document PWM example better

src/main/scala/pwm/PWM.scala

@@ -14,9 +14,14 @@ class PWM(implicit p: Parameters) extends Module {
     val tl = (new ClientUncachedTileLinkIO).flip
   }
 
+  // How many clock cycles in a PWM cycle?
   val period = Reg(UInt(width = 64))
+  // For how many cycles should the clock be high?
   val duty = Reg(UInt(width = 64))
+  // Is the PWM even running at all?
   val enable = Reg(init = Bool(false))
+
+  // The counter should count up until period is reached
   val counter = Reg(UInt(width = 64))
 
   when (counter >= period) {
@@ -25,11 +30,28 @@ class PWM(implicit p: Parameters) extends Module {
     counter := counter + UInt(1)
   }
 
+  // If PWM is enabled, pwmout is high when counter < duty
+  // If PWM is not enabled, it will always be low
   io.pwmout := enable && (counter < duty)
 
+  // One entry queue to hold the acquire message
   val acq = Queue(io.tl.acquire, 1)
-  val addr = Cat(acq.bits.addr_block, acq.bits.addr_beat)
 
+  // We assume that the TileLink interface is 64 bits wide
+  require(io.tl.tlDataBits == 64)
+
+  // Then addr_block and addr_beat together form the word address
+  val full_addr = Cat(acq.bits.addr_block, acq.bits.addr_beat)
+  // Since we have 3 registers, we only need the lower two bits to distinguish them
+  val index = full_addr(1, 0)
+
+  // Make sure the acquires we get are only the types we expect
+  assert(!acq.valid ||
+    acq.bits.isBuiltInType(Acquire.getType) ||
+    acq.bits.isBuiltInType(Acquire.putType),
+    "PWM: unexpected acquire type")
+
+  // Base the grant on the stored acquire
   io.tl.grant.valid := acq.valid
   acq.ready := io.tl.grant.ready
   io.tl.grant.bits := Grant(
@@ -38,20 +60,20 @@ class PWM(implicit p: Parameters) extends Module {
     client_xact_id = acq.bits.client_xact_id,
     manager_xact_id = UInt(0),
     addr_beat = acq.bits.addr_beat,
-    data = MuxLookup(addr, UInt(0), Seq(
+    // For gets, map the index to the three registers
+    data = MuxLookup(index, UInt(0), Seq(
       UInt(0) -> period,
       UInt(1) -> duty,
       UInt(2) -> enable)))
 
+  // If this is a put, update the registers according to the index
   when (acq.fire() && acq.bits.hasData()) {
-    switch (addr) {
+    switch (index) {
       is (UInt(0)) { period := acq.bits.data }
       is (UInt(1)) { duty   := acq.bits.data }
       is (UInt(2)) { enable := acq.bits.data(0) }
     }
   }
-
-  require(io.tl.tlDataBits == 64)
 }
 
 trait PeripheryPWM extends LazyModule {