util: Do BlackBox Async Set/Reset Registers more properly (#305)

* util: Do Set/Reset Async Registers more properly

The way BlackBox "init" registers were coded before was
not really kosher verilog for most synthesis tools.
Also, the enable logic wasn't really pushed down into the flop.

This change is more explicit about set/reset flops,
again this is only a 'temporary' problem that would go away
with parameterizable blackboxes (or general async reset support).

* Tabs, not spaces, in Makefiles

* util: Fix typos in Async BB Reg Comments

src/main/scala/uncore/util/BlackBoxRegs.scala

@@ -5,13 +5,17 @@ import Chisel._
 import cde.{Parameters}
 
 /** This black-boxes an Async Reset
-  * Reg.
+  *  (or Set)
+  * Register.
   *  
   * Because Chisel doesn't support
   * parameterized black boxes, 
   * we unfortunately have to 
   * instantiate a number of these.
   *  
+  *  We also have to hard-code the set/
+  *  reset behavior.
+  *  
   *  Do not confuse an asynchronous
   *  reset signal with an asynchronously
   *  reset reg. You should still 
@@ -22,29 +26,26 @@ import cde.{Parameters}
   *  @param q Data Output
   *  @param clk Clock Input
   *  @param rst Reset Input
-  *  
-  *  @param init Value to write at Reset. 
-  *  This is a constant, 
-  *  but this construction
-  *  will likely make backend flows
-  *  and lint tools unhappy.
+  *  @param en Write Enable Input
   *  
   */
 
-class AsyncResetReg  extends BlackBox {
+abstract class AbstractBBReg extends BlackBox {
 
   val io = new Bundle {
     val d = Bool(INPUT)
     val q = Bool(OUTPUT)
+    val en = Bool(INPUT)
 
     val clk = Clock(INPUT)
     val rst = Bool(INPUT)
-
-    val init = Bool(INPUT)
   }
 
 }
 
+class AsyncResetReg  extends AbstractBBReg
+class AsyncSetReg extends AbstractBBReg
+
 class SimpleRegIO(val w: Int) extends Bundle{
 
   val d = UInt(INPUT, width = w)
@@ -60,30 +61,38 @@ class AsyncResetRegVec(val w: Int, val init: BigInt) extends Module {
 
   val bb_d = Mux(io.en, io.d, io.q)
 
-  val async_regs = List.fill(w)(Module (new AsyncResetReg))
+  val async_regs: List[AbstractBBReg] = List.tabulate(w)(
+    i => Module (
+      if (((init >> i) % 2) > 0)
+        new AsyncSetReg
+      else
+        new AsyncResetReg)
+  )
 
   io.q := async_regs.map(_.io.q).asUInt
 
   for ((reg, idx) <- async_regs.zipWithIndex) {
     reg.io.clk := clock
     reg.io.rst := reset
-    reg.io.init := Bool(((init >> idx) & 1) == 1)
     reg.io.d   := bb_d(idx)
+    reg.io.en  := io.en
   }
 
 }
 
 object AsyncResetReg {
-  def apply(d: Bool, clk: Clock, rst: Bool, init: Bool): Bool = {
-    val reg = Module(new AsyncResetReg)
+  def apply(d: Bool, clk: Clock, rst: Bool, init: Boolean): Bool = {
+    val reg: AbstractBBReg =
+      if (init) Module (new AsyncSetReg)
+      else Module(new AsyncResetReg)
     reg.io.d := d
     reg.io.clk := clk
     reg.io.rst := rst
-    reg.io.init := init
+    reg.io.en  := Bool(true)
     reg.io.q
   }
 
-  def apply(d: Bool, clk: Clock, rst: Bool): Bool = apply(d, clk, rst, Bool(false))
+  def apply(d: Bool, clk: Clock, rst: Bool): Bool = apply(d, clk, rst, false)
 
   def apply(updateData: UInt, resetData: BigInt, enable: Bool): UInt = {
     val w = updateData.getWidth max resetData.bitLength
@@ -93,9 +102,9 @@ object AsyncResetReg {
     reg.io.q
   }
 
-  def apply(updateData: UInt, resetData: BigInt): UInt = apply(updateData, resetData, Bool(true))
+  def apply(updateData: UInt, resetData: BigInt): UInt = apply(updateData, resetData, enable=Bool(true))
 
-  def apply(updateData: UInt, enable: Bool): UInt = apply(updateData, BigInt(0), enable)
+  def apply(updateData: UInt, enable: Bool): UInt = apply(updateData, resetData=BigInt(0), enable)
 
-  def apply(updateData: UInt): UInt = apply(updateData, BigInt(0), Bool(true))
+  def apply(updateData: UInt): UInt = apply(updateData, resetData=BigInt(0), enable=Bool(true))
 }

vsim/Makefrag

@@ -8,6 +8,7 @@ bb_vsrcs = $(base_dir)/vsrc/DebugTransportModuleJtag.v \
 	$(base_dir)/vsrc/jtag_vpi.v \
 	$(base_dir)/vsrc/AsyncMailbox.v \
 	$(base_dir)/vsrc/AsyncResetReg.v \
+	$(base_dir)/vsrc/AsyncSetReg.v \
 	$(base_dir)/vsrc/ClockDivider.v \
 	$(base_dir)/vsrc/ClockToSignal.v \
 	$(base_dir)/vsrc/SignalToClock.v \

vsrc/AsyncResetReg.v

@@ -8,6 +8,8 @@
   * we unfortunately have to 
   * instantiate a number of these.
   *  
+  * We also have to hard-code the set/reset.
+  *  
   *  Do not confuse an asynchronous
   *  reset signal with an asynchronously
   *  reset reg. You should still 
@@ -18,32 +20,25 @@
   *  @param q Data Output
   *  @param clk Clock Input
   *  @param rst Reset Input
-  *  
-  *  @param init Value to write at Reset. 
-  *  This is a constant, 
-  *  but this construction
-  *  will likely make backend flows
-  *  and lint tools unhappy.
+  *  @param en Write Enable Input
   *  
   */
 
 module AsyncResetReg (
                       input      d,
                       output reg q,
+                      input      en,
 
                       input      clk,
-                      input      rst,
-                      
-                      input      init);
+                      input      rst);
    
    always @(posedge clk or posedge rst) begin
 
       if (rst) begin
-         q <= init;
-      end else begin
+         q <= 1'b0;
+      end else if (en) begin
          q <= d;
       end
-   
    end
    
 

vsrc/AsyncSetReg.v

@@ -0,0 +1,47 @@
+
+
+/** This black-boxes an Async Set
+  * Reg.
+  *  
+  * Because Chisel doesn't support
+  * parameterized black boxes, 
+  * we unfortunately have to 
+  * instantiate a number of these.
+  *  
+  * We also have to hard-code the set/reset.
+  *  
+  *  Do not confuse an asynchronous
+  *  reset signal with an asynchronously
+  *  reset reg. You should still 
+  *  properly synchronize your reset 
+  *  deassertion.
+  *  
+  *  @param d Data input
+  *  @param q Data Output
+  *  @param clk Clock Input
+  *  @param rst Reset Input
+  *  @param en Write Enable Input
+  * 
+  */
+
+module AsyncSetReg (
+                      input      d,
+                      output reg q,
+                      input en,
+                    
+                      input      clk,
+                      input      rst);
+   
+   always @(posedge clk or posedge rst) begin
+
+      if (rst) begin
+         q <= 1'b1;
+      end else if (en) begin
+         q <= d;
+      end
+   
+   end
+   
+
+endmodule // AsyncSetReg
+