partner: targets: add fomu target

The `fomu` target represents a generic target that supports the Fomu
48 MHz crystal, with or without a PLL.

It does not yet include a BaseSoC, since that requires USB and
up5kspram, neither of which are present yet.

Signed-off-by: Sean Cross <sean@xobs.io>

litex_boards/partner/targets/fomu.py

@@ -0,0 +1,112 @@
+from litex_boards.partner.platforms import netv2
+
+from migen import Module, Signal, Instance, ClockDomain, If
+from migen.genlib.resetsync import AsyncResetSynchronizer
+
+# CRG ----------------------------------------------------------------------------------------------
+
+class _CRG(Module):
+    def __init__(self, platform, use_pll=True):
+        clk48_raw = platform.request("clk48")
+        clk12_raw = Signal()
+        clk48 = Signal()
+        clk12 = Signal()
+
+        reset_delay = Signal(13, reset=4095)
+        self.clock_domains.cd_por = ClockDomain()
+        self.reset = Signal()
+
+        self.clock_domains.cd_sys = ClockDomain()
+        self.clock_domains.cd_usb_12 = ClockDomain()
+        self.clock_domains.cd_usb_48 = ClockDomain()
+
+        platform.add_period_constraint(self.cd_usb_48.clk, 1e9/48e6)
+        platform.add_period_constraint(self.cd_sys.clk, 1e9/12e6)
+        platform.add_period_constraint(self.cd_usb_12.clk, 1e9/12e6)
+        platform.add_period_constraint(clk48, 1e9/48e6)
+        platform.add_period_constraint(clk48_raw, 1e9/48e6)
+        platform.add_period_constraint(clk12_raw, 1e9/12e6)
+
+        # POR reset logic- POR generated from sys clk, POR logic feeds sys clk
+        # reset.
+        self.comb += [
+            self.cd_por.clk.eq(self.cd_sys.clk),
+            self.cd_sys.rst.eq(reset_delay != 0),
+            self.cd_usb_12.rst.eq(reset_delay != 0),
+        ]
+
+        if use_pll:
+
+            # Divide clk48 down to clk12, to ensure they're synchronized.
+            # By doing this, we avoid needing clock-domain crossing.
+            clk12_counter = Signal(2)
+
+            self.clock_domains.cd_usb_48_raw = ClockDomain()
+
+            platform.add_period_constraint(self.cd_usb_48_raw.clk, 1e9/48e6)
+
+            # POR reset logic- POR generated from sys clk, POR logic feeds sys clk
+            # reset.
+            self.comb += [
+                self.cd_usb_48.rst.eq(reset_delay != 0),
+            ]
+
+            self.comb += self.cd_usb_48_raw.clk.eq(clk48_raw)
+            self.comb += self.cd_usb_48.clk.eq(clk48)
+
+            self.sync.usb_48_raw += clk12_counter.eq(clk12_counter + 1)
+
+            self.comb += clk12_raw.eq(clk12_counter[1])
+            self.specials += Instance(
+                "SB_GB",
+                i_USER_SIGNAL_TO_GLOBAL_BUFFER=clk12_raw,
+                o_GLOBAL_BUFFER_OUTPUT=clk12,
+            )
+
+            self.specials += Instance(
+                "SB_PLL40_CORE",
+                # Parameters
+                p_DIVR = 0,
+                p_DIVF = 3,
+                p_DIVQ = 2,
+                p_FILTER_RANGE = 1,
+                p_FEEDBACK_PATH = "PHASE_AND_DELAY",
+                p_DELAY_ADJUSTMENT_MODE_FEEDBACK = "FIXED",
+                p_FDA_FEEDBACK = 15,
+                p_DELAY_ADJUSTMENT_MODE_RELATIVE = "FIXED",
+                p_FDA_RELATIVE = 0,
+                p_SHIFTREG_DIV_MODE = 1,
+                p_PLLOUT_SELECT = "SHIFTREG_0deg",
+                p_ENABLE_ICEGATE = 0,
+                # IO
+                i_REFERENCECLK = clk12,
+                o_PLLOUTGLOBAL = clk48,
+                i_BYPASS = 0,
+                i_RESETB = 1,
+            )
+        else:
+            self.specials += Instance(
+                "SB_GB",
+                i_USER_SIGNAL_TO_GLOBAL_BUFFER=clk48_raw,
+                o_GLOBAL_BUFFER_OUTPUT=clk48,
+            )
+            self.comb += self.cd_usb_48.clk.eq(clk48)
+
+            clk12_counter = Signal(2)
+            self.sync.usb_48 += clk12_counter.eq(clk12_counter + 1)
+
+            self.comb += clk12_raw.eq(clk12_counter[1])
+            self.specials += Instance(
+                "SB_GB",
+                i_USER_SIGNAL_TO_GLOBAL_BUFFER=clk12_raw,
+                o_GLOBAL_BUFFER_OUTPUT=clk12,
+            )
+
+        self.comb += self.cd_sys.clk.eq(clk12)
+        self.comb += self.cd_usb_12.clk.eq(clk12)
+
+        self.sync.por += \
+            If(reset_delay != 0,
+                reset_delay.eq(reset_delay - 1)
+            )
+        self.specials += AsyncResetSynchronizer(self.cd_por, self.reset)