HOW-TO GUIDE
How to Trigger an Oscilloscope on a Falling Edge
By the OhmSavvy Research Team ·
When a falling edge is the event you care about
Plenty of signals sit high when idle and announce something by going low. If you trigger on the rising edge instead, the scope still locks, but the moment you wanted to study ends up somewhere off to the side, or off screen entirely on a fast timebase. These are the usual reasons to switch the slope. If you are new to triggers in general, start with how to trigger an oscilloscope and come back here.
| Signal | Idle state | What the falling edge means |
|---|---|---|
| SPI chip-select (CS) | High | The controller has selected the device and a transfer is starting |
| Reset line (active-low) | High | The chip is being held in reset; the drop is the start of the reset pulse |
| Interrupt request (open-drain) | High via pull-up | A peripheral is asking for attention |
| UART TX/RX | High | The start bit of a new character |
| MOSFET or relay drive | High while on | The switch is being turned off; watch for the inductive kick that follows |
| Supply rail at shutdown | At nominal voltage | The rail starting to collapse once input power is removed |
Rising vs falling on the same signal
On a plain square wave either slope gives a stable trace; changing it simply swaps which transition sits on the center line. The two screens below show the same 5-division-period wave with the level (T) at 0 V. Notice that nothing about the signal changed, only where the scope anchors each capture.
1Set the source and change the slope to falling
Open the trigger menu, confirm the type is Edge and the source is the channel carrying the active-low line, then change the slope from rising to falling. Scopes show this as a downward arrow or a small falling-step icon, and many repeat it in the status bar beside the trigger level readout, which makes it easy to confirm at a glance. If you trigger on one line and view others, the slope applies only to the trigger source; the remaining channels are simply displayed relative to that moment.
2Place the level where the signal actually falls through
For logic, roughly half the supply is a safe choice: about 1.65 V on a 3.3 V line or 2.5 V on a 5 V line. That keeps the crossing on the steep part of the edge and away from any ringing at the top. A narrow active-low pulse, like the chip-select below, only spends a short time low, so a level that sits too close to the high state can be fooled by noise on the idle line, while one too close to ground may miss a pulse that never gets all the way down.
Supply rails are different. To catch the instant a 12 V rail starts sagging, place the level slightly below its normal value; to see when it crosses a brownout or undervoltage threshold, put the level at that threshold voltage instead.
3Use the trigger point to look before the edge
A digital scope keeps recording while it waits, so the trigger point does not have to be the left edge of the screen. By default many models put it at the horizontal center, which gives you equal time before and after the drop. Turn the horizontal position (sometimes called delay) to slide it: move the trigger point right to see more of what led up to the edge, or left to see more of the aftermath. That pre-trigger view is often the whole point, for example checking whether a rail dipped before a reset was asserted.
4Use Single for one-off drops, Normal for repeating ones
A shutdown or a reset happens once, so Auto mode is the wrong tool: it will keep drawing untriggered sweeps and overwrite the event a moment later. Press Single, then cut the power or press the reset button. The scope arms, catches the first qualifying high-to-low crossing and freezes. For a chip-select that toggles every few milliseconds, Normal mode is better, since it refreshes only on valid falling edges and holds the last capture between bursts.
01
Find it in Auto
Check the idle-high level and scale
02
Falling slope
Level partway down the expected drop
03
Position
Slide trigger point for pre-trigger view
04
Single
Arm, cause the event, read the frozen trace
5Add holdoff so serial data locks on the start bit
A UART character starts with a falling start bit, but the data bits that follow can contain more high-to-low transitions, and an edge trigger cannot tell them apart. The display then hops between alignments. Holdoff solves it: set it a little longer than one frame and the scope ignores everything until the line has returned to idle. At 9600 baud each bit lasts about 104 µs, so a 10-bit frame (start, eight data, stop) is roughly 1.04 ms; a holdoff a bit above that works for back-to-back characters with a gap between them.
Rule of thumb
Troubleshooting a falling-edge trigger
| Symptom | Likely cause | Fix |
|---|---|---|
| Locks, but the drop is at the screen edge | Slope still on rising | Switch the slope to falling, or check it is not set to either edge |
| Single never fires on power-down | Level below where the rail stops, or source wrong | Raise the level to just under nominal and confirm the trigger channel |
| Trace jumps between positions on serial data | Data bits also have falling edges | Increase holdoff to just over one frame time |
| Triggers on idle-line noise | Level too close to the high state | Move the level toward mid-swing, or try noise-reject trigger coupling if offered |
| Nothing before the edge is visible | Trigger point at the far left of the screen | Adjust horizontal position to bring the trigger point back toward center |
Switching circuits off safely
Scopes for edge triggering
Every digital scope offers a falling-edge trigger, so the differences that matter here are channel count for watching other signals around the edge and bandwidth for timing fast transitions accurately.
Scopes for falling-edge captures
4 channels at 50MHz: trigger on a chip-select or reset line and watch three more signals at once.
100MHz bandwidth for faster logic edges, with 4 channels and 1 GSa/s.
Compact 100MHz two-channel scope; its 25 MHz generator can supply a test signal while you learn triggers.
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Frequently asked questions
Does the falling-edge level need to be at exactly 50%?
No. Mid-swing is a sensible default because the edge is steep there and noise moves the crossing time least, but you can put the level anywhere the signal genuinely passes through on its way down. For a supply rail you may want it just below the normal voltage to catch the very start of a sag, or near a brownout threshold to see when the rail reaches it.
Why does my falling-edge trigger lock on the wrong edge of a serial byte?
A byte of data contains several high-to-low transitions, and an edge trigger accepts any of them. Raise holdoff until it is a little longer than one complete frame so the scope ignores the data bits and re-arms during the idle time, which leaves the start bit as the first falling edge it sees.
Can I trigger on a falling edge on CH1 and watch other channels?
Yes, and it is one of the most useful setups. Put the active-low control line, such as a chip-select or reset, on the trigger channel and the related clock, data or supply on the others. Every channel is shown relative to the moment that control line dropped.
What does an either-edge or alternating slope setting do?
Some scopes offer a setting that triggers on both rising and falling crossings. It can help when you only want to know that a line changed, but because consecutive captures can align on opposite edges, the picture may look like two overlapping traces. For studying one specific drop, choose falling only.
How do I measure the fall time once the scope triggers on the edge?
Spread the edge across several divisions with a faster timebase, then use the automatic Fall Time measurement, which normally times the drop from 90% to 10% of the swing, or place cursors at those levels yourself. Use a 10x probe with a short ground. A scope can't show an edge much faster than about 0.35 divided by its bandwidth.
