soc: cores: add up5kspram module

The ICE40UP5K has 128 kB of SPRAM that's designed to be used
as memory for a softcore.  This memory is actually 4 16-bit
chunks that we can gang together to give us either 64 kB or
128 kB.

Add a module that will allow us to use this memory in an ICE40.
Signed-off-by: Sean Cross <sean@xobs.io>

litex/soc/cores/up5kspram.py

+from migen import *
+from litex.soc.interconnect import wishbone
+
+# ICE40 UltraPlus family-specific Wishbone interface to the Single Port RAM
+# (SPRAM) primitives. Because SPRAM is much more coarse grained than Block
+# RAM resources, this RAM is only minimally configurable at present (64kB or
+# 128kB). Because it is single port, this module is meant to be used as the
+# CPU's RAM region, leaving block RAM free for other use.
+
+class Up5kSPRAM(Module):
+    def __init__(self, width=32, size=64*1024):
+
+        # Right now, LiteX only supports 32-bit CPUs. To get a 32-bit data bus,
+        # we must width-cascade 2 16-bit SPRAMs. We've already used 2 out of 4
+        # SPRAMs for this, so the only other valid config is using all 4 SPRAMs
+        # by depth-cascading.
+        if width != 32:
+            raise ValueError("Width of Up5kSPRAM must be 32 bits")
+        if size != 64*1024 and size != 128*1024:
+            raise ValueError("Size of Up5kSPRAM must be 64kB or 128kB.")
+
+        self.bus = wishbone.Interface(width)
+
+        bytesels = []
+        for i in range(0, 2):
+            datain = Signal(16)
+            dataout = Signal(16)
+            maskwren = Signal(4)
+            wren = Signal(1)
+
+            # 64k vs 128k-specific routing signals.
+            datain0 = Signal(16)
+            dataout0 = Signal(16)
+            maskwren0 = Signal(4)
+
+            if size == 128 * 1024:
+                datain1 = Signal(16)
+                dataout1 = Signal(16)
+                maskwren1 = Signal(4)
+
+            # Generic routing common to all depths.
+            for j in range(16):
+                self.comb += [self.bus.dat_r[16*i + j].eq(dataout[j])]
+
+            self.comb += [
+                datain.eq(self.bus.dat_w[16*i:16*i+16]),
+                # MASKWREN is nibble-based, interestingly enough.
+                maskwren.eq(
+                    Cat(
+                        Replicate(self.bus.sel[2*i], 2),
+                        Replicate(self.bus.sel[2*i + 1], 2),
+                    )
+                ),
+                wren.eq(self.bus.we & self.bus.stb & self.bus.cyc)
+            ]
+
+            # Signals which must be routed differently based on depth.
+            # 64kB
+            if size == 64*1024:
+                self.comb += [
+                    datain0.eq(datain),
+                    dataout.eq(dataout0),
+                    maskwren0.eq(maskwren)
+                ]
+            # 128kB
+            else:
+                self.comb += [
+                    If(self.bus.adr[14],
+                        datain1.eq(datain),
+                        dataout.eq(dataout1),
+                        maskwren1.eq(maskwren)
+                    ).Else(
+                        datain0.eq(datain),
+                        dataout.eq(dataout0),
+                        maskwren0.eq(maskwren)
+                    )
+                ]
+
+            self.specials.spram = Instance("SB_SPRAM256KA",
+                i_ADDRESS=self.bus.adr[0:14], i_DATAIN=datain0,
+                i_MASKWREN=maskwren0,
+                i_WREN=wren,
+                i_CHIPSELECT=C(1,1),
+                i_CLOCK=ClockSignal("sys"),
+                i_STANDBY=C(0,1),
+                i_SLEEP=C(0,1),
+                i_POWEROFF=C(1,1),
+                o_DATAOUT=dataout0
+            )
+
+            # We need to depth cascade if using 128kB.
+            if size == 128*1024:
+                self.specials.spram = Instance("SB_SPRAM256KA",
+                    i_ADDRESS=self.bus.adr[0:14], i_DATAIN=datain1,
+                    i_MASKWREN=maskwren1,
+                    i_WREN=wren,
+                    i_CHIPSELECT=C(1,1),
+                    i_CLOCK=ClockSignal("sys"),
+                    i_STANDBY=C(0,1),
+                    i_SLEEP=C(0,1),
+                    i_POWEROFF=C(1,1),
+                    o_DATAOUT=dataout1
+                )
+
+        self.sync += [
+            self.bus.ack.eq(0),
+            If(self.bus.stb & self.bus.cyc & ~self.bus.ack,
+                self.bus.ack.eq(1)
+            )
+        ]