ECE 3110: Introduction to Digital SystemsChapter 5Combinational Logic Design Practices Documentation Standards (contd.)
Previous… • Block diagram • Schematics Diagram • Gate symbols • Signal names • Active levels for signal names
Active Levels for Pins • When we draw the outline of a logic symbol, we think of the given logic function as occurring inside that outline. • In logic gates and logic structures the inversion bubble indicates the active level of the signal • Examples:- 2-to- 4 Decoder- EN_L is active low- A and B are active high- Y0_L, Y1_L, Y2_L,Y3_L are active low EN_L EN Y0 Y0_L Y1 Y1_L A A Y2 Y2_L B B Y3 Y3_L
More ways of obtaining AND/OR functions (Generalized AND/OR function). • Four ways of AND function: output is asserted if both of its inputs are asserted. • AND gate (74x08): active-high input/output • NAND gate (74x00): active-high input, active-low output • NOR gate (74x02): active-low input, active-high output • OR gate (74x32): active-low input/output • Four ways of OR function: output is asserted if either of its inputs are asserted. • OR gate (74x32): active-high input/output • NOR gate (74x02): active-high input, active -low output • NAND gate(74x00): active-low inputs, active-high output • AND gate (74x08): active-low inputs/output
Bubble-to-Bubble Logic Design • Purpose : To make it easy to understand the function of the Logic circuit by choosing appropriate logic symbols and signal names including active-level designators. FAIL_L FAIL_L ERROR ERROR OVERFLOW_L OVERFLOW_L
Bubble-to-Bubble Logic Design Rules • - The active level of the output signal of a logic device should match the active level of the device’s output pin. Active-low if the device symbol has an inversion bubble, active-high if not. - If the active level of an input signal is the same as that of the device’s input pin to which it’s connected, then the logic function inside the symbolic outline is activated when the signal is asserted. Most common case. - If the active level of an input signal is the opposite of that of the input pin to which it’s connected, then the logic function inside the symbolic outline is activated when the signal is negated. Should be avoided. ERROR ERROR OVERFLOW HALT_L READY READY_L ERROR REQUEST REQUEST FAIL_L ERROR ENABLE_L ENABLE OVERFLOW_L
Drawing Layouts • Inputs to the left, outputs to the right. • Signals flow from left to right. • Signal paths should be connected. • Broken signal paths should be flagged to indicate the source or destination and direction. • Crossing lines/Connected lines (T-type connection) • Multiple pages schematics:- Flat Structure.- Hierarchical Structure.
Some rules to avoid common errors • Use exactly the same name for same signal. Use different names for different signals. (especially cross pages) • Use appropriate active levels for signal names • Use “T” convention for connected lines.
DATA5 DATA6 DATA[0-7] Buses • Buses should be named : DATA[0-7], CONTROL • A bus name may use brackets and a colon to denote a range • Buses are drawn with thicker lines than ordinary signals. • Individual signals are put into or pulled out of the bus by connecting an ordinary signal line to the bus and writing the signal name. (A special connection dot is often used.) • A signal extracted from a bus should be named • Inter-page signal/Bus Flags : • Uni-direction • Bi-direction • Example: Figure 5-16, pp327 (next slide)
Schematic diagram/Logic Diagram • Logic Diagram Schematic Diagram • Bubble-to-Bubble Logic • IC-Type-Logic Family • Pin numbers- Pin Diagram • Reference designator- Unit Number 74LS04 74LS00 1 2 1 A A_L 3 74LS00 U2 2 F 10 F U1 8 3 4 B B_L 9 74LS00 4 U1 6 74LS04 U2 5 U1
Complete schematic diagram • IC types: a part number identifying the IC that performs a given logic function. Also defines the device’s logic family and speed. Eg. 74HCT00, 74LS00 • Reference designators: a particular instance of that IC type installed in the system. U1,U2… • Pin numbers: used to locate individual logic signal numbers on its pins.
Pinouts for SSI ICs in standard dual-inline packages (pp. 329) Small elements in 74x03 indicate an open-drain or open-collector output
Combinational SSI devices (Contd.) Small elements in 74x14 indicate hysteresis
Combinational SSI devices (Contd.) Small elements in 74x266 indicate an open-drain or open-collector output
Next… • Timing • Reading Wakerly CH-5.2-5.3