2-19
Architecture
Lattice Semiconductor LatticeECP/EC Family Data Sheet
Table 2-8. An Example of Sign Extension
OVERFLOW Flag from MAC
The sysDSP block provides an overflow output to indicate that the accumulator has overflowed. When two
unsigned numbers are added and the result is a smaller number then accumulator roll over is said to occur and
overflow signal is indicated. When two positive numbers are added with a negative sum and when two negative
numbers are added with a positive sum, then the accumulator “roll-over” is said to have occurred and an overflow
signal is indicated. Note when overflow occurs the overflow flag is present for only one cycle. By counting these
overflow pulses in FPGA logic, larger accumulators can be constructed. The conditions overflow signal for signed
and unsigned operands are listed in Figure 2-22.
Figure 2-22. Accumulator Overflow/Underflow Conditions
ispLEVER Module Manager
The user can access the sysDSP block via the ispLEVER Module Manager, which has options to configure each
DSP module (or group of modules) or through direct HDL instantiation. Additionally Lattice has partnered Math-
works to support instantiation in the Simulink tool, which is a Graphical Simulation Environment. Simulink works
with ispLEVER and dramatically shortens the DSP design cycle in Lattice FPGAs.
Number Unsigned
Unsigned
9-bit
Unsigned
18-bit Signed
Two’s Complement
Signed 9-Bits
Two’s Complement
Signed 18-bits
+5 0101 000000101 000000000000000101 0101 000000101 000000000000000101
-6 0110 000000110 000000000000000110 1010 111111010 111111111111111010
000000000
000000001
000000010
000000011
111111101
111111110
111111111
Overflow signal is generated
for one cycle when this
boundary is crossed
0
+1
+2
+3
-3
-2
-1
Unsigned Operation
Signed Operation
0101111111
0101111110
0101111101
0101111100
1010000010
1010000001
1010000000
255
254
253
252
254
255
256
000000000
000000001
000000010
000000011
111111101
111111110
111111111
Carry signal is generated for
one cycle when this
boundary is crossed
0
1
2
3
509
510
511
0101111111
0101111110
0101111101
0101111100
1010000010
1010000001
1010000000
255
254
253
252
258
257
256
