Digital hardware is built from two types of hardware primitives:
- Combinatorial gates (inverters, and, or, xor, 1-bit full adders, 1-bit multiplexers...) These logic gates perform a simple boolean computation on their inputs and produce an output. Each time one of their inputs changes, they start propagating electrical signals and, after a short delay, the output stabilizes to the resulting value. The propagation delay is important because it is strongly related to the speed at which the digital circuit can run, that is, its maximum clock frequency.
- Memory elements (latches, D-flip-flops, RAMs...). Contrary to the combinatorial logic gates, memory elements do not react immediately to the change of any of their inputs. They have data inputs, control inputs and data outputs. They react on a particular combination of control inputs, not on any change of their data inputs. The rising-edge triggered D-flip-flop (DFF), for instance, has a clock input and a data input. On every rising edge of the clock, the data input is sampled and copied to the data output that remains stable until the next rising edge of the clock, even if the data input changes in between.
A digital hardware circuit is a combination of combinatorial logic and memory elements. Memory elements have several roles. One of them is to allow reusing the same combinatorial logic for several consecutive operations on different data. Circuits using this are frequently referred to as sequential circuits. The figure below shows an example of a sequential circuit that accumulates integer values using the same combinatorial adder, thanks to a rising-edge triggered register. It is also our first example of a block diagram.
Pipe-lining is another common use of memory elements and the basis of many micro-processor architectures. It aims at increasing the clock frequency of a circuit by splitting a complex processing in a succession of simpler operations, and at parallelizing the execution of several consecutive processing:
The block diagram is a graphical representation of the digital circuit. It helps making the right decisions and getting a good understanding of the overall structure before coding. It is the equivalent of the recommended preliminary analysis phases in many software design methods. Experienced designers frequently skip this design phase, at least for simple circuits. If you are a beginner in digital hardware design, however, and if you want to code a digital circuit in VHDL, adopting the 10 simple rules below to draw your block diagram should help you getting it right:
- Surround your drawing with a large rectangle. This is the boundary of your circuit. Everything that crosses this boundary is an input or output port. The VHDL entity will describe this boundary.
- Clearly separate edge-triggered registers (e.g. square blocks) from combinatorial logic (e.g. round blocks). In VHDL they will be translated into processes but of two very different kinds: synchronous and combinatorial.
- Do not use level-triggered latches, use only rising-edge triggered registers. This constraint does not come from VHDL, which is perfectly usable to model latches. It is just a reasonable advice for beginners. Latches are less frequently needed and their use poses many problems which we should probably avoid, at least for our first designs.
- Use the same single clock for all of your rising-edge triggered registers. There again, this constraint is here for the sake of simplicity. It does not come from VHDL, which is perfectly usable to model multi-clock systems. Name the clock
clock. It comes from the outside and is an input of all square blocks and only them. If you wish, do not even represent the clock, it is the same for all square blocks and you can leave it implicit in your diagram.
- Represent the communications between blocks with named and oriented arrows. For the block an arrow comes from, the arrow is an output. For the block an arrow goes to, the arrow is an input. All these arrows will become ports of the VHDL entity, if they are crossing the large rectangle, or signals of the VHDL architecture.
- Arrows have one single origin but they can have several destinations. Indeed, if an arrow had several origins we would create a VHDL signal with several drivers. This is not completely impossible but requires special care in order to avoid short-circuits. We will thus avoid this for now. If an arrow has several destinations, fork the arrow as many times as needed. Use dots to distinguish connected and non-connected crossings.
- Some arrows come from outside the large rectangle. These are the input ports of the entity. An input arrow cannot also be the output of any of your blocks. This is enforced by the VHDL language: the input ports of an entity can be read but not written. This is again to avoid short-circuits.
- Some arrows go outside. These are the output ports. In VHDL versions prior 2008 the output ports of an entity can be written but not read. An output arrow must thus have one single origin and one single destination: the outside. No forks on output arrows, an output arrow cannot be also the input of one of your blocks. If you want to use an output arrow as an input for some of your blocks, insert a new round block to split it in two parts: the internal one, with as many forks as you wish, and the output arrow that comes from the new block and goes outside. The new block will become a simple continuous assignment in VHDL. A kind of transparent renaming. Since VHDL 2008 ouptut ports can also be read.
- All arrows that do not come or go from/to the outside are internal signals. You will declare them all in the VHDL architecture.
- Every cycle in the diagram must comprise at least one square block. This is not due to VHDL. It comes from the basic principles of digital hardware design. Combinatorial loops shall absolutely be avoided. Except in very rare cases, they do not produce any useful result. And a cycle of the block diagram that would comprise only round blocks would be a combinatorial loop.
Do not forget to carefully check the last rule, it is as essential as the others but it may be a bit more difficult to verify.
Unless you absolutely need features that we excluded for now, like latches, multiple-clocks or signals with multiple drivers, you should easily draw a block diagram of your circuit that complies with the 10 rules. If not, the problem is probably with the circuit you want, not with VHDL or the logic synthesizer. And it probably means that the circuit you want is not digital hardware.
Applying the 10 rules to our example of a sequential circuit would lead to a block diagram like:
- The large rectangle around the diagram is crossed by 3 arrows, representing the input and output ports of the VHDL entity.
- The block diagram has two round (combinatorial) blocks - the adder and the output renaming block - and one square (synchronous) block - the register.
- It uses only edge-triggered registers.
- There is only one clock, named
clock and we use only its rising edge.
- The block diagram has five arrows, one with a fork. They correspond to two internal signals, two input ports and one output port.
- All arrows have one origin and one destination except the arrow named
Sum that has two destinations.
Clock arrows are our two input ports. They are not output of our own blocks.
Data_out arrow is our output port. In order to be compatible with VHDL versions prior 2008, we added an extra renaming (round) block between
Data_out has exactly one source and one destination.
Next_sum are our two internal signals.
- There is exactly one cycle in the graph and it comprises one square block.
Our block diagram complies with the 10 rules. The Coding example will detail how to translate this type of block diagrams in VHDL.