A Tour of the Digital Gates
1. The standard gates in the Digital LOG library provide all
of the logic functions you will need to build a digital
circuit. If you need a function that is not provided, you
can design it yourself with the LOGED program, or using
2. All the standard library gates use consistent conventions for
their pins. For example, when a row of pins carries a binary
number, the most significant bit is on the left or bottom end
of the row. Signals are active-high unless a "bubble" shows
3. Input/output devices and controls:
SWITCH Looks like a small pointed box. It is a voltage
source that drives either One or Zero onto its
output. The center lights up black or red to
indicate the state of the switch. Touch the switch
to change its state.
SWITCH2 A smaller switch suitable for packing into tight
PULSE Pulse switch. Looks like a double-pointed box.
It acts much like SWITCH, except that when you
touch it it goes to One only briefly, then back
LED Digital display; small square box. It lights up
according to the value being driven on it: red for
One (or weak One), black for Zero, and background
gray for undriven wires (None). It has connection
points all around so you can connect to it from any
direction. (Note: This means that placing two
LED's right next to each other will short their
LED2 Structurally the same as LED, but the input pins
are treated a little differently. In LED, all
eight pins are connected together. Thus touching
LED's will short each other out. In LED2, the
eight pins are independent. If any of them is
driven high, the LED2 glows red. If any is driven
low, it glows black. If some drive high and some
low at the same time, the LED2 does not glow.
Since the pins are not internally connected, you
can let LED2's touch and they won't mind.
LED3 This is a miniature LED useful for tightly packed
situations. Connect to it by running the wire
right through the center.
EDGE Edge detector. This is the differently-pointed box.
It lights up red whenever it sees any change on its
input, and only resets to black when you touch it.
KEYPAD Numeric keypad. This is a 16-key keypad that generates
a binary number from 0000 to 1111 when you press one
of the 16 keys on the keypad. The lowest output is
most significant. There is also a "strobe" output
on the bottom, that generates a pulse each time a
key is pressed.
7SEG Numeric display. This is a 7-segment LED display,
which displays the symbols 0 through F for inputs
0000 through 1111. It has two equivalent sets of
inputs, for your convenience. The most significant
bit is on the left, and at the bottom. (The two
sets of pins are equivalent; you may use either.)
ASCKBD ASCII keyboard. Analogous to KEYPAD, but with an
alphanumeric keyboard instead which generates
8-bit ASCII codes. Tap a key to generate its
ASCII code (plus a strobe signal). Tap SH, then
a key, to shift that key. CT is the Control key.
Configure the gate and select Key-Codes mode to
generate positional codes for all the keys instead
of ASCII. This lets you handle Shift and Control
yourself, if you like.
ASCDISP ASCII terminal display. Shows 16x64 characters.
To print a character, put its ASCII code on the
data pins and pulse the Strobe pin. Above Strobe
is the Clear pin, which resets the terminal. This
terminal understands most standard Chipmunk control
codes such as Carriage Return and Line Feed. Form
Feed clears the screen. Certain variations on the
control codes can be had by Configuring the gate.
CLOCK This is a two- or four-phase clock generator or
oscillator. By default, it is a two-phase square
wave generator in which the outputs are CLK and
not-CLK; the output changes on every time step
(i.e., once per standard gate delay). You can
reconfigure it to take several time steps per clock
phase, or to clock according to real, physical time,
or to be a "synchronous" clock which changes as
soon as the rest of the circuit has settled down
from the last change. Also, you can change the
waveform to four-phase mode, where the outputs are
completely non-overlapping CLK1 and CLK2, each with
25% duty cycle.
BREAK Breakpoint. This is a box with an X in it. This
gate is the circuit equivalent of a software
"breakpoint." When its inputs see a positive- or
negative-going edge (respectively), it turns the
simulation off so you can examine what was the state
of the system at the instant the transition
TIE Pullup resistor. This generates a weak One, which
can be overridden without conflict by an open-
collector output pulling down.
TIEGND Pulldown resistor. Generates a weak Zero.
