diff --git a/src/devices/machine/z80ctc.cpp b/src/devices/machine/z80ctc.cpp
index 840855a6ee0..7be4ecb62b3 100644
--- a/src/devices/machine/z80ctc.cpp
+++ b/src/devices/machine/z80ctc.cpp
@@ -518,6 +518,7 @@ TIMER_CALLBACK_MEMBER(z80ctc_channel_device::timer_callback)
 	}
 
 	// generate the clock pulse
+	// FIXME: should only be cleared after one cycle of the channel input clock
 	m_device->m_zc_cb[m_index](1);
 	m_device->m_zc_cb[m_index](0);