@Stefan Schippers I am facing issue in simulating DFF cell from sky130 standard cell library in xschem. Can you please suggest what might be wrong in this testbench.

You need to add a VCC supply and assign VCC and GND as supplies to the flip flop. This is done via attributes. Click the dff and do a 'q' and assign GND and VCC to ground and supply rails as well as p and n wells. Modified circuit attached. The .include line is not needed.

Thanks Stefan, I am getting message

xterm: Can't execvp 'ngspice': No such file or directory

while doing

Netlist -> Simulate

for updated D flip-flop schematic in interactive mode. Why this error is coming ?

This means ngspice is not found. FInd the exact location of the

ngspice

executable in your system and replace

ngspice

with

/full/path/to/ngspice

in xschem

simulation->Configure simulators and tools

Ngspice simulation was functioning correctly initially. I could execute the simulation and observe the plots within the internal xschem viewer. However, after making some modifications to the schematic, the viewer failed to display the plots. Additionally, the

.raw

file isn't being generated in the

.xschem/simulations

directory. I'm using ngspice in interactive mode. Please suggest what could be the issue here ?

I don't know. Your schematic works perfectly here.

Sorry, I shared the schematic which was working fine. Schematic having issue after update to measure VCC current after adding

ammeter.sym

is attached now. No

.raw

file is getting generated with this schematic.

ok the solution is simple: add a

remzerovec

command before

write

Adding

remzerovec

worked for current through

ammeter.sym

only but node voltages were missing. To plot those I added

save all

before

tran

and then the whole testbench is working fine. Thanks for your help.

Yes this is correct. Ammeters by default add a

.save

in the netlist for automatically saving their currents. So you need an explicit

save all

for plotting all other nodes.

@Stefan Schippers Trying to build div-by-8 frequency divider using

DFFIQ

flip-flop from sky130 std. cell library. But it seems spice model for this cell is not available in following spice file:

/usr/local/share/pdk/sky130A/libs.ref/sky130_fd_sc_hd/spice/sky130_fd_sc_hd.spice

Can you please confirm if my interpretation is correct or not ? If yes, what is the alternate way to build D-FF based frequency divider in sky130?

@vks you need to use cells from library sky130_stdcells, use

sky130_stdcells/dfxbp_2.sym

Thanks @Stefan Schippers. Is there any documentation or resource available to know about features of std. cells like port functionality, current drive, timing etc. ?

I think the .lib file has all the details. In general the _n suffix of the cells indicates the driving strength, _1: 1 standard loads, _2: 2 standard loads. How this is exactly defined I don't know. For sky130 Look at .../pdk/sky130A/libs.ref/sky130_fd_sc_hd/lib/sky130_fd_sc_hd__tt_025C_1v80.lib

I do not see

lib

directory in installed pdk path at my end. Can you please suggest why is it so ?

This is strange. Hopefully @Tim Edwards knows why the

lib/

directory is not installed in your system...

My suspicion is that this problem is caused by switching some of the repositories from the github/google originals to the github/efabless forks. The repository was probably cloned into sources/ and the update routine isn't smart enough to figure out that the repository changed locations. One major difference in the repositories is that instead of building the .lib files from bits and pieces, I pre-built them and then put them in the repository as split files to avoid the github file size limit. I think that's why the whole lib/ section is missing. If my guess is correct, then you should cd to open_pdks/sources/ and delete everything there, then rebuild open_pdks so that it will clone all the sources from the correct repositories.

@Stefan Schippers is am getting this xterm: can’t execvp : no file or directory after updating ngspice from 40 to 42.

You need to ensure the ngspice executable exists. Try to start xschem from a terminal:

$ ngspice

If the executable is not found try to locate it"

$ find / -type f  -name ngspice 2>&1 |grep -v 'Permission denied'

If you find the location of the executable go in xschem,

Simulation->Configure Simulators and tools

and set the path accordingly and click

Accept,Save and Close

.

I tried all three paths. But still issue is same

install xterm:

sudo apt install xterm

I have changed later to &

do you have xterm installed? type xterm on the terminal, does it open an xterm window?

IMG_3371.jpg

@Atif Khan Bring up the

Simulation->Configure simulators and tools

, try these options in the "Ngspice interactive" text box: •

xterm -e {ngspice -i "$N" -a || sh}

•

xterm -e {/usr/local/bin/ngspice -i "$N" -a}

•

xterm -e /usr/local/bin/ngspice -i "$N" -a

•

gnome-terminal e /usr/local/bin/ngspice -i "$N" -a