Games / Command flash cards / Liberty timing libraries / Reading a library

Reading a library

Each card shows real tool output to read. A missed card returns later in the round and starts the schedule over. A card answered correctly moves up the schedule and next appears after 1, 3, 7, then 21 days. Keys 1 to 4 choose an option, h shows a hint, and Enter moves on. A hint keeps the card where it is on the schedule. An ordering card is answered by clicking its items in order, and one wrong click ends it. Time values are in nanoseconds. Progress is stored in this browser.

SourceLiberty is the format that carries a cell library's timing and power. These cards are checked against the Nangate45 library that OpenROAD-flow-scripts ships, and the excerpts are quoted from it. Liberty is a Synopsys format whose specification is licensed; the behaviour described here is what the open timer OpenSTA implements, and answers link to its command reference. Reference: the OpenSTA command reference (Liberty on GitHub).

All 7 cards in this section, with answers

What does this tell you about the cell, before any timing is read?
report excerpt
```
  cell (DFF_X1) {

	drive_strength     	: 1;

	ff ("IQ" , "IQN") {
		next_state         	: "D";
		clocked_on         	: "CK";
	}

	area               	: 4.522000;
```
It is sequential: the ff group gives its state and its clock.

Hint: One group here only appears on a certain kind of cell.

The ff group marks the cell as a flip-flop, names the internal state IQ and its complement, says the next state comes from D, and says CK clocks it. area is in library units and feeds utilization; drive_strength is the relative label that appears in the cell's name. Liberty reference
Which check is this, and between which two pins?
report excerpt
```
	pin (D) {

		direction		: input;
		related_power_pin		: "VDD";
		related_ground_pin		: "VSS";
		capacitance		: 1.140290;
		fall_capacitance	: 1.062342;
		rise_capacitance	: 1.140290;

		timing () {

			related_pin	   : "CK";
			timing_type	   : hold_rising;
			fall_constraint(Hold_3_3) {
				index_1 ("0.00117378,0.0449324,0.198535");
```
A hold check on D against the rising edge of CK.

Hint: Look at related_pin and timing_type together, and note which pin group they sit in.

The group sits inside pin (D), so the check is on D; related_pin names CK as the reference and timing_type says it is a hold check on the rising edge. The values come as a constraint table indexed by the two pins' transitions, not by load. Liberty reference
Why does this cell have more than one leakage value?
report excerpt
```
	cell_leakage_power 	: 79.112308;

	leakage_power () {
		when           : "!CK & !D & !Q & QN";
		value          : 73.812310;
	}
	leakage_power () {
		when           : "!CK & !D & Q & !QN";
		value          : 77.650980;
	}
	leakage_power () {
		when           : "!CK & D & !Q & QN";
```
Leakage depends on the state the cell is holding.

Hint: Read the when conditions: they describe the cell's state, not an event.

Which transistors are off depends on the stored state and the inputs, so each state leaks differently. cell_leakage_power is the average used when the state probabilities are unknown; report_power uses the state-dependent values when it has activity data. Liberty reference
A delay table in this library holds the value 0.0189. What is that?
report excerpt
```
  delay_model             		: table_lookup;
  in_place_swap_mode      		: match_footprint;
  library_features        		  (report_delay_calculation,report_power_calculation);
  /* Units Attributes */
  time_unit               		: "1ns";
  leakage_power_unit      		: "1nW";
  voltage_unit            		: "1V";
  current_unit            		: "1mA";
  pulling_resistance_unit 		: "1kohm";
  capacitive_load_unit    		  (1,ff);
```
0.0189 nanoseconds, since the library's time unit is 1 ns.

Hint: The file declares its units once, at the top.

Values in Liberty are bare numbers, and the header says what they mean: time in nanoseconds, capacitance in femtofarads, power in nanowatts here. A tool reporting in different units converts on the way out, which report_units shows. Liberty reference
What kind of cell is this, and what does the timer allow it to do?
report excerpt
```
  cell (DLH_X1) {
    latch ("IQ" , "IQN") {
      data_in : "D";
      enable  : "G";
    }
    area : 2.660000;
  }
```
A transparent latch, so late data can borrow time through it.

Hint: Compare the group's keywords against a flip-flop's.

The latch group with data_in and enable is the signature. While G is active the cell is transparent, so the timer checks the data against the closing edge and permits borrowing up to the limit set by set_max_time_borrow. The two names are the state and its complement. Liberty reference
This cell's state table ends with the row "H - - : - : N". What does that row say?
report excerpt
```
  statetable ("CK E SE","IQ") {
       table : "L L L : - : L ,\
                L L H : - : H ,\
                L H L : - : H ,\
                L H H : - : H ,\
                H - - : - : N " ;
  }
  clock_gating_integrated_cell : latch_posedge_precontrol;
```
While the clock is high the cell holds its state, whatever the enables do.

Hint: The three inputs are CK, E, and SE, and N is not a logic level.

N means no change. The enable is captured only while the clock is low, so a late enable cannot cut a pulse that has already started. That is the whole point of an integrated clock gating cell, and the reason set_clock_gating_check times the enable against the opposite edge. Liberty reference
The clock reaching this pin has a transition of about 0.045 nanoseconds. What does this table then require?
report excerpt
```
  timing () {
      related_pin    : "CK";
      timing_type    : min_pulse_width;
      fall_constraint(Pulse_width_3) {
          index_1 ("0.00117378,0.0449324,0.198535");
          values ("0.075346,0.087068,0.198733");
      }
  }
```
The clock must stay low for at least 0.087 nanoseconds.

Hint: Find 0.045 on the axis and read the value beneath it.

It is a fall constraint, so it governs the low phase, and 0.0449 is the second point on the axis, giving 0.087068. A sharper edge would need only 0.075. This is why a clock with a degraded transition can fail a pulse width check that a clean one passes at the same frequency. Liberty reference