled.
t
ZX1
Output buffer enable delay with the slow slew rate logic
option turned off and V
CCIO
= 5.0 V. The delay required for
the output signal to appear at the output pin after the
tri-state buffer’s enable control is enabled.
t
ZX2
Output buffer enable delay with the slow slew rate logic
option turned off and V
CCIO
= 3.3 V. The delay required for
the output signal to appear at the output pin after the
tri-state buffer’s enable control is enabled.
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t
ZX3
Output buffer enable delay with the slow slew rate logic
option turned on and V
CCIO
= 5.0 V or 3.3 V. The delay
required for the output signal to appear at the output pin
after the tri-state buffer’s enable control is enabled.
Interconnect Timing Parameters
The following list defines the interconnect timing parameters for the
FLEX 8000 device family.
t
DIN_D
Dedicated input data delay. The time required for a signal
(used as a data input) to reach a logic element (LE) from a
dedicated input pin. The
t
DIN_D
delay is a function of
fan-out and the distance between the source pin and
destination LEs. The value shown in the
FLEX 8000
Programmable Logic Device Family Data Sheet
is the longest
delay possible for a pin with a fan-out of four LEs. However,
the value generated by the MAX+PLUS II Timing Analyzer
is more accurate because it includes considerations of the
fan-out and the relative locations of the source pin and
destination LEs of the design.
t
DIN_C
Dedicated input control delay. The delay of a signal coming
from a dedicated input pin that is used as an LE register
control. These signals include the clock, clear, and preset
inputs to the LE register.
t
DIN_IO
Dedicated input I/O control delay. The delay of a signal
from a dedicated input pin that is used as an IOE register
control. These signals include the clock and clear inputs to
the IOE register and the output enable control of the IOE’s
tri-state buffer.
t
COL
FastTrack Interconnect column delay. The delay incurred by
a signal that requires routing through a column channel in
the FastTrack Interconnect.
t
ROW
FastTrack Interconnect row delay. The delay incurred by a
signal that requires routing through a row channel in the
FastTrack Interconnect. The
t
ROW
delay is a function of
fan-out and the distance between the source and destination
LEs. The value shown in the
FLEX 8000 Programmable Logic
Device Family Data Sheet
is the longest delay possible for an
LE with a fan-out of four LEs. However, the value generated
by the MAX+PLUS II Timing Analyzer is more accurate
because it includes considerations of the fan-out and the
relative locations of the source and destination LEs of the
design.
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t
LOCAL
Local interconnect delay. The delay incurred by a signal
entering a logic array block (LAB) or routed between LEs in
the same LAB.
t
LABCARRY
Carry chain delay to the next LAB. The delay incurred by a
carry-out signal that carries into the next LAB in the row.
t
LABCASC
Cascade chain delay to the next LAB. The delay incurred by
a cascade-out signal that cascades into the next LAB in the
row.
Logic Element Timing Parameters
The following list defines the LE timing parameters for the FLEX 8000
device family.
t
LUT
Look-up table (LUT) delay. The delay incurred by
generating an LUT output from an LAB local interconnect
signal.
t
RLUT
LUT using LE feedback delay. The time required for the
output of an LE register to be fed back and used to generate
the LUT output in the same LE. This parameter is used in
one of the LE counter modes.
t
CLUT
Carry chain LUT delay. The delay incurred by a carry chain
signal that is used to generate the LUT output.
t
CGEN
Carry-out generation delay. The delay incurred by
generating a carry-out signal from an LAB local interconnect
signal.
t
CGENR
Carry-out generation using LE feedback delay. The time
required for the output of an LE register to be fed back and
used to generate the carry-out signal in the same LE.
t
CICO
Carry-in to carry-out delay. The delay incurred by
generating a carry-out signal that uses the carry-in signal
from the previous LE.
t
C
Register control delay. The time required for a signal to be
routed to the clock, preset, or clear input of an LE register.
t
GATE
Cascade gate delay. The time required for a signal to pass
through the cascade-generating
AND
gate in the LE. This
delay is incurred whether or not the cascade output is used.
