wishbone: fix that RoundRobin might assert CYC indefinitely
[nmigen-soc.git] / nmigen_soc / scheduler.py
1 from nmigen import *
2
3
4 __all__ = ["RoundRobin"]
5
6
7 class RoundRobin(Elaboratable):
8 """A round-robin scheduler.
9
10 Parameters
11 ----------
12 n : int
13 Maximum number of requests to handle.
14
15 Attributes
16 ----------
17 request : Signal(n)
18 Signal where a '1' on the i-th bit represents an incoming request from the i-th device.
19 grant : Signal(range(n))
20 Signal that equals to the index of the device which is currently granted access.
21 stb : Signal()
22 Strobe signal to enable granting access to the next device requesting. Externally driven.
23 """
24 def __init__(self, n):
25 self.n = n
26 self.request = Signal(n)
27 self.grant = Signal(range(n))
28 self.stb = Signal()
29
30 def elaborate(self, platform):
31 m = Module()
32
33 with m.If(self.stb):
34 with m.Switch(self.grant):
35 for i in range(self.n):
36 with m.Case(i):
37 with m.If(~self.request[i]):
38 for j in reversed(range(i+1, i+self.n)):
39 # If i+1 <= j < n, then t == j; (after i)
40 # If n <= j < i+n, then t == j - n (before i)
41 t = j % self.n
42 with m.If(self.request[t]):
43 m.d.sync += self.grant.eq(t)
44
45 return m