Insert Lockup Cells in DFT ECO

In DFT flows, lockup latches or flops are typically inserted in two key locations:

• Between flops crossing different clock domains to ensure timing consistency and avoid metastability.
• At scan output ports to maintain scan integrity during scan shifts in ATPG modes.

This article discusses a hypothetical scenario where a metal-only ECO is required to insert lockup flops at scan output ports. The same approach can also be applied to other cases, such as inserting isolation cells. In this scenario, the design initially lacked lockup flops at the scan outputs, necessitating correction through an ECO process.

Figure 1: Lockup flop insertion to scan output

Implementation Overview

Lockup flops are inserted using a GOF ECO script. The process involves identifying scan output ports, inserting negedge-triggered flops, and mapping these to gate array spare cells.

Why Use Gate Array Spare Cells?

• Flexibility: Unlike standard spare cells, gate array spare cells can be re-configured into various logic types, including negedge-triggered flops.
• Resource Efficiency: Standard spare cells often lack sufficient latch or negedge flop types, whereas gate array cells provide versatile coverage.

This flexibility simplifies and enhances the efficiency of metal-only ECO implementation, particularly for complex, timing-critical scan paths or specialized flop requirements.

Detail Script

GOF utilizes ECO APIs to locate scan output ports and insert negedge-triggered flops.

# GOF script, dft_insert_lockup.pl
use strict;
read_library("tsmc.lib");# Read in standard library
read_design("-imp", "implementation.gv");# Read in Implementation Netlist Which is under ECO
set_top("SOC_TOP"); # Set the top to the most top module SOC_TOP
my @so_ports = get_ports("scan_out*");
my $so_num = scalar(@so_ports);
foreach my $so (@so_ports){
  my @drv = get_driver($so, "-nonbuf");
  my $drv_flop = $drv[0];
  my $drv_pin = $drv[1];
  my $lockup_inst = $so;
  $lockup_inst =~ s/\W/_/g;
  $lockup_inst .= "_lockup";
  gprint("Inserting negedge flop as lockup to SO $so driven by $drv_flop/$drv_pin\n");
  change_pin("$drv_flop/$drv_pin", "GDFFNQ_X1M_A9TH50", $lockup_inst, "$drv_flop/CK,-"); 
}
read_lef("cell_lib.lef");
read_def("soc_top.def");
save_session("dft_insert_lockup");
get_spare_cells("-gate_array", "G*", "-gate_array_filler", "GFILL*");
map_spare_cells(); 
report_eco();
write_verilog("eco_verilog.v");# Write out ECO result in Verilog
exit; 

Results

• Total Scan Outputs Modified: 24 ports
• Tiles Utilized: 96 gate array tiles (GFILL4_A9TH50)

# ECO log file

...

FILLER_impl1_594 (GFILL4_A9TH50) @(156.18, 12.6) 4 Tiles used by the following instances. Fully used, deleted

   u_gmii_tio/scan_out_12__lockup (GDFFNQ_X1M_A9TH50) TILE X 4

FILLER_impl1_14075 (GFILL4_A9TH50) @(70.49, 185.22) 4 Tiles used by the following instances. Fully used, deleted

   u_mactop/u_prov_rst/scan_out_16__lockup (GDFFNQ_X1M_A9TH50) TILE X 4

 FILLER_impl1_47975 (GFILL4_A9TH50) @(129.39, 579.6) 4 Tiles used by the following instances. Fully used, deleted

   u_mactop/u_creg/u_rdl/scan_out_22__lockup (GDFFNQ_X1M_A9TH50) TILE X 4

FILLER_impl0_376 (GFILL8_A9TH50) @(91.96, 16.38) 4 Tiles used by the following instances. Changed to GFILL4_A9TH50

   u_gre/u_dpth/u_fifo/scan_out_13__lockup (GDFFNQ_X1M_A9TH50) TILE X 4

Added Gates Used Total Tile Numbers: 96

The actual placement confirms that both inserted lockup flops are positioned optimally within the design.

Figure 2: Lockup flops on the placement

Conclusion

This metal-only ECO approach effectively corrects missing lockup flops in scan outputs. By leveraging gate array spare cells, it simplifies the insertion process, ensuring robust DFT implementation without major design disruptions.