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t
CASC
Cascade chain delay. The time required for a cascade-out
signal to be routed to the next LE in the same LAB. This
delay, along with
t
LABCASC
, is also used to calculate the
delay for a cascade-out signal to be routed to an LE in the
next LAB in the row.
t
CO
LE clock-to-output delay. The delay from the rising edge of
the LE register’s clock to the time the data appears at the
register output.
t
COMB
Combinatorial output delay. The time required for a
combinatorial signal to bypass the LE register and become
the output of the LE.
t
SU
LE register setup time for data and enable signals before
clock. The minimum time a signal is required to be stable at
the LE register’s data and enable inputs before the register
clock’s rising edge. This parameter ensures that the register
correctly stores the input data.
t
H
LE register hold time for data and enable signals after clock.
The minimum time a signal is required to be stable at the LE
register’s data and enable inputs after the register clock’s
rising edge. This parameter ensures that the register
correctly stores the input data.
t
PRE
LE register preset delay. The delay from the assertion of the
LE register’s asynchronous preset input to the time the
register output stabilizes at a logic high.
t
CLR
LE register clear delay. The delay from the assertion of the
LE register’s asynchronous clear input to the time the
register output stabilizes at a logic low.
t
CH
Minimum LE register clock-high time. The minimum time
the LE register’s clock input must remain at a stable logic
high state before the falling edge of the clock.
t
CL
Minimum LE register clock-low time. The minimum time
the LE register’s clock input must remain at a stable logic
low state before the rising edge of the clock.
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External Timing
Parameters
External timing parameters represent actual pin-to-pin timing
characteristics. Each external timing parameter consists of a combination
of internal delay elements. They are worst-case values, derived from
extensive performance measurements and are ensured by device testing
or characterization. For example,
t
DRR
is the AC operating specification.
All external timing parameters are shown in bold type. Other external
timing parameters can be estimated by using the timing model in
Figure 2.
t
DRR
Register-to-register delay. The time required for the signal
from one register to pass through four LEs via three row
interconnects and four local interconnects to reach the
D
input of a second register. The test circuit used for this
parameter is a register with an output that goes through
three
LCELL
primitives in two different LABs; the last
LCELL
feeds another register in another LAB. Figure 1 shows this
path. The test circuit file is available from Altera
Applications.
t
ODH
Output data hold time after clock. The minimum time a
registered output pin will remain at its previous value after
a rising edge is applied to the clock input pin. This
parameter applies to global and non-global clocking, and
for logic element and I/O element registers.
Figure 1. Path for t
DRR
Circuit for 21-Column FLEX 8000 Devices
FLEX 8000
Timing Model
Timing models are simplified block diagrams that illustrate the
propagation delays through Altera devices. Logic can be implemented on
different paths. You can trace the actual paths used in your FLEX 8000
device by examining the equations listed in the MAX+PLUS II Report File
(
.rpt
) for the project. You can then add up the appropriate internal timing
parameters to calculate the approximate propagation delays through the
FLEX 8000 device. However, the MAX+PLUS II Timing Analyzer
provides the most accurate timing information. Figure 2 shows the timing
model for FLEX 8000 devices.
