U Verilogu je uvijek blok jedan od proceduralnih blokova. Naredbe unutar uvijek bloka izvršavaju se sekvencijalno.
Uvijek se blok uvijek izvršava, za razliku od početnih blokova koji se izvršavaju samo jednom na početku simulacije. Blok uvijek trebao bi imati osjetljivu listu ili odgodu povezanu s njom
Osjetljiva lista je ona koja bloku uvijek govori kada treba izvršiti blok koda.
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Sintaksa
The Verilog uvijek blokirajte sljedeću sintaksu
always @ (event) [statement] always @ (event) begin [multiple statements] end
Primjeri
Simbol @ iza rezervirane riječi stalno , označava da će se blokada pokrenuti na uvjet u zagradi iza simbola @.
always @ (x or y or sel) begin m = 0; if (sel == 0) begin m = x; end else begin m = y; end end
U gornjem primjeru opisujemo 2:1 mux, s ulazom x i y. The ovaj je odabir ulaza, i m je mux izlaz.
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U bilo kojoj kombinacijskoj logici, izlaz se mijenja kad god se mijenja ulaz. Kada se ova teorija primijeni na uvijek blokove, tada se kod unutar uvijek blokova mora izvršiti kad god se promijene ulazne ili izlazne varijable.
NAPOMENA: Može pokretati tipove podataka reg i integer, ali ne može pokretati tipove podataka žice.
Postoje dvije vrste osjetljivih popisa u Verilogu, kao što su:
- Osjetljivo na razinu (za kombinacijske sklopove).
- Osjetljivo na rubove (za japanke).
Kôd ispod isti je 2:1 mux, ali izlaz m je sada flip-flop izlaz.
always @ (posedge clk ) if (reset == 0) begin m <= 0; end else if (sel="=" 0) begin m <="x;" pre> <h4>NOTE: The always block is executed at some particular event. A sensitivity list defines the event.</h4> <h3>Sensitivity List</h3> <p>A sensitivity list is an expression that defines when the always block executed, and it is specified after the @ operator within the parentheses ( ). This list may contain either one or a group of signals whose value change will execute the always block.</p> <p>In the code shown below, all statements inside the always block executed whenever the value of signals x or y change.</p> <pre> // execute always block whenever value of 'x' or 'y' change always @ (x or y) begin [statements] end </pre> <p> <strong>Need of Sensitivity List</strong> </p> <p>The always block repeats continuously throughout a simulation. The sensitivity list brings a certain sense of timing, i.e., whenever any signal in the sensitivity list changes, the always block is triggered.</p> <p>If there are no timing control statements within an always block, the simulation will hang because of a zero-delay infinite loop.</p> <p>For example, always block attempts to invert the value of the signal clk. The statement is executed after every 0-time units. Hence, it executes forever because of the absence of a delay in the statement.</p> <pre> // always block started at time 0 units // But when is it supposed to be repeated // There is no time control, and hence it will stay and // be repeated at 0-time units only and it continues // in a loop and simulation will hang always clk = ~clk; </pre> <p>If the sensitivity list is empty, there should be some other form of time delay. Simulation time is advanced by a delay statement within the always construct.</p> <pre> always #10 clk = ~clk; </pre> <p>Now, the clock inversion is done after every 10-time units. That's why the real Verilog design code always requires a sensitivity list.</p> <h4>NOTE: Explicit delays are not synthesizable into logic gates.</h4> <h3>Uses of always block</h3> <p>An always block can be used to realize combinational or sequential elements. A sequential element like flip flop becomes active when it is provided with a clock and reset.</p> <p>Similarly, a combinational block becomes active when one of its input values change. These hardware blocks are all working concurrently independently of each other. The connection between each is what determines the flow of data.</p> <p>An always block is made as a continuous process that gets triggered and performs some action when a signal within the sensitivity list becomes active.