diff --git a/lib/rcircuit/vcd.rb b/lib/rcircuit/vcd.rb
index 5257566..aaf6989 100644
--- a/lib/rcircuit/vcd.rb
+++ b/lib/rcircuit/vcd.rb
@@ -1,6 +1,9 @@
 ## Creates VCD files from port states
 class VCD
 
+  # @!attribute [w] fd
+  #   @api private
+  attr_writer :fd
   # @param filename [String] The filename to output to
   # @param timescale [String] The timescale
   def initialize(filename, timescale="1ps")
@@ -28,7 +31,12 @@ class VCD
       write_port_state(portname)
     end
     @fd.puts "$end"
-    @fd.puts "\#0"
+    @fd.puts "#0"
+    @portmap.each do |name,port|
+      port.add_callback do |value|
+        write_port_state(name)
+      end
+    end
   end
 
   # Stop logging
@@ -49,16 +57,13 @@ class VCD
     @idmap[portname] = @id
     @id=@id.next()
     @portmap[portname] = port
-    port.add_callback do |value|
-      write_port_state(portname)
-    end
   end
 
   # Advance x timesteps
   # @param timesteps [Integer] The number of timesteps to advance
   def advance(timesteps)
     @time += timesteps
-    @fd.puts "\##{@time.to_s}"
+    @fd.puts "##{@time.to_s}"
   end
 
   private
diff --git a/spec/vcd_spec.rb b/spec/vcd_spec.rb
new file mode 100644
index 0000000..e330bef
--- /dev/null
+++ b/spec/vcd_spec.rb
@@ -0,0 +1,43 @@
+describe VCD do
+  it "Outputs VCD files" do
+    a=Port.new(1)
+    b=Port.new(1)
+    gate=XorGate.new(a,b)
+    vcd=VCD.new("vcd_test.vcd")
+    obj=double()
+    allow(obj).to receive(:puts)
+    allow(obj).to receive(:close)
+    vcd.fd=obj
+    `rm vcd_test.vcd`
+    vcd.attach(a,"a")
+    vcd.attach(b,"b")
+    vcd.attach(gate.out, "out")
+    expect(obj).to receive(:puts).with("$scope module TOP $end")
+    expect(obj).to receive(:puts).with("$var wire 1 a a $end")
+    expect(obj).to receive(:puts).with("$var wire 1 b b $end")
+    expect(obj).to receive(:puts).with("$var wire 1 c out $end")
+    expect(obj).to receive(:puts).with("$upscope $end")
+    expect(obj).to receive(:puts).with("$enddefinitions $end")
+    expect(obj).to receive(:puts).with("$dumpvars")
+    expect(obj).to receive(:puts).with("0a")
+    expect(obj).to receive(:puts).with("0b")
+    expect(obj).to receive(:puts).with("0c")
+    expect(obj).to receive(:puts).with("$end")
+    expect(obj).to receive(:puts).with("#0")
+    vcd.start
+    expect(obj).to receive(:puts).with("1a")
+    expect(obj).to receive(:puts).with("1c")
+    a.setval(1)
+    expect(obj).to receive(:puts).with("#1")
+    vcd.advance(1)
+    expect(obj).to receive(:puts).with("1b")
+    expect(obj).to receive(:puts).with("0c")
+    b.setval(1)
+    expect(obj).to receive(:puts).with("#2")
+    vcd.advance(1)
+    expect(obj).to receive(:puts).with("0a")
+    expect(obj).to receive(:puts).with("1c")
+    a.setval(0)
+    vcd.stop
+  end
+end
diff --git a/todo.txt b/todo.txt
index c9bca61..7039733 100644
--- a/todo.txt
+++ b/todo.txt
@@ -1,2 +1,2 @@
-Must add tests for VCD and Device
-Parts of Port must get tests
+Must add tests for Device
+Must add tests for parts of Port
diff --git a/tst.vcd b/tst.vcd
deleted file mode 100644
index 0dea325..0000000
--- a/tst.vcd
+++ /dev/null
@@ -1,26 +0,0 @@
-$version
-RCircuit VCD Generator Version 0.0
-$end
-$date
-Thu Jul 12 07:57:29 2018
-$end
-$timescale 1ps $end
-$scope module TOP $end
-$var wire 8 a a[0:7] $end
-$var wire 8 b b[0:7] $end
-$upscope $end
-$enddefinitions $end
-$dumpvars
-b0 a
-b0 b
-$end
-#0
-b1000 a
-b1010 b
-#1
-b10000 a
-b11101 b
-#2
-b0 a
-b0 b
-#3
diff --git a/vcd_test.rb b/vcd_test.rb
deleted file mode 100644
index 8296ea7..0000000
--- a/vcd_test.rb
+++ /dev/null
@@ -1,27 +0,0 @@
-#xor_test.rb
-
-require_relative "lib/rcircuit/port.rb"
-require_relative "lib/rcircuit/gate.rb"
-require_relative "lib/rcircuit/xor.rb"
-require_relative "lib/rcircuit/vcd.rb"
-
-in_a = Port.new(1, "A")
-in_b = Port.new(1, "B")
-gate = XorGate.new(in_a, in_b)
-
-vcd = VCD.new("vcd_test.vcd").attach(in_a)
-                             .attach(in_b)
-                             .attach(gate.out, "OUT")
-
-in_a.setval(0)
-in_b.setval(0)
-vcd.start
-in_a.setval(1)
-vcd.advance(1000).write
-in_b.setval(1)
-vcd.advance(1000).write
-in_a.setval(0)
-vcd.advance(1000).write
-vcd.advance(1000)
-vcd.finish
-