out_1
PRN
CLRN
D
DFF LCELL
data_1
clk
9 LABs Apart4 LABs Apart3 LABs Apart Same LAB
QLCELL LCELL DFF
PRN
CLRN
DQ
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Figure 2. FLEX 8000 Timing Model
t
CO
t
COMB
t
SU
t
H
t
PRE
t
CLR
Register
Delays
LUT Delay
t
LUT
t
RLUT
t
CLUT
Carry Chain
Delay
Register
Control
Carry-In from
Previous LE Cascade-In from
Previous LE
LE-Out
t
LABCASC
t
LABCARRY
Cascade-Out
to Next LE in
Next LAB
Carry-Out
to Next LE
in Next
LAB
t
C
LE
Dedicated
Input Delays
I/O Pin
t
IOC
t
IOE
t
IN
t
IOD
I/O Register
Delays
Output Data
Delay
IOE
Control
Input
Delay
Output
Delays
t
IOCO
t
IOCOMB
t
IOSU
t
IOH
t
IOCLR
t
OD1
t
OD3
t
XZ
t
ZX1
t
ZX2
t
ZX3
IOE
Data-In
t
COL
t
ROW
t
LOCAL
Carry-Out
to Next LE
in Same
LAB
Cascade-Out
to Next LE in
Same LAB
Cascade
Gate Delay
t
GATE
t
CGEN
t
CGENR
t
CICO
t
DIN_D
t
DIN_C
t
DIN_IO
t
CASC
Cascade
Chain Delay
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Calculating
Timing Delays
You can calculate approximate pin-to-pin timing delays for FLEX 8000
devices with the timing model shown in Figure 2 and the internal timing
parameters in the
FLEX 8000 Programmable Logic Device Family Data Sheet
in this data book. Each timing delay is calculated from a combination of
internal timing parameters. Figure 3 shows the FLEX 8000 device family
LE timing delays. To calculate the delay for a signal that follows a
different path through the FLEX 8000 device, refer to the FLEX 8000
timing model to determine which internal timing delays to add together.
Figure 3. Logic Element Timing Delays (Part 1 of 4)
Combinatorial Delay
t1
t2
From Row I/O Inputs:
t1=
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
+
t
COMB
+
t
COL
+
t
IOD
+
t
IOCOMB
+
t
OD1
t2=
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
+
t
COMB
+
t
ROW
+
t
IOD
+
t
IOCOMB
+
t
OD1
From Dedicated Inputs:
t1=
t
DIN_D
+
t
LOCAL
+
t
LUT
+
t
GATE
+
t
COMB
+
t
COL
+
t
IOD
+
t
IOCOMB
+
t
OD1
t2=
t
DIN_D
+
t
LOCAL
+
t
LUT
+
t
GATE
+
t
COMB
+
t
ROW
+
t
IOD
+
t
IOCOMB
+
t
OD1
Row I/O Row I/O
Column I/O
Row I/O Combinatorial
Logic
Combinatorial
Logic
t1
t2
Dedicated Input Row I/O
Column I/O
Dedicated Input Combinatorial
Logic
Combinatorial
Logic
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Figure 3. Logic Element Timing Delays (Part 2 of 4)
Tri-State Enable/Disable Delay
From Row I/O Inputs through logic:
tXZ, tZX =
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
+
t
COMB
+
t
ROW
+
t
IOE
+ (
t
XZ
or
t
ZX1
)
Directly from Dedicated Inputs:
tXZ, tZX =
t
DIN_IO
+
t
IOE
+ (
t
XZ
or
t
ZX1
)
Directly from Row I/O Inputs:
tXZ, tZX =
t
IN
+
t
ROW
+
t
IOE
+ (
t
XZ
or
t
ZX1
)
tXZ or tZX
Row I/O Combinatorial
Logic Any I/O
Dedicated Input
Any I/O
Row I/O
Any I/O
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Figure 3. Logic Element Timing Delays (Part 3 of 4)
LE Register Clear & Preset Time
From Row I/O Inputs to Row or Column Outputs:
tCLR =
t
IN
+
t
ROW
+
t
LOCAL
+
t
C
+
t
CLR
+ (
t
ROW
or
t
COL
) +
t
IOD
+
t
IOCOMB
+
t
OD1
tPRE =
t
IN
+
t
ROW
+
t
LOCAL
+
t
C
+
t
PRE
+ (
t
ROW
or
t
COL
) +
t
IOD
+
t
IOCOMB
+
t
OD1
From Dedicated Inputs to Row or Column Outputs:
tCLR =
t
DIN_C
+
t
C
+
t
CLR
+ (
t
ROW
or
t
COL
) +
t
IOD
+
t
IOCOMB
+
t
OD1
tPRE =
t
DIN_C
+
t
C
+
t
PRE
+ (
t
ROW
or
t
COL
) +
t
IOD
+
t
IOCOMB
+
t
OD1
