# Myhdl BlockError

**URL:** <https://discourse.myhdl.org/t/myhdl-blockerror/184>\
**Category:** Support\
**Created:** [May 2, 2017, 8:50am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184 "2017-05-02T08:50:37Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![ycgeng](https://avatars.discourse-cdn.com/v4/letter/y/278dde/32.png) [@ycgeng](https://discourse.myhdl.org/u/ycgeng)\
**Post date:** [May 2, 2017, 8:50am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/1 "2017-05-02T08:50:37Z")

</div>

Hi,

Just started myhdl couples of days ago. It is amazing and I went through the documents and examples those days. After that I started to practice with my first sample code. To implement a simple fifo with myhdl on my own.

However problem arise and I can not solve it on my own even after refer back to the sample code many times. So would like to seek help here.

Ok, this is the source code I am working on.

[fifo.py](http://fifo.py)

from myhdl import \*

@block  
def fifo(clk,rst,wr,re,data,q,status):

```
wr_pt = Signal(modbv(0)[9:])
rd_pt = Signal(modbv(0)[9:])

full = Signal(bool(0))
empty = Signal(bool(1))

mem = [Signal(intbv(0)[8:]) for i in range(256)]

#status[0] = full
#status[1] = empty

@always_comb
def status():
    full.next = (wr_pt - rd_pt == 256)
    empty.next = (wr_pt - rd_pt == 0)	

@always_seq(clk.posedge, reset=rst)
def write():
if(wr):
        if(not full):
            mem[wr_pt].next = data
            wr_pt.next = wr_pt + 1
        else:
            status[0].next = 1

@always_seq(clk.posedge,reset=rst)
def read():
    if(re):
        if(not empty):
        	q = mem[rd_pt]
        	rd_pt.next = rd_pt + 1
        else:
            status[1].next = 1

return status,read,write

```

@block  
def bench():

```
clk = Signal(bool(0))
rst = ResetSignal(0, active=0, async=True)

wr = Signal(bool(0))
re = Signal(bool(0))
data = Signal(0)
q	= Signal(0)
status = Signal(0)

dut = fifo(clk,rst,wr,re,data,q,status)

HALF_PERIOD = delay(10)

@always(HALF_PERIOD)
def clockGen():
    clk.next = not clock

@block
def write(d):
    yield clock.negedge
    data.next = d
    wr.next = 1
    yield clock.negedge
    wr.next = 0

@instance
def stimulate():
    rst.next = 0
    for i in range(3):
        yield clk.negedge

    rst.next = 1
    write(0x44)

    yield clk.negedge
    raise StopSimulation()

return dut,clockGen,write,stimulate

```

def main():  
tb = bench()  
tb.run\_sim()

if **name** == ‘ **main** ’:  
main()

After run python3 [fifo.py](http://fifo.py) ( My working environment is ubuntu python3.5), I got the following message.

Traceback (most recent call last):  
File “[fifo.py](http://fifo.py)”, line 92, in   
main()  
File “[fifo.py](http://fifo.py)”, line 88, in main  
tb = bench()  
File “/usr/local/lib/python2.7/dist-packages/myhdl-1.0.dev0-py2.7.egg/myhdl/\_block.py”, line 125, in **call**  
self.srcline, \*args, \*\*kwargs)  
File “/usr/local/lib/python2.7/dist-packages/myhdl-1.0.dev0-py2.7.egg/myhdl/\_block.py”, line 159, in **init**  
self.\_verifySubs()  
File “/usr/local/lib/python2.7/dist-packages/myhdl-1.0.dev0-py2.7.egg/myhdl/\_block.py”, line 174, in \_verifySubs  
raise BlockError(\_error.ArgType % ([self.name](http://self.name),))  
myhdl.BlockError: bench\_1: A block should return block or instantiator objects

---

<div class="post-metadata">

**Author:** ![josyb](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/josyb/32/5_2.png) [@josyb](https://discourse.myhdl.org/u/josyb)\
**Post date:** [May 2, 2017, 9:03am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/2 "2017-05-02T09:03:45Z")

</div>

I guess you intended to write an _@instance_ instead of an _@block_ to decorate the _def write(d):_ in the test-bench?