4. Connection gates:
VDD Power supply. Provides a constant One signal.
GND Ground. Provides a constant Zero signal.
TO/FROM Terminals. These are the two arrow-shaped gates
in the Catalog. They are equivalent except for
appearance. They are used for putting names on
signals: To name a signal FOO, connect a terminal
to it, then tap the space next to the arrow and type
FOO. If there are several terminals in the
circuit with the same names, they are all
electrically connected as if by wires. TO/FROM
names can be used to pass signals between circuit
pages. Signal names are also used with the Scope
mode, to be described below. Dropping a TO/FROM
into the menu area preserves its programmed name;
if the name ends in a number, each fresh gate pulled
from the slot gets a new number. Predefined signal
names include "Gnd" and "Vdd", equivalent to the
GND and VDD gates, and "Reset", which is normally
grounded but receives a pulse every time you do a
Reset command (in the Misc menu).
5. Standard digital gates:
AND, OR, NAND, NOR, XOR, XNOR
Standard two-input logic gates. Also, AND3, NOR4,
etc. are multiple-input gates, and INV is an
ANDX, ORX etc. These are versions of AND, OR, etc. whose
pictures have been transformed according to
DeMorgan's theorem of logic, which says that an AND
gate is just like an OR gate with its output and all
its inputs inverted.
COMPL This is a complementary-output buffer. Its two
outputs provide the input and its complement, with
identical propagation delays.
DNEG, DPOS, JKNEG, JKPOS, TNEG, TPOS
Flip-flops. Each kind comes in negative- and
positive-edge-triggered flavors. There is a choice
of D flip-flops (which latch a Data bit on each
clock), or J-K flip-flops (which set, reset, toggle,
or stay the same on each clock), or Toggle flip-
flops (which toggle or stay the same on each
LATCH A level-sensitive latch. When G is One, the output
follows the D input. When G is Zero, the output
freezes at the last D input value that was followed.
SHIFT A four-bit shift register. On a falling clock
transition, it either loads from the parallel inputs
(if M is One), or shifts in the direction of the
arrow (if M is Zero).
SRAM8K Static RAM or ROM, storing 8K bytes of 8 bits.
Address inputs are on the left (MSB lowest). Data
input/output pins are at the bottom (MSB on the
the left). Control pins are Chip Enable (CE), Read,
and Output Enable (OE). If Read is high and CE and
OE are low, the addressed byte is driven on the Data
pins. If Read and CE are low (regardless of OE),
the value on the Data pins is stored in the RAM.
Otherwise, the Data pins are undriven and ignored.
(To make a ROM, simply leave Read unconnected.)
Configuring the SRAM8K allows you to examine or
change any byte in the memory, control whether the
memory contents should be recorded in saved circuit
files, and save or load the memory to ".hex" data
files. SRAM8K has the same unit propagation delay
as all Digital LOG gates---bear in mind that real
memory chips are much slower!
6. There is also a large number of 7400's series TTL gates to choose
from in the gate library. These are all intended to be close
models of their true counterparts, but there are no guarantees.
In particular, remember all propagation delays are the same.
7. A set of gates for simulating digital VLSI transistors exists.
All of these gates begin with "V_" and are described below.
8. The gates whose names begin with "A_" are for use with the ACTEL
design system. This is a special box which takes network files
produced by LOGNTK from circuits consisting of "A_" gates, and
programs gate array chips while-U-wait to produce fast, custom
digital chips for prototyping. Use of the ACTEL system is beyond
the scope of this document.
9. The gates DIGH, FORCEDRV, and INST1 through INST5 are intended
for use with hierarchical definitions, described in the next
10. To view the "program" that the simulator uses for a gate, press
the shift-D key, then touch the gate in the circuit diagram.
A small window pops up containing the program. Press somewhere
not on a gate to exit this mode. To erase the definitions, just
hit the space bar to refresh the screen. Notations like "#4"
refer to pin numbers; letters like "E" refer to internal state
variables of the gate. For further documentation, see the
- john [dot] lazzaro [at] gmail [dot] com