Setup Time from a Global Clock & Row I/O Data Input
tSU = (
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
) – (
t
DIN_C
+
t
C
) +
t
SU
Row I/O
Dedicated clock
LE Register
Combinatorial
Logic
tCLR
tPRE
Row I/O
Any I/O
Any I/O
Any I/O
Any I/O
LE Register
Row I/O
LE Register
tCLR
tPRE
Dedicated Input
LE Register
Dedicated Input
LE Register
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Figure 3. Logic Element Timing Delays (Part 4 of 4)
Asynchronous Setup Time from a Row I/O Clock & Row I/O Data Input
tASU = (
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
) – (
t
IN
+
t
ROW
+
t
LOCAL
+
t
C
) +
t
SU
Asynchronous Hold Time from a Row I/O Clock & Row I/O Data Input
tAH = (
t
IN
+
t
ROW
+
t
LOCAL
+
t
C
) – (
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
) +
t
H
Clock-to-Output Delay from a Global Clock to Any Output
tCO =
t
DIN_C
+
t
C
+
t
CO
+ (
t
ROW
or
t
COL
)
+
t
IOD
+
t
IOCOMB
+
t
OD1
Clock-to-Output Delay from a Row I/O Clock to Any Output
tACO =
t
IN
+
t
ROW
+
t
LOCAL
+
t
C
+
t
CO
+ (
t
ROW
or
t
COL
)
+
t
IOD
+
t
IOCOMB
+
t
OD1
Combinatorial
Logic
Combinatorial
Logic
Row I/O
Dedicated Clock
LE Register
Row I/O
Row I/O
LE Register
Row I/O
LE Register
Any I/O
LE Register
Row I/O Any I/O
Hold Time from a Global Clock & Row I/O Data Input
tH= (
t
DIN_C
+
t
C
) – (
t
IN
+
t
ROW
+
t
LOCAL
+
t
LUT
+
t
GATE
) +
t
H
Row I/O
Dedicated Clock
LE Register
Combinatorial
Logic
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Figure 4 shows the FLEX 8000 device family I/O element timing delays.
To calculate the delay for a signal that follows a different path through the
FLEX 8000 device, refer to the FLEX 8000 timing model to determine
which internal timing parameters to add together.
Figure 4. I/O Element Timing Delays (Part 1 of 2)
I/O Element Clear Time
From Row I/O Inputs:
tCLR =
t
IN
+
t
ROW
+
t
IOC
+
t
IOCLR
+
t
OD1
From Dedicated Inputs:
tCLR =
t
DIN_IO
+
t
IOC
+
t
IOCLR
+
t
OD1
Setup Time from a Global Clock & Any I/O Data Input
tSU =
t
IN
– (
t
DIN_IO
+
t
IOC
) +
t
IOSU
Hold Time from a Global Clock & Any I/O Data Input
tH= (
t
DIN_IO
+
t
IOC
) –
t
IN
+
t
IOH
Any I/O
Dedicated Clock
IOE Input Register
Any I/O
Dedicated Clock
IOE Output Register
Any I/O
Row I/O
IOE Output Register
Any I/O
Dedicated Input
IOE Input Register
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Figure 4. I/O Element Timing Delays (Part 2 of 2)
Timing Model
vs.
MAX+PLUS II
Timing
Analyzer
Hand calculations based on the timing model provide a good estimate of
a design’s performance. However, the MAX+PLUS II Timing Analyzer
always provides the most accurate information on the performance of a
design, because it takes into account three secondary factors that influence
the tROW and tDIN_D parameters:
■Fan-out for each signal in the delay path
■Positions of other loads relative to the source and destination
■Distance between signal source and destination
Fan-Out
The more loads a signal has to drive, the longer the delay across tROW and
tDIN_D. These delays are functions of the number of LABs that a signal
source has to drive as well as of the number of LEs in the LAB that use the
signal. The number of LABs that a signal drives has a greater effect on the
delay than the number of cells in the LAB that use the signal.
