add support for Winbond W9825G6KH-6 at 50MHz 1:2 rate

litedram/modules.py

@@ -535,6 +535,46 @@ class M12L16161A(SDRModule):
     technology_timings = _TechnologyTimings(tREFI=64e6/4096, tWTR=(2, None), tCCD=(1, None), tRRD=(None, 10))
     speedgrade_timings = {"default": _SpeedgradeTimings(tRP=15, tRCD=15, tWR=15, tRFC=(None, 55), tFAW=None, tRAS=40)}
 
+class W9825G6KH6(SDRModule):
+    """
+    Winbond W9825G6KH-6 chip on Mister SDRAM XS board v2.2
+    This is the smallest and cheapest module.
+    running it at 100MHz (1:2 if system clock is 50MHz)
+    works well on my SoCKit and all 32MB test error free
+    I get a number of data errors if I run it at 50MHz,
+    so this defaults to 1:2. If you want to use a higher
+    system clock (eg 100MHz), you might want to consider
+    using 1:1 clocking, because the -6 speedgrade
+    can be clocked up to 166MHz (CL3) or 133MHz (CL2)
+    Nevertheless I could not get it to work flawlessly
+    at 1:1 rate, so currently it only runs well at
+    50 MHz and 1:2 rate.
+    """
+    # geometry
+    nbanks = 4
+    nrows  = 8192
+    ncols  = 512
+
+    @staticmethod
+    def clock_cycles_to_ns(cycles, clk_freq, sdram_rate) -> float:
+        d = {
+            "1:1" : 1,
+            "1:2" : 2,
+            "1:4" : 4
+        }
+        return cycles / (d[sdram_rate] * clk_freq) / 1e-9
+
+    def __init__(self, clk_freq, sdram_rate):
+        # The datasheet specifies tWr in clock cycles, not in
+        # ns but to me it looks like litedram expects
+        # ns for these two parameters, so I have to convert them
+        # to ns first.
+        tWr  = self.clock_cycles_to_ns(2, clk_freq, sdram_rate)
+        tRRD = self.clock_cycles_to_ns(2, clk_freq, sdram_rate)
+        self.technology_timings = _TechnologyTimings(tREFI=64e6/8000, tWTR=(2, None), tCCD=(1, None), tRRD=(None, tRRD))
+        self.speedgrade_timings = {"default": _SpeedgradeTimings(tRP=15, tRCD=15, tWR=tWr, tRFC=(None, 60), tFAW=None, tRAS=42)}
+        super().__init__(clk_freq, sdram_rate)
+
 # DDR ----------------------------------------------------------------------------------------------
 
 class DDRModule(SDRAMModule):                     memtype = "DDR"