GUI Examples

Each circuit is in a folder of its own in circuits/, with the files it uses, like the Verilog file of a Verilog component or the circuits of its subcircuits. They come with DigSim: File > Examples copies one to a folder of yours and opens it (see Example circuits). In a copy of the repository, open a circuit with --load, then press Start Simulation (or Space):

uv run -m digsim.app --load circuits/yosys_counter/counter_yosys_netlist.circuit

You can also use File > Open in the application. See GUI Reference for how to work with the editor.

Verilog counter

counter_yosys_netlist.circuit and counter_yosys_verilog.circuit are the same circuit: the Verilog counter as a Verilog component, loaded from the netlist or directly from counter.v. Notes in the circuit explain each part:

• a 5 Hz clock drives the counter and an LED,
• a push button resets the counter (shortcut 1),
• a switch enables counting (shortcut 2),
• a hex digit shows the counter value,
• a logic analyzer shows the clock, button, switch and the four counter bits, connected through Wire Sources/Sinks and a bus splitter,
• a buzzer sounds while the reset button is pressed.

Turn on the switch to start counting.

74162 counter

74162_counter.circuit rebuilds a classic counter board with two ICs from the built-in IC library: a 74162 synchronous decade counter and a 7448 BCD to 7-segment decoder.

• A 5 Hz clock drives the counter, and the switch enables counting (ENP and ENT).
• The 7448 decodes the counter value to the 7-segment display.
• The LED shows the counter's ripple-carry output (RCO), which is high on the count of 9.
• The DIP switch sets a value that is loaded with one push button (Load), and another push button clears the counter (Clear). Both inputs are active low, hence the NOT gates.
• The third push button drives the 7448 lamp-test input (lt).

Shift register

shift_register.circuit is a 6-bit shift register: six D flip-flops in a chain, clocked by a 5 Hz clock, with an LED on each output. Press the push button to shift ones into the register and watch them move along the LEDs.

Tristate bus

tristate.circuit has four static values (3, 7, 10 and 14) that share one 4-bit bus to a hex digit, each through a tristate buffer. The four switches of the DIP switch enable one buffer each.

• With one buffer enabled, the hex digit shows its value.
• With no buffer enabled, the bus is not driven (high impedance) and blue. With two or more enabled, it is undefined and red. The hex digit is blank in both cases.

Bidirectional port

bidirectional_port.circuit shows a bidirectional bus for a first look: two walkie-talkies, the same Verilog module (walkie_talkie.v) twice, share one 4-bit bus wire. The bus port of each, drawn as a diamond, is an inout port: it can send on the wire and listen to it. Each walkie-talkie has four switches for its message, a Talk button, and a hex digit for what it hears on the bus.

• Hold Talk A (shortcut 1): A sends its message, and both hex digits show it.
• Hold Talk B (shortcut 2) instead: the same wire carries B's message the other way.
• Hold both, with different messages: the two collide, and the bus is undefined (red).
• With nobody talking the bus is not driven (blue), and the hex digits are blank.

Traffic light

traffic_light.circuit is the traffic light state machine as a Verilog component, made from traffic_light.sv with a 4 Hz clock and red, yellow and green LEDs.

• Push the upper button (shortcut 1) to reset it to red.
• Push the lower button (shortcut 2) to request green: red and yellow, green, yellow, and red again.

Buzzers

buzzers.circuit has five push buttons, each connected to a buzzer with its own tone (C, D, E, F and G). Press the buttons, or the shortcut keys 1 to 5, while the simulation runs to play a tune.

RV32E CPU system

rv32e_demo.circuit in circuits/rv32e/ is a small computer: a RISC-V RV32E processor that runs a C program and writes to an HD44780 character LCD. Start the simulation and press Reset (shortcut 1). After a few seconds of simulated time the LCD shows "Hello RV32E!" between two hearts (a custom character), and the time since reset on the second line.

• The processor is the cpu subcircuit (cpu.circuit): the RV32E core in Verilog (rv32e.v, a Verilog component that runs one instruction per clock cycle) with its memory system, an arbiter, a GPIO register and a timer, each a subcircuit of its own. Double-click it to see inside it, the memory map is in a note there.
• A 100 Hz clock drives it. The ROM holds the program, rv32e_code/lcd.bin, and the RAM has a byte enable input for the byte strobes of the processor's stores.
• The program drives the LCD in 4-bit mode through byte 0 of the GPIO register: bits 3:0 are D7:D4, bit 4 is RS and bit 5 is E. Once the text is written, byte 1 (bits 15:8) is a running bar that fills up and empties again. The register writes only the bytes of a store, so the program writes each byte by itself, as a field of a struct.
• Two LED bars show GPIO bits 7:0 and 15:8, a hex display the program counter, and an LED lights when the processor halts (EBREAK, ECALL or a trap).
• rv32e_code/ has the program's source: lcd.c, the startup code crt0.S and the linker script link.ld. make there builds lcd.bin again with the RISC-V GCC (riscv64-unknown-elf-gcc).

Adders from subcircuits

half_adder.circuit is a half adder, with In ports for a and b and Out ports for s and c. full_adder.circuit is a full adder from two half adder subcircuits and an OR gate, and adder_4bit.circuit a 4-bit adder from four full adder subcircuits. The DIP switches are A and B, the hex digit shows A + B and the LED the carry. Right-click a subcircuit and choose Edit Subcircuit to open its file in a tab: a change to the half adder, once it is saved, is in every adder that uses it.