# VHDL301: Forbidden std_logic values should not be used

## Summary

```{raw} html
<div class="coding-quality-issue-table"><div class="coding-quality-issue-toolbar"><p>Count: <strong class="coding-quality-visible">1</strong></p><button type="button" class="coding-quality-clear-filters">Clear filters</button></div><table class="docutils coding-quality-issues coding-quality-issue-list"><thead><tr class="coding-quality-headers"><th scope="col">Reliability</th><th scope="col">Security</th><th scope="col">Maintainability</th><th scope="col">Code attribute</th><th scope="col">Effort to fix</th><th scope="col">Tags</th></tr><tr class="coding-quality-filters"><th><input type="search" placeholder="Filter" aria-label="Filter Reliability"></th><th><input type="search" placeholder="Filter" aria-label="Filter Security"></th><th><input type="search" placeholder="Filter" aria-label="Filter Maintainability"></th><th><input type="search" placeholder="Filter" aria-label="Filter Code attribute"></th><th><input type="search" placeholder="Filter" aria-label="Filter Effort to fix"></th><th><input type="search" placeholder="Filter" aria-label="Filter Tags"></th></tr></thead><tbody><tr><td>HIGH</td><td>-</td><td>-</td><td>Intentionality &#124; Logical</td><td>20min</td><td>synthesis, vscode, linter, beta</td></tr></tbody></table></div>
```

## Description

```{raw} html
<p>
  The following <code>std_logic</code> values should not be used in synthesis:
</p>
<ul>
  <li><code>U</code>: Unitialized</li>
  <li><code>X</code>: Forcing Unknown</li>
  <li><code>W</code>: Weak Unknown</li>
  <li><code>L</code>: Weak <code>0</code></li>
  <li><code>H</code>: Weak <code>1</code></li>
  <li><code>-</code>: Don't care</li>
</ul>
<h4>Noncompliant code example</h4>
<pre>
  i1 : in std_ulogic := 'U';
</pre>
```

## Parameters

No parameters.

```{raw} html

<style>
.coding-quality-issue-toolbar { display: flex; align-items: center; justify-content: space-between; gap: 12px; margin-bottom: 8px; }
.coding-quality-clear-filters { margin-left: auto; cursor: pointer; border: 0; border-radius: 8px; padding: 5px 12px; background: var(--linty-teal, #1d6f78); color: #f4fbfd; font-size: 12px; font-weight: 600; }
.coding-quality-issue-table input { width: 100%; box-sizing: border-box; }
.coding-quality-issues tr.coding-quality-filters th { font-weight: normal; text-transform: none; letter-spacing: normal; }
.coding-quality-issues tr.coding-quality-filters input {
  font-size: 1rem !important;
  font-weight: normal;
  letter-spacing: normal;
  text-transform: none;
}
.coding-quality-issues th[scope="col"] {
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 1rem !important;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}
.coding-quality-issues th.coding-quality-nosort { cursor: default; }
.coding-quality-sort {
  display: inline-block;
  margin-left: 0.35em;
  opacity: 0.55;
  font-size: 0.85em;
}
.coding-quality-issues th[aria-sort="ascending"] .coding-quality-sort,
.coding-quality-issues th[aria-sort="descending"] .coding-quality-sort {
  opacity: 1;
}

a.coding-quality-rule-link,
.rst-content a.coding-quality-rule-link,
.rst-content a.coding-quality-rule-link:visited {
  color: var(--linty-cyan-strong);
  font-family: var(--font-mono);
  font-size: inherit;
  text-decoration: none;
  border-bottom: 1px dashed color-mix(in srgb, var(--linty-cyan-strong) 35%, transparent);
  cursor: pointer;
}
a.coding-quality-rule-link:hover,
.rst-content a.coding-quality-rule-link:hover {
  color: #0a5d6d;
  border-bottom-color: var(--linty-cyan);
}

</style>
<script>
document.addEventListener("DOMContentLoaded", function () {
  document.querySelectorAll("table.coding-quality-issues").forEach(function (table) {
    if (table.dataset.enhanced) return;
    table.dataset.enhanced = "true";
    var headers = table.querySelectorAll("thead tr.coding-quality-headers th");
    var inputs = table.querySelectorAll("thead tr.coding-quality-filters input");
    var tbody = table.querySelector("tbody");
    var count = table.parentElement.querySelector(".coding-quality-visible");
    var clear = table.parentElement.querySelector(".coding-quality-clear-filters");
    var directions = {};

    function rows() {
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    }

    function applyFilters() {
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        return input.value.trim().toLowerCase();
      });
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      rows().forEach(function (row) {
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          if (text.indexOf(queries[index]) === -1) {
            match = false;
            break;
          }
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        if (match) visible += 1;
      });
      if (count) count.textContent = String(visible);
    }

    function sortBy(index) {
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      directions = {};
      directions[index] = direction;
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        var active = headerIndex === index;
        header.setAttribute("aria-sort", active ? (direction === "asc" ? "ascending" : "descending") : "none");
        var mark = header.querySelector(".coding-quality-sort");
        if (mark) mark.textContent = active ? (direction === "asc" ? " \u25B2" : " \u25BC") : " \u21C5";
      });
      rows().sort(function (left, right) {
        var leftText = (left.children[index].textContent || "").trim();
        var rightText = (right.children[index].textContent || "").trim();
        var result = leftText.localeCompare(rightText, undefined, {numeric: true, sensitivity: "base"});
        return direction === "asc" ? result : -result;
      }).forEach(function (row) {
        tbody.appendChild(row);
      });
    }

    Array.prototype.forEach.call(headers, function (header, index) {
      if ((header.textContent || "").trim() === "Details") {
        header.classList.add("coding-quality-nosort");
        return;
      }
      header.setAttribute("aria-sort", "none");
      header.title = "Sort";
      var mark = document.createElement("span");
      mark.className = "coding-quality-sort";
      mark.textContent = " \u21C5";
      header.appendChild(mark);
      header.addEventListener("click", function () { sortBy(index); });
    });
    Array.prototype.forEach.call(inputs, function (input) {
      input.addEventListener("input", applyFilters);
    });
    if (clear) {
      clear.addEventListener("click", function () {
        Array.prototype.forEach.call(inputs, function (input) { input.value = ""; });
        applyFilters();
      });
    }
  });
});
</script>

```