1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
|
// See LICENSE for license details.
package chiselTests
import chisel3._
import chisel3.testers.BasicTester
import chisel3.util._
import chisel3.util.random.{PRNG, LFSR}
/**
* This test creates two 4 sided dice.
* Each cycle it adds them together and adds a count to the bin corresponding to that value
* The asserts check that the bins show the correct distribution.
*/
//scalastyle:off magic.number
class LFSRDistribution(gen: => UInt, cycles: Int = 10000) extends BasicTester {
val rv = gen
val bins = Reg(Vec(8, UInt(32.W)))
// Use tap points on each LFSR so values are more independent
val die0 = Cat(Seq.tabulate(2) { i => rv(i) })
val die1 = Cat(Seq.tabulate(2) { i => rv(i + 2) })
val (trial, done) = Counter(true.B, cycles)
val rollValue = die0 +& die1 // Note +& is critical because sum will need an extra bit.
bins(rollValue) := bins(rollValue) + 1.U
when(done) {
printf(p"bins: $bins\n") // Note using the printable interpolator p"" to print out a Vec
// test that the distribution feels right.
assert(bins(1) > bins(0))
assert(bins(2) > bins(1))
assert(bins(3) > bins(2))
assert(bins(4) < bins(3))
assert(bins(5) < bins(4))
assert(bins(6) < bins(5))
assert(bins(7) === 0.U)
stop()
}
}
class LFSRMaxPeriod(gen: => UInt) extends BasicTester {
val rv = gen
val started = RegNext(true.B, false.B)
val seed = withReset(!started) { RegInit(rv) }
val (_, wrap) = Counter(started, math.pow(2.0, rv.getWidth).toInt - 1)
when (rv === seed && started) {
chisel3.assert(wrap)
stop()
}
val last = RegNext(rv)
chisel3.assert(rv =/= last, "LFSR last value (0b%b) was equal to current value (0b%b)", rv, last)
}
/** Check that the output of the new LFSR is the same as the old LFSR */
class MeetTheNewLFSR16SameAsTheOldLFSR16 extends BasicTester {
val en = Counter(2).value.asBool
/** This is the exact implementation of the old LFSR16 algorithm */
val oldLfsr = {
val width = 16
val lfsr = RegInit(1.U(width.W))
when (en) {
lfsr := Cat(lfsr(0)^lfsr(2)^lfsr(3)^lfsr(5), lfsr(width-1,1))
}
lfsr
}
/** The new LFSR16 uses equivalent taps and a reverse so that it can use LFSR(16) under the hood. */
val newLfsr = LFSR16(en)
val (_, done) = Counter(true.B, 16)
assert(oldLfsr === newLfsr)
when (done) {
stop()
}
}
class LFSRSpec extends ChiselPropSpec {
property("LFSR16 can be used to produce pseudo-random numbers, this tests the distribution") {
assertTesterPasses{ new LFSRDistribution(LFSR16()) }
}
property("LFSR16 period tester, Period should 2^16 - 1") {
assertTesterPasses{ new LFSRMaxPeriod(LFSR16()) }
}
property("New LFSR16 is the same as the old LFSR16") {
assertTesterPasses{ new MeetTheNewLFSR16SameAsTheOldLFSR16 }
}
}
|