Clock-to-Output Delay from a Global Clock to Any Output
tCO =
t
DIN_IO
+
t
IOC
+
t
IOCO
+
t
OD1
Clock-to-Output Delay from a Row I/O Clock to Any Output
tACO =
t
IN
+
t
ROW
+
t
IOC
+
t
IOCO
+
t
OD1
Row I/O Any I/O
Dedicated Clock
IOE Output Register
Any I/O
IOE Output Register
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Load Distribution
The load distribution relative to the source and destination also affects the
tROW and tDIN_D delays. Consider a signal s1 that feeds destination d1 and
logic elements y[4..1]. If y[4..1] are in different LABs, s1 has four
additional loads. However, if the LEs are all in the same LAB, s1 has four
shorter-delay loads. Therefore, the row interconnect delay from s1 to d1
is greater when each load y[4..1] is in a different LAB. Figure 5
illustrates the change in the tROW and tDIN_D delays caused by variations
in the position of d1 and the distribution of y[4..1].
Figure 5. Delay from s1 to d1 as a Function of Relative Position & Load Distribution
Shorter Delay
Longer Delay
d1
y1
y2
y3
y4
s1
s1 y4y3y2y1
s1
LAB
d1y4y3y2y1s1
d1
y1
y2
y3
y4
d1
LAB LAB
LAB LAB LAB LAB LAB LAB
LAB LAB LAB
LAB LAB LAB LAB LAB LAB
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Distance
The distance between the source and destination LEs also affects the tROW
and tDIN_D parameters. For example, if s1 drives an LE in the same row,
the delay from s1 to the LE increases as the distance from s1 to the LE
increases.
Examples The following examples show how to use internal timing parameters to
estimate the delays for real applications.
Example 1: 4-Bit Equality Comparator with a Cascade Chain
You can analyze the timing delays for circuits that have been subjected to
minimization and logic synthesis. The synthesized equations are available
in your project’s MAX+PLUS II Report File (.rpt). These equations are
structured so that you can quickly determine the logic implementation of
any signal. For example, Figure 6 shows a 4-bit equality comparator.
Figure 6. 4-Bit Equality Comparator Circuit
CASCADE
a0
b0
a1
b1
a2
b2
a3
b3
eq
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The MAX+PLUS II Report File for the circuit shown in Figure 6 gives the
equations for eq, the output of the comparator:
eq = _LC2_B1;
_LC2_B1 = LCELL( _EQ002C);
_EQ002C = _EQ002 & CASCADE( _EQ001C);
_EQ002 = a2 & a3 & b2 & b3
# a2 & !a3 & b2 & !b3
# !a2 & a3 & !b2 & b3
# !a2 & !a3 & !b2 & !b3;
_LC1_B1 = LCELL( _EQ001C);
_EQ001C = _EQ001;
_EQ001 = a0 & a1 & b0 & b1
# a0 & !a1 & b0 & !b1
# !a0 & a1 & !b0 & b1
# !a0 & !a1 & !b0 & !b1;
Figure 7 shows a synthesized 4-bit equality comparator.
Figure 7. Synthesized 4-Bit Equality Comparator
The output pin eq is the output of the second LE of a cascade chain. The
LUT of _LC1_B1 implements the comparison of the first two bits. The
comparison of the second two bits is implemented in the LUT of _LC2_B1.
The outputs of these two LUTs are then cascaded together to form the
output of _LC2_B1.
If a2 and eq are both row I/O pins, the timing delay from a2 to eq can be
estimated by adding the following parameters:
tIN + tROW + tLOCAL + tLUT + tGATE + tCOMB + tROW + tIOD + tIOCOMB + tOD1
a0
b0
_EQ002
a1
b1
LUT
a2
b2
a3
b3
LUT eq
_EQ001C
_EQ002C
_LC2_B1
_LC1_B1
Altera Corporation 915
AN 76: Understanding FLEX 8000 Timing
If a0 is a row I/O pin, the timing delay from a0 to eq can be estimated by
adding the following parameters:
tIN + tROW + tLOCAL + tLUT + tGATE + tCASC + tGATE + tCOMB + tROW +
tIOD + tIOCOMB + tOD1
Example 2: 3-Bit Adder Using a Carry Chain
FLEX 8000 devices have specialized resources that implement complex
arithmetic functions. For instance, adders and counters require a carry
function to determine whether or not to increment the next significant bit.