---

<div class="post-metadata">

**Author:** ![ycgeng](https://avatars.discourse-cdn.com/v4/letter/y/278dde/32.png) [@ycgeng](https://discourse.myhdl.org/u/ycgeng)\
**Post date:** [May 2, 2017, 9:20am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/3 "2017-05-02T09:20:38Z")

</div>

I do not quite understand the difference among those decorate. such as block, instance. Could you help me clarify the difference a little bit?

And if I change the write from block to instance. it report another error:

File “[fifo.py](http://fifo.py)”, line 64, in bench  
@instance  
File “/usr/local/lib/python3.5/dist-packages/myhdl-1.0.dev0-py3.5.egg/myhdl/\_instance.py”, line 80, in instance  
raise InstanceError(\_error.NrOfArgs)  
myhdl.InstanceError: decorated generator function should not have arguments

---

<div class="post-metadata">

**Author:** ![josyb](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/josyb/32/5_2.png) [@josyb](https://discourse.myhdl.org/u/josyb)\
**Post date:** [May 2, 2017, 9:44am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/4 "2017-05-02T09:44:26Z")

</div>

I was a bit quick …  
the _def write(d):_ is just a simple function and should not have any decorator at all.

---

<div class="post-metadata">

**Author:** ![ycgeng](https://avatars.discourse-cdn.com/v4/letter/y/278dde/32.png) [@ycgeng](https://discourse.myhdl.org/u/ycgeng)\
**Post date:** [May 2, 2017, 10:08am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/5 "2017-05-02T10:08:28Z")

</div>

ok, Thanks. This time at least the simulation run without problem. However it is not as expected.

I change the [fifo.py](http://fifo.py) test bench part to the follow.

[fifo.py](http://fifo.py)

from myhdl import \*

@block  
def fifo(clk,rst,wr,re,data,q,status):  
… Does not changed

```
  return status,read,write

```

clk = Signal(bool(0))  
rst = ResetSignal(0, active=0, async=True)

wr = Signal(bool(0))  
re = Signal(bool(0))  
data = Signal(0)  
q = Signal(0)  
status = Signal(0)

dut = fifo(clk,rst,wr,re,data,q,status)

def tb\_write(d):

```
print("enter tb_write : d=%s" % d)
yield clk.negedge
data.next = d
wr.next = 1
yield clk.negedge
wr.next = 0

```

@block  
def bench():

```
HALF_PERIOD = delay(10)

@always(HALF_PERIOD)
def clockGen():
    clk.next = not clk

@instance
def stimulate():
    rst.next = 0
    for i in range(3):
        yield clk.negedge

    rst.next = 1
    print("write to fifo")
    tb_write(0x44)
    print("write to fifo finished")
    

    yield clk.negedge
    for i in range(6):
        yield clk.negedge
    
    raise StopSimulation()

return clockGen,stimulate

```

Then I run it again. The simulation goes fine and vcd waveform also got dumped. as the log below shows.

$python3 [fifo.py](http://fifo.py)  
write to fifo  
write to fifo finished

but seems tb\_write does not get executed at all. “enter tb\_write ： d = %s” %d" message do not got printed out. Why?

---

<div class="post-metadata">

**Author:** ![ycgeng](https://avatars.discourse-cdn.com/v4/letter/y/278dde/32.png) [@ycgeng](https://discourse.myhdl.org/u/ycgeng)\
**Post date:** [May 2, 2017, 11:39am UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/6 "2017-05-02T11:39:45Z")

</div>

Ok, managed to solve the problem myself. I used a generator for write which run parallel to stimulate. solved this issue. However still interest if it is possible to call a function or kind of block of code in instance decorator.

---

<div class="post-metadata">

**Author:** ![josyb](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/josyb/32/5_2.png) [@josyb](https://discourse.myhdl.org/u/josyb)\
**Post date:** [May 2, 2017, 5:00pm UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/7 "2017-05-02T17:00:11Z")

</div>

You have to precede a call to a function with a _yield_: as in: `yield tb_write(0x44)`

---

<div class="post-metadata">

**Author:** ![ycgeng](https://avatars.discourse-cdn.com/v4/letter/y/278dde/32.png) [@ycgeng](https://discourse.myhdl.org/u/ycgeng)\
**Post date:** [May 2, 2017, 6:54pm UTC](https://discourse.myhdl.org/t/myhdl-blockerror/184/8 "2017-05-02T18:54:00Z")

</div>

Thanks very much, It worked!