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IHP-GmbH / IHP-Open-PDK

130nm BiCMOS Open Source PDK, dedicated for Analog, Mixed Signal and RF Design
Apache License 2.0
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large increase in area for ff compared to buf for upsize #186

Open maliberty opened 3 weeks ago

maliberty commented 3 weeks ago

@precisionmoon noticed this large jump in area when upsizing a sequential compared to a buffer. Is this expected?

PDK BUFX1 BUFX2 BUFX1 -> BUFX2 area increase DFFX1 DFFX2 DFFX1 -> DFFX2 area increase DFF1->2 /BFF1->2
ihp sg13g2_buf_1 sg13g2_buf_2   sg13g2_dfrbp_1 sg13g2_dfrbp_2
   7.2576 9.072 1.8144 47.1744 54.432 7.2576 4
KrzysztofHerman commented 3 weeks ago

@maliberty thank you for reporting this issue. At a first glance it should not be the case since as shown in the table (I also checked it in LEF and LIB files) the areas of the cells does not increase much as you change the cell type. I have also checked the spice netlist of all 4 circuits and they have identically sized transistors at the output, what should result in equal drive strengths for buffers and FF with the same postfix. I suppose that the increase in the area produces while placing the cells. Could you please mention the most critical parameters which are driving placer/resizer ?

maliberty commented 3 weeks ago

Drive strength, pin capacitance, and area are the most critical. In other PDKs we observe an smaller ratio of increase. To me this suggests some inefficiency in the cell layout.