uart: Eliminate systemic baud rate error with low divisor values

This refactors the receiver logic to compensate for the case of the baud
rate divisor not being multiple of the oversampling period.

Previously, the bit time was effectively rounded to (s * floor(div / s))
cycles, where "s" is the oversampling factor - the number of
intermediate samples for each data bit.  The remainder r = (div % s) was
ignored, thereby resulting in gradually accumulated drift that became
significant for divisor values on the same order of magnitude as "s".

The revised approach inserts the required additional delay by extending
the last "r" samples of a given data bit by one cycle each.

diff --git a/src/main/scala/devices/uart/UART.scala b/src/main/scala/devices/uart/UART.scala
index 449f897819a57427a8a8e978569df70745bebe2b..d0220e7cbe2966eecd26b6d901dccdef68ae37b2 100644 (file)
--- a/src/main/scala/devices/uart/UART.scala
+++ b/src/main/scala/devices/uart/UART.scala
@@ -104,46 +104,38 @@ class UARTRx(c: UARTParams)(implicit p: Parameters) extends UARTModule(c)(p) {
   val debounce_max = (debounce === UInt(3))
   val debounce_min = (debounce === UInt(0))
 
-  val prescaler = Reg(init = UInt(0, uartDivisorBits - uartOversample))
+  val prescaler = Reg(UInt(width = uartDivisorBits - uartOversample + 1))
   val start = Wire(init = Bool(false))
-  val busy = Wire(init = Bool(false))
-  val pulse = (prescaler === UInt(0)) && busy
+  val pulse = (prescaler === UInt(0))
 
-  when (busy) {
-    prescaler := prescaler - UInt(1)
-  }
-  when (start || pulse) {
-    prescaler := io.div >> uartOversample
-  }
+  private val dataCountBits = log2Floor(uartDataBits) + 1
 
-  val sample = Reg(Bits(width = uartNSamples))
-  val voter = Majority(sample.toBools.toSet)
-  when (pulse) {
-    sample := Cat(sample, io.in)
-  }
+  val data_count = Reg(UInt(width = dataCountBits))
+  val data_last = (data_count === UInt(0))
+  val sample_count = Reg(UInt(width = uartOversample))
+  val sample_mid = (sample_count === UInt((uartOversampleFactor - uartNSamples + 1) >> 1))
+  val sample_last = (sample_count === UInt(0))
+  val countdown = Cat(data_count, sample_count) - UInt(1)
 
-  private val delay0 = (uartOversampleFactor + uartNSamples) >> 1
-  private val delay1 = uartOversampleFactor
+  // Compensate for the divisor not being a multiple of the oversampling period.
+  // Let remainder k = (io.div % uartOversampleFactor).
+  // For the last k samples, extend the sampling delay by 1 cycle.
+  val remainder = io.div(uartOversample-1, 0)
+  val extend = (sample_count < remainder) // Pad head: (sample_count > ~remainder)
+  val restore = start || pulse
+  val prescaler_in = Mux(restore, io.div >> uartOversample, prescaler)
+  val prescaler_next = prescaler_in - Mux(restore && extend, UInt(0), UInt(1))
 
-  val timer = Reg(UInt(width = uartOversample + 1))
-  val counter = Reg(UInt(width = log2Floor(uartDataBits) + 1))
+  val sample = Reg(Bits(width = uartNSamples))
+  val voter = Majority(sample.toBools.toSet)
   val shifter = Reg(Bits(width = uartDataBits))
-  val expire = (timer === UInt(0)) && pulse
-
-  val sched = Wire(init = Bool(false))
-  when (pulse) {
-    timer := timer - UInt(1)
-  }
-  when (sched) {
-    timer := UInt(delay1-1)
-  }
 
   val valid = Reg(init = Bool(false))
   valid := Bool(false)
   io.out.valid := valid
   io.out.bits := shifter
 
-  val (s_idle :: s_start :: s_data :: Nil) = Enum(UInt(), 3)
+  val (s_idle :: s_data :: Nil) = Enum(UInt(), 2)
   val state = Reg(init = s_idle)
 
   switch (state) {
@@ -154,36 +146,29 @@ class UARTRx(c: UARTParams)(implicit p: Parameters) extends UARTModule(c)(p) {
       when (!io.in) {
         debounce := debounce + UInt(1)
         when (debounce_max) {
-          state := s_start
-          start := Bool(true)
-          timer := UInt(delay0-1)
-        }
-      }
-    }
-
-    is (s_start) {
-      busy := Bool(true)
-      when (expire) {
-        sched := Bool(true)
-        when (voter) {
-          state := s_idle
-        } .otherwise {
           state := s_data
-          counter := UInt(uartDataBits)
+          start := Bool(true)
+          prescaler := prescaler_next
+          data_count := UInt(uartDataBits+1)
+          sample_count := UInt(uartOversampleFactor - 1)
         }
       }
     }
 
     is (s_data) {
-      busy := Bool(true)
-      when (expire) {
-        counter := counter - UInt(1)
-        when (counter === UInt(0)) {
-          state := s_idle
-          valid := Bool(true)
-        } .otherwise {
-          shifter := Cat(voter, shifter >> 1)
-          sched := Bool(true)
+      prescaler := prescaler_next
+      when (pulse) {
+        sample := Cat(sample, io.in)
+        data_count := countdown >> uartOversample
+        sample_count := countdown(uartOversample-1, 0)
+
+        when (sample_mid) {
+          when (data_last) {
+            state := s_idle
+            valid := Bool(true)
+          } .otherwise {
+            shifter := Cat(voter, shifter >> 1)
+          }
         }
       }
     }