Not bound to synthesizable node exception in chisel memory

Question

I get the following exception in a chisel code.

[info] - should correctly write and read data *** FAILED ***
[info]   chisel3.core.Binding$BindingException: 'this' (chisel3.core.UInt@d7): Not bound to synthesizable node, currently only Type description
[info]   at chisel3.core.Binding$.checkSynthesizable(Binding.scala:184)
[info]   at chisel3.core.Data.connect(Data.scala:139)
[info]   at chisel3.core.Data.$colon$eq(Data.scala:204)
[info]   at Common.OnChipMemory$$anonfun$1.apply(memory.scala:88)
[info]   at Common.OnChipMemory$$anonfun$1.apply(memory.scala:60)
[info]   at scala.collection.immutable.Range.foreach(Range.scala:166)
[info]   at Common.OnChipMemory.<init>(memory.scala:60)
[info]   at Common.memoryTester$$anonfun$3$$anonfun$apply$1$$anonfun$apply$mcV$sp$1.apply(memoryTest.scala:32)
[info]   at Common.memoryTester$$anonfun$3$$anonfun$apply$1$$anonfun$apply$mcV$sp$1.apply(memoryTest.scala:32)
[info]   at chisel3.core.Module$.do_apply(Module.scala:35)

From this stack trace and some trial and error testing I could find that the line,

read_data := chipMem(data_idx) //line 88

is causing the problem. Code that immediately precedes this is posted below.

val lsb_idx = log2Up(4) // index of lsb in address

val chipMem = Mem(Vec(4, UInt(width = 8)), num_lines)   // memory

val data_idx = req_addr >> UInt(lsb_idx)    //req_addr is a UInt

val read_data = Bits()

After that I've had no luck finding the cause of the problem. I tried changing read_data to a Vec of UInt and using read() to read from memory.

Jack Koenig Jack Koenig · Accepted Answer · 2017-01-28T18:16:59

The issue is with the declaration of read_data. Bits() simply constructs a type rather than an actual hardware value. You need to make read_data an actual Wire instead of just to the Bits type. Also note that the type of read_data needs to be the same as the type of the Mem, thus you should declare read_data as follows:

val read_data = Wire(Vec(4, UInt(8.W))

Not bound to synthesizable node exception in chisel memory

1 Answers