shuttler: change 0th order accumulator width

It now truncates the LSBs instead of the MSBs.

artiq/coredevice/shuttler.py

@@ -12,7 +12,7 @@ def shuttler_volt_to_mu(volt):
     """Return the equivalent DAC code. Valid input range is from -10 to
     10 - LSB.
     """
-    return round((1 << 14) * (volt / 20.0)) & 0x3fff
+    return round((1 << 16) * (volt / 20.0)) & 0xffff
 
 
 class Config:
@@ -25,7 +25,7 @@ class Config:
     output data with pre-DAC gain, then adds the offset.
 
     .. note::
-        The DAC code is capped at 0x1fff and 0x2000.
+        The DAC code is capped at 0x7fff and 0x8000.
 
     :param channel: RTIO channel number of this interface.
     :param core_device: Core device name.
@@ -152,8 +152,8 @@ class Volt:
 
             a_3 &= p_3T^3
         
-        :math:`a_0`, :math:`a_1`, :math:`a_2` and :math:`a_3` are 14, 30, 46
+        :math:`a_0`, :math:`a_1`, :math:`a_2` and :math:`a_3` are 16, 32, 48
-        and 46 bits in width respectively. See :meth:`shuttler_volt_to_mu` for
+        and 48 bits in width respectively. See :meth:`shuttler_volt_to_mu` for
         machine unit conversion.
 
         Note: The waveform is not updated to the Shuttler Core until
@@ -238,8 +238,8 @@ class Dds:
 
             c_2 &= r_2T^2
         
-        :math:`b_0`, :math:`b_1`, :math:`b_2` and :math:`b_3` are 14, 30, 46
+        :math:`b_0`, :math:`b_1`, :math:`b_2` and :math:`b_3` are 16, 32, 48
-        and 46 bits in width respectively. See :meth:`shuttler_volt_to_mu` for
+        and 48 bits in width respectively. See :meth:`shuttler_volt_to_mu` for
         machine unit conversion. :math:`c_0`, :math:`c_1` and :math:`c_2` are
         16, 32 and 32 bits in width respectively.
 

artiq/examples/kasli_shuttler/repository/shuttler.py

@@ -26,15 +26,15 @@ def shuttler_volt_amp_mu(volt):
 
 @portable
 def shuttler_volt_damp_mu(volt_per_us):
-    return round(float(2) ** 30 * (volt_per_us / 20) / DAC_Fs_MHZ)
+    return round(float(2) ** 32 * (volt_per_us / 20) / DAC_Fs_MHZ)
 
 @portable
 def shuttler_volt_ddamp_mu(volt_per_us_square):
-    return round(float(2) ** 46 * (volt_per_us_square / 20) * 2 / (DAC_Fs_MHZ ** 2))
+    return round(float(2) ** 48 * (volt_per_us_square / 20) * 2 / (DAC_Fs_MHZ ** 2))
 
 @portable
 def shuttler_volt_dddamp_mu(volt_per_us_cube):
-    return round(float(2) ** 46 * (volt_per_us_cube / 20) * 6 / (DAC_Fs_MHZ ** 3))
+    return round(float(2) ** 48 * (volt_per_us_cube / 20) * 6 / (DAC_Fs_MHZ ** 3))
 
 @portable
 def shuttler_dds_amp_mu(volt):

artiq/gateware/shuttler.py

@@ -124,16 +124,18 @@ class Dac(Module):
         ]
 
         # Infer signed multiplication
-        data_raw = Signal((14, True))
+        data_raw = Signal((16, True))
-        data_buf = Signal(16)
+        # Buffer data should have 2 more bits than the desired output width
+        # It is to perform overflow/underflow detection
+        data_buf = Signal(18)
         self.sync.rio += [
             data_raw.eq(reduce(add, [sub.data for sub in subs])),
             # Extra buffer for timing for the DSP
             data_buf.eq(((data_raw * Cat(self.gain, ~self.gain[-1])) + (self.offset << 16))[16:]),
             If(overflow,
-                self.data.eq(0x1fff),
+                self.data.eq(0x7fff),
             ).Elif(underflow,
-                self.data.eq(0x2000),
+                self.data.eq(0x8000),
             ).Else(
                 self.data.eq(data_buf),
             ),
@@ -350,7 +352,7 @@ class Shuttler(Module, AutoCSR):
                 dac.clear.eq(self.cfg.clr[idx]),
                 dac.gain.eq(self.cfg.gain[idx]),
                 dac.offset.eq(self.cfg.offset[idx]),
-                self.dac_interface.data[idx // 2][idx % 2].eq(dac.data)
+                self.dac_interface.data[idx // 2][idx % 2].eq(dac.data[2:])
             ]
 
             for i in dac.i: