Driving Unusual Resolutions: Custom Timing, EDID and Scaler Configuration

Bar displays, stretched formats and portrait panels all break the assumption that a display will present a standard video mode. The host asks for 1920×1080, the panel wants 1920×360, and the result is either no image or a stretched one. Fixing this is a configuration problem with a defined workflow, and the earlier it is planned the cheaper it is.

Part of the Interfaces, Touch & Signal Integrity guide — 39 articles on RGB, SPI, LVDS, MIPI, eDP and the touch controller.

Why bar and stretched panels break standard timings

Video standards describe aspect ratios the panel does not have.

Standard video modes come with fixed horizontal and vertical totals, blanking intervals and refresh rates. A bar panel with a 1920×360 array has a horizontal resolution matching a full-HD panel and a vertical resolution closer to a character display, so no standard mode maps onto it. Feeding a standard mode to such a panel produces either a lost image, a partial image or a scaled one, depending on where the scaling is done. The panel’s own timing table is the document that resolves this.

Obtaining the timing table early – with pixel clock, front and back porch, sync widths and polarities – is the single most useful thing a buyer can request from a display supplier. CDTech’s notes on display timing parameters explain each field in practical terms.

Pixel clock, blanking and sync polarity in practice

These four numbers describe the whole video signal.

The pixel clock sets the rate at which pixels are transmitted; the total horizontal and vertical counts include the active area plus blanking; the porches define where sync pulses sit within the blanking; and the sync polarity tells the panel where the active period begins. Getting any of them wrong produces characteristic symptoms – a shifted image, a rolling picture, a black border or no signal at all – which is why a timing table is more useful than a resolution figure.

Where the host’s output is configurable, the timing table can be entered directly. Where it is not, a bridge or scaler that accepts a standard mode and generates the panel’s timing is required.

EDID: what it declares and where it fails

EDID tells the host what the display can accept.

An EDID block lists the display’s supported modes, its preferred timing, and information such as the manufacturer and physical size. A host reads it and configures its output accordingly. Failures occur when the EDID does not include a mode the host insists on, when a non-standard timing is not expressible in the standard mode list, or when the EDID is corrupted or unreadable so the host falls back to a default that the panel cannot display. Emulating a known-good EDID is a common way to make a host behave, with the timing conversion handled downstream.

For custom formats, the practical route is often a controller board that presents a standard EDID to the host while generating the panel’s native timing, rather than trying to describe an unusual mode in an EDID that some hosts will ignore.

Scaler versus direct drive

One decision removes most of the difficulty.

Scaler and direct drive compared

Approach	Advantages	Costs
Direct drive from the host	No extra board; lowest latency; simplest BOM	Host must generate native timing; changes if the panel changes
Controller with scaler	Accepts standard video; handles EDID and conversion	Adds cost, latency and another firmware element
Timing controller without scaling	Accepts native timing from a standard interface	Host must still output the correct timings

Where the host’s output is fixed – a PC, a media player, a standard SoC – a scaler is the only practical route for an unusual format. Where the host is designed for the product, direct drive removes a board and a failure mode.

Writing a working custom timing entry

Start from the panel’s table, then verify each field.

1. Enter the active resolution and the total horizontal and vertical counts.
2. Set the front porch, back porch and sync widths from the panel data.
3. Set the sync polarities as specified rather than by assumption.
4. Choose the refresh rate within the panel’s supported range and matching the source content.
5. Verify the pixel clock stays inside the interface’s capability at that refresh rate.
6. Test at the extremes of temperature if the display operates across a range.

A timing entry that works at room temperature can fail at the extremes if the pixel clock is at the edge of the interface’s capability, so margin is worth leaving.

Rotating portrait panels without losing quality

Rotation is a scaling decision, not a mechanical one.

A portrait-mounted panel driven by a landscape-oriented host requires the image to be rotated. Doing it in the host costs processing but preserves quality; doing it in a scaler adds latency and, if the rotation is not an exact multiple of the panel’s pixel grid, softening. Where the content is text, rotation artefacts are visible; where it is a photograph, they are less so. The cleanest approach is to render the content in the correct orientation rather than rotate afterwards.

Mechanically, rotation also changes the viewing-angle behaviour and the polariser orientation, which affects how the display appears through polarised sunglasses and how colour shifts with angle.

Colour depth, subsampling and bandwidth limits

Colour depth consumes bandwidth quickly.

Increasing from 6-bit to 8-bit per colour raises the data rate by a third, and adding frame-rate or resolution increases it further. Where the interface is near its limit, the usual casualties are refresh rate or colour depth rather than resolution, and the symptoms are banding in gradients or a lower frame rate than expected. Subsampling schemes that reduce chroma resolution save bandwidth at the cost of colour detail in fine text and edges.

The calculation should be done once, at design time, with the panel’s native resolution and the required refresh rate, rather than discovered when the image banding appears.

Debugging out-of-range and no-signal problems

Four symptoms, four likely causes.

• No signal at all: check EDID reading, hot-plug detection and the host’s fallback mode.
• Image shifted or wrapped: check porches and sync polarity against the panel table.
• Rolling or unstable image: check the pixel clock and refresh rate against the panel’s range.
• Intermittent loss at temperature extremes: check timing margin and link training behaviour.

Logging the host’s detected mode alongside the panel’s required timing resolves most of these cases quickly, and it is worth building that diagnostic into the product during development.

Verifying timing stability over temperature

Timing margin is a thermal question.

The interface electronics and the panel’s own driver ICs change behaviour with temperature, and a link that runs at the edge of its margin when cold may fail when hot, or vice versa. Verifying the timing at the extremes – with the display running its worst-case content – is a short test that prevents a seasonal field problem. Where the link includes training (as eDP and some serial links do), the training result at temperature extremes is the parameter to record.

A simple check of captured frames or error counters at each extreme is usually enough to show whether margin exists.

Checklist for the display supplier’s timing table

Ask for these fields explicitly.

Timing data to request with a display module

Item	Purpose
Active resolution and scan direction	Confirms orientation and pixel grid
Pixel clock range	Sets the achievable refresh rate and margin
Horizontal and vertical totals with porches	Needed to build a working timing entry
Sync widths and polarities	Determines image position and stability
Supported refresh range	Prevents out-of-range operation
Interface data format and lane configuration	Matches the host output to the panel
EDID requirement and preferred timing	Determines whether emulation is needed
Behaviour on loss of signal	Defines the fail state for the product

Interface specifications for these links are published by MIPI and HDMI.org, and practical timing and scaling behaviour is documented in component vendor material from TI and Analog Devices. CDTech’s notes on stretched versus standard panels cover the format side of the same problem.