The Liberty File

Section 2.2 ran synthesis and compared results. Section 2.3 opens the files that made those results possible.

Every -liberty flag so far in this module, read_liberty, dfflibmap -liberty, abc -liberty, points at the same kind of file, and it is time to open one. A .lib (Liberty) file is the single source synthesis consults for every cost decision: which cell is faster, which is smaller, which drives more downstream logic without slowing down. Without it, ABC has no way to compare sky130_fd_sc_hd__nand2_1 against sky130_fd_sc_hd__nand2_4; they would be two names.

What Liberty describes

A Liberty file is a text-based (though machine-generated and machine-read) description of every cell in a library, characterised under specific process, voltage and temperature conditions (a PVT corner, which Module 7 returns to for multi-corner STA). For each cell it holds roughly four categories of data:

• Function: the Boolean behaviour of the cell, so tools can verify a substitution preserves logic.
• Input pin capacitance: how much load the cell’s inputs present to whatever drives them.
• Timing arcs: for every input-to-output path through the cell, a delay model, typically a lookup table indexed by input transition time and output load.
• Power: leakage, and dynamic switching power per transition.

Synthesis reads all four. Mapping (Section 2.1) leans mainly on function and timing arcs to pick a covering; the area-versus-delay bias of SYNTH_STRATEGY (Section 2.2) is a bias in how heavily the cost function weighs area (implied by cell size and drive strength) against those timing-arc delays.

Reading a cell entry

A representative entry, in the structure of the real Liberty syntax, for a 2-input NAND:

cell (sky130_fd_sc_hd__nand2_1) {
    area : 3.75 ;
    pin(A) {
        direction : input ;
        capacitance : 0.00218 ;
    }
    pin(B) {
        direction : input ;
        capacitance : 0.00206 ;
    }
    pin(Y) {
        direction : output ;
        function : "(A * B)'" ;
        timing() {
            related_pin : "A" ;
            timing_sense : negative_unate ;
            cell_rise(delay_template_7x7) {
                index_1 ("0.0100, 0.0400, ...");   /* input slew */
                index_2 ("0.0050, 0.0200, ...");   /* output load */
                values ("0.0421, 0.0512, ...", \
                        "0.0509, 0.0601, ...");
            }
        }
    }
}

Two fields to be able to point at without hesitation:

• Pin capacitance: pin(A) { capacitance : 0.00218 ; }. This is the load, in picofarads, that the A input presents to whatever drives it. Every input pin on every cell has one.
• Delay: inside cell_rise, the values table gives propagation delay in nanoseconds from A to Y, indexed by two variables: how slow the incoming edge is (index_1, input slew) and how much capacitance the output has to drive (index_2, output load). Real delay is not one number; it depends on both.

That two-dimensional table is the mechanism behind a fact you will rely on constantly from Module 4: a gate’s delay is not fixed. It depends on what drives it and what it drives. A nand2_1 feeding one nearby gate is faster than the same nand2_1 feeding a long, high-capacitance net across the die. Same cell, same entry, a different point in the same table.

One thing not to confuse: the area field is a relative sizing number useful for area estimates. It says nothing about a cell’s physical shape or pin locations. Liberty describes timing, power and function, and nothing about where pins sit or how big the footprint is on silicon. That is a different file, and the next article.

Practice Exercise

Given this trimmed entry for a buffer:

cell (sky130_fd_sc_hd__buf_2) {
    area : 6.25 ;
    pin(A) {
        direction : input ;
        capacitance : 0.00394 ;
    }
    pin(X) {
        direction : output ;
        function : "A" ;
    }
}

What is the input pin capacitance, and what does function : "A" tell you about what the cell does logically, as opposed to what it is for?

Liberty tells synthesis how fast and how power-hungry a cell is, but nothing about where its pins sit or how big a rectangle it occupies. In the next article we open the file that does: LEF.