2 Drawing a Schematic

Getting started with logic design.

This section contains an introduction to the components in CLB Synthesizer and getting started with a logic design.

2.1 Components in CLB Synthesizer

Components available in the CLB Synthesizer.

2.1.1 Ports

Port components represent the interface between a hierarchical layer and the layer above and are strictly input or output.

An input port is an input to the CLB:

An output port is an output from the CLB:

There are two main categories of ports:
  • I/O ports (global)
  • Off-sheet ports (hierarchical)

Global Input Ports

Global I/O ports represent the interface to other peripherals on the MCU. To place an input port, drag and drop the input port symbol onto the canvas and select the I/O option in the Properties panel.

The selection box specifies to which signal the input is mapped.

Note: Selecting a signal to use as an input does not configure the actual source of that signal. Make sure to configure the peripheral or clock source to produce the signal.

Input ports from other peripherals have one of four synchronizer options selected in the Input Synchronizer options box. For more information, see Input Synchronizers.

Input ports from the CPU do not have synchronizer options. For more information, see Using CLBSWIN.

Usually, the global ports are used only at the top level (main), but there are cases in which it makes sense to use global input ports on sub-sheets. It is also possible to use the same input signal with an alternative synchronizer option, in which case a second input port symbol must be used, and the alternative synchronizer option applied to that input.

Global Output Ports

To place an output port, drag and drop the output port symbol onto the canvas and select the I/O option in the Properties panel.

The selection box specifies to which signal the output is mapped.

Note: Selecting a signal to be used as an output does not configure the actual destination of that signal. Make sure to configure the peripheral to use the signal as its input.

For more information on output signals, see CLB Outputs to Other Peripherals.

Off-Sheet I/O Ports

Off-sheet ports represent the interface from the module defined on that particular sheet to the sheets in which the module is used. To place an off-sheet port, drag and drop the input or output symbol onto the canvas, select the Off-Sheet Port option in the Properties panel, and give the signal an appropriate name.

Note: Verilog keywords and other reserved words cannot be used as sheet or signal names.

For more information on hierarchical design, see Using Hierarchical Modules.

2.1.2 Logic Gates

Building blocks of a configurable logic design.

The CLB Synthesizer provides a palette of common logic gates in the left hand panel.

To use a logic gate, drag and drop the symbol from the palette onto the canvas and connect it as required.

Note: Input and output ports of logic gates must not be left unconnected.

2.1.3 Flip-Flops

Building blocks for synchronous logic designs.

The CLB Synthesizer provides four basic types of flip-flops to use in synchronous designs.

Note:
  1. The CLB architecture has only one type of flip-flop, identical to its BLEs.
  2. It is not possible to provide an input clock to any flip-flops. For more information, refer to Clocking the Configurable Logic Block.

2.1.4 Multiplexers

Switching between signals.

The CLB Synthesizer provides two multiplexers (MUXes) for logic designs—one with a single-bit select input and one with a 2-bit select input.

Note: The CLB architecture does not contain actual MUXes as they are implemented in the CLB by LUTs.

2.1.5 Tables

Manually configured logic functions.

The CLB Synthesizer provides a 4-bit Table implementation.

To use the Table, drag and drop the Table symbol onto the canvas and set the output (Q) states in the properties panel according to the required logic function.

Note: Although the CLB architecture consists of four input LUTs, there is no ensurement that a manually entered Table will be implemented entirely in one BLE LUT in the same way it was drawn. The synthesis process will always produce the logic equivalent to this Table, which may be simplified or distributed over several actual BLEs.

2.1.6 Constants

A source of a fixed signal.

The CLB Synthesizer provides several ways to give a fixed signal to an input:

  • Individual constant output bits can be connected to inputs

  • Multi-signal constants

  • Configure the number of bits and their values in the Properties panel

  • A Zero port can also be used to provide a constant by using an input MUX position

Note: Although constants can be useful in many cases, they can also be a source of problems. The input logic is optimized into a logical equivalent during the synthesis process. When using constants, this has a major impact. The “zero port” is an exception since it is treated as any other input signal with an unknown value. For more information, see Synthesis and Place-and-Route Process.

2.1.7 Net Labels

Using the same signal in many places in a design.

A netlabel component dispenses a signal to be used in several places in a logic design, which can increase the readability of a schematic by reducing the number of crossing lines.

For example, then inverted CLBIN1PPS signal is used immediately (connection not shown) and routed to a netlabel SWITCH.

The SWITCH signal can then be used elsewhere in the design as a source.

2.2 Drawing a Basic Schematic

How to get started with a logic design.

The CLB Synthesizer GUI simplifies the process of creating a logic design.

Step 1: Add and Configure Logic Input Sources

  • Drag-and-drop an input port from the left side panel onto the canvas

  • Select the input port and configure its source properties in the right-hand panel

  • Repeat as needed

Step 2: Add and Configure Logic Outputs

  • Drag and drop an output port from the left side panel onto the canvas

  • Select the output port and configure its destination properties in the right-hand panel

  • Repeat as needed

Step 3: Add Logic Content

  • Drag and drop logic gates, flip-flops, multiplexers, LUTs and constants from the palette on the left side panel onto the canvas

  • Connect them up by dragging connections from outputs to inputs, or vice versa

  • Add comments for future understanding

Step 4: Synthesize

  • Click the Synthesize button, and the logic design is converted into a bit stream using a Microchip online service