</p> <p>In the following example, all statements within the always block executed at every positive edge of the signal clk</p> <pre> // execute always block at the positive edge of signal 'clk' always @ (posedge clk) begin [statements] end </pre> <h3>Sequential Element Design</h3> <p>The below code defines a module called <strong> <em>tff</em> </strong> that accepts a data input, clock, and active-low reset. Here, the always block is triggered either at the positive edge of the <strong> <em>clk</em> </strong> or the negative edge of <strong> <em>rstn</em> </strong> .</p> <p> <strong>1. The positive edge of the clock</strong> </p> <p>The following events happen at the positive edge of the clock and are repeated for all positive edge of the clock.</p> <p> <strong>Step 1:</strong> First, if statement checks the value of active-low reset <strong> <em>rstn</em> </strong> .</p> <ul> <li>If <strong> <em>rstn</em> </strong> is zero, then output q should be reset to the default value of 0.</li> <li>If <strong> <em>rstn</em> </strong> is one, then it means reset is not applied and should follow default behavior.</li> </ul> <p> <strong>Step 2:</strong> If the previous step is false, then</p> <ul> <li>Check the value of d, and if it is found to be one, then invert the value of q.</li> <li>If d is 0, then maintain value of q.</li> </ul> <pre> module tff (input d, clk, rstn, output reg q); always @ (posedge clk or negedge rstn) begin if (!rstn) q <= 0; else if (d) q <="~q;" end endmodule pre> <p> <strong>2. Negative edge of reset</strong> </p> <p>The following events happen at the negative edge of <strong> <em>rstn</em> </strong> .</p> <p> <strong>Step 1:</strong> First, if statement checks the value of active-low reset <strong> <em>rstn</em> </strong> . At the negative edge of the signal, its value is 0.</p> <ul> <li>If the value of <strong> <em>rstn</em> </strong> is 0, then it means reset is applied, and output should be reset to the default value of 0.</li> <li>And if the value of <strong> <em>rstn</em> </strong> is 1, then it is not considered because the current event is a negative edge of the <strong> <em>rstn</em> </strong> .</li> </ul> <h3>Combinational Element Design</h3> <p>An always block can also be used in the design of combinational blocks.</p> <p>For example, the digital circuit below represents three different logic gates that provide a specific output at signal o.</p> <img src="//techcodeview.com/img/verilog-tutorial/39/verilog-always-block.webp" alt="Verilog Always Block"> <p>The code shown below is a module with four input ports and a single output port called o. The always block is triggered whenever any of the signals in the sensitivity list changes in value.</p> <p>The output signal is declared as type <strong> <em>reg</em> </strong> in the module port list because it is used in a procedural block. All signals used in a procedural block should be declared as type <strong> <em>reg</em> </strong> .</p> <pre> module combo (input a, input b, input c, input d, output reg o); always @ (a or b or c or d) begin o <= ~((a & b) | (c^d)); end endmodule < pre> <p>The signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly, o becomes 0 when RHS is false.</p> <hr></=></pre></=></pre></=>
Potreba za popisom osjetljivosti
Blok uvijek se kontinuirano ponavlja tijekom simulacije. Popis osjetljivosti donosi određeni osjećaj vremena, tj. kad god se bilo koji signal na popisu osjetljivosti promijeni, uvijek se aktivira blokada.
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Ako unutar uvijek bloka nema naredbi za kontrolu vremena, simulacija će stati zbog beskonačne petlje bez odgode.
Na primjer, uvijek blokirajte pokušaje invertiranja vrijednosti signala clk. Naredba se izvršava nakon svakih 0-vremenskih jedinica. Stoga se izvršava zauvijek zbog nepostojanja kašnjenja u iskazu.
// always block started at time 0 units // But when is it supposed to be repeated // There is no time control, and hence it will stay and // be repeated at 0-time units only and it continues // in a loop and simulation will hang always clk = ~clk;
Ako je popis osjetljivosti prazan, trebao bi postojati neki drugi oblik vremenske odgode. Vrijeme simulacije je unaprijedeno naredbom kašnjenja unutar konstrukcije uvijek.
always #10 clk = ~clk;
Sada se inverzija sata vrši nakon svakih 10 vremenskih jedinica. Zato pravi Verilog dizajn kod uvijek zahtijeva popis osjetljivosti.
