<p><b>@maribu</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/8842#discussion_r262461854">cpu/atmega_common/periph/rtt.c</a>:</p>
<pre style='color:#555'>> +
+ISR(TIMER2_OVF_vect) {
+    __enter_isr();
+
+    rtt_state.ext_cnt++;
+
+    /* Enable RTT alarm if overflowed enough times */
+    if (rtt_state.ext_comp <= rtt_state.ext_cnt) {
+        /* Clear interrupt flag */
+        TIFR2 = (1 << OCF2A);
+
+        /* Enable interrupt */
+        TIMSK2 |= (1 << OCIE2A);
+    }
+
+#if MODULE_PERIPH_RTC
</pre>
<p>While this seems to work, I would prefer <code>#ifdef MODULE_PERIPH_RTC</code>. This would be consistent to the negation of the macro, as</p>
<div class="highlight highlight-source-c"><pre>#<span class="pl-k">ifndef</span> MODULE_PERIPH_RTC</pre></div>
<p>will work but</p>
<div class="highlight highlight-source-c"><pre>#<span class="pl-k">if</span> !MODULE_PERIPH_RTC</pre></div>
<p>will not work. (See this as a suggestion, I will not block the PR on this.)</p>

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