The FLEX 8000 architecture has a built-in carry chain that performs this
function. This example explains how to estimate the delay for a 3-bit
adder that uses a carry chain (see Figure 8).
Figure 8. 3-Bit Adder Implemented with a Carry Chain
b0
CARRY
a0 sum0
CARRY
b1
a1 sum1
CARRY
b2
a2 sum2
cout
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AN 76: Understanding FLEX 8000 Timing
The MAX+PLUS II Report File contains the following equations for the
3-bit adder in Figure 8:
cout = _LC5_B1;
sum0 = _LC2_B1;
sum1 = _LC3_B1;
sum2 = _LC4_B1;
_LC2_B1 = LCELL( _EQ001);
_EQ001 = !a0 & b0
# a0 & !b0;
_LC2_B1_CARRY = CARRY( _EQ002);
_EQ002 = a0 & b0;
_LC3_B1 = LCELL( _EQ003);
_EQ003 = a1 & !b1 & !_LC2_B1_CARRY
# !a1 & !b1 & _LC2_B1_CARRY
# a1 & b1 & _LC2_B1_CARRY
# !a1 & b1 & !_LC2_B1_CARRY;
_LC3_B1_CARRY = CARRY( _EQ004);
_EQ004 = a1 & _LC2_B1_CARRY
# a1 & b1
# b1 & _LC2_B1_CARRY;
_LC4_B1 = LCELL( _EQ005);
_EQ005 = a2 & !b2 & !_LC3_B1_CARRY
# !a2 & !b2 & _LC3_B1_CARRY
# a2 & b2 & _LC3_B1_CARRY
# !a2 & b2 & !_LC3_B1_CARRY;
_LC5_B1 = LCELL( _LC4_B1_CARRY);
_LC4_B1_CARRY = CARRY( _EQ006);
_EQ006 = a2 & _LC3_B1_CARRY
# a2 & b2
# b2 & _LC3_B1_CARRY;
Altera Corporation 917
AN 76: Understanding FLEX 8000 Timing
Figure 9 shows a synthesized 3-bit adder.
Figure 9. Synthesized 3-Bit Adder
In Figure 9, LE _LC2_B1 generates sum0 and a carry-out signal
(_LC2_B1_CARRY) that feeds the carry-in of _LC3_B1. _LC3_B1 generates
sum1 and a carry-out signal (_LC3_B1_CARRY) that feeds the carry-in of
_LC4_B1. _LC4_B1 generates sum2 and cout using a2, b2, and
_LC3_B1_CARRY. The cout signal must pass through _LC5_B1 because a
carry buffer cannot directly feed a pin.
a0
b0
_LC2_B1
sum0
_EQ001
LUT
Carry Chain _LC2_B1_CARRY
sum1
a1
_LC3_B1
b1 LUT
Carry Chain _LC3_B1_CARRY
_EQ003
_LC5_B1
cout
LUT
sum2
a2
_LC4_B1
b2 LUT
Carry Chain _LC4_B1_CARRY
_EQ005
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AN 76: Understanding FLEX 8000 Timing
If a0 and sum1 are row I/O pins, the timing delay from a0 to sum1 can be
estimated by adding the following microparameters:
tIN + tROW + tLOCAL + tCGEN + tCICO + tCLUT + tGATE + tCOMB + tROW +
tIOD + tIOCOMB + tODI
If a0 and cout are row I/O pins, the timing delay from a0 to cout can be
estimated by adding the following microparameters:
tIN + tROW + tLOCAL + tCGEN + tCICO + tCICO +tCLUT + tGATE +
tCOMB + tROW + tIOD + tIOCOMB + tODI
Conclusion The FLEX 8000 device architecture has predictable internal timing delays
that can be estimated based on signal synthesis and placement. The
MAX+PLUS II Timing Analyzer provides the most accurate timing
information. However, you can also use the FLEX 8000 timing model
shown in Figure 2, along with the timing parameters listed in the
FLEX 8000 Programmable Logic Device Family Data Sheet in this data book,
to estimate a design’s performance before compilation. Both methods
enable you to accurately predict your design’s in-system timing
performance.
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