NAPOMENA: Eksplicitna kašnjenja ne mogu se sintetizirati u logička vrata.
Upotreba uvijek blokirati
Uvijek blok se može koristiti za realizaciju kombinacijskih ili sekvencijalnih elemenata. Sekvencijalni element kao što je japanka postaje aktivan kada ima sat i resetiranje.
Slično, kombinacijski blok postaje aktivan kada se jedna od njegovih ulaznih vrijednosti promijeni. Svi ovi hardverski blokovi rade istodobno neovisno jedan o drugom. Veza između svakog od njih je ono što određuje tijek podataka.
Uvijek se blokira kao kontinuirani proces koji se pokreće i izvodi neku radnju kada signal unutar popisa osjetljivosti postane aktivan.
U sljedećem primjeru, sve naredbe unutar bloka always izvode se na svakom pozitivnom rubu signala clk
// execute always block at the positive edge of signal 'clk' always @ (posedge clk) begin [statements] end
Dizajn sekvencijalnih elemenata
Donji kod definira modul tzv tff koji prihvaća unos podataka, sat i aktivno nisko resetiranje. Ovdje se uvijek blok pokreće ili na pozitivnom rubu clk ili negativni rub od rstn .
1. Pozitivni rub sata
ručno testiranje
Sljedeći događaji događaju se na pozitivnom rubu sata i ponavljaju se za sve pozitivne rubove sata.
Korak 1: Prvo, naredba if provjerava vrijednost aktivnog niskog resetiranja rstn .
- Ako rstn je nula, onda izlaz q treba vratiti na zadanu vrijednost 0.
- Ako rstn je jedan, onda to znači da se resetiranje ne primjenjuje i treba slijediti zadano ponašanje.
Korak 2: Ako je prethodni korak lažan, onda
- Provjerite vrijednost d, i ako se ustanovi da je jedan, obrnite vrijednost q.
- Ako je d 0, onda zadržite vrijednost q.
module tff (input d, clk, rstn, output reg q); always @ (posedge clk or negedge rstn) begin if (!rstn) q <= 0; else if (d) q <="~q;" end endmodule pre> <p> <strong>2. Negative edge of reset</strong> </p> <p>The following events happen at the negative edge of <strong> <em>rstn</em> </strong> .</p> <p> <strong>Step 1:</strong> First, if statement checks the value of active-low reset <strong> <em>rstn</em> </strong> . At the negative edge of the signal, its value is 0.</p> <ul> <li>If the value of <strong> <em>rstn</em> </strong> is 0, then it means reset is applied, and output should be reset to the default value of 0.</li> <li>And if the value of <strong> <em>rstn</em> </strong> is 1, then it is not considered because the current event is a negative edge of the <strong> <em>rstn</em> </strong> .</li> </ul> <h3>Combinational Element Design</h3> <p>An always block can also be used in the design of combinational blocks.</p> <p>For example, the digital circuit below represents three different logic gates that provide a specific output at signal o.</p> <img src="//techcodeview.com/img/verilog-tutorial/39/verilog-always-block.webp" alt="Verilog Always Block"> <p>The code shown below is a module with four input ports and a single output port called o. The always block is triggered whenever any of the signals in the sensitivity list changes in value.</p> <p>The output signal is declared as type <strong> <em>reg</em> </strong> in the module port list because it is used in a procedural block. All signals used in a procedural block should be declared as type <strong> <em>reg</em> </strong> .</p> <pre> module combo (input a, input b, input c, input d, output reg o); always @ (a or b or c or d) begin o <= ~((a & b) | (c^d)); end endmodule < pre> <p>The signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly, o becomes 0 when RHS is false.</p> <hr></=></pre></=>