Choosing a TFT Display Interface: RGB, SPI, MCU, LVDS, MIPI DSI, eDP and HDMI Compared

Interface choices are usually made once, early, and then constrain the panel, the connector, the PCB layer count and the maximum cable length for the life of the product. Choosing on bandwidth alone produces designs that fail on cable length; choosing on cost alone produces designs that cannot be upgraded. The useful approach is to match the interface to resolution, distance and volume together.

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

The seven interfaces and the problems each one solves

Each family exists for a reason.

Parallel RGB remains the simplest way to drive a small panel from a microcontroller or an FPGA, with one wire per colour bit plus timing signals. SPI and MCU interfaces reduce pin count further and suit small displays where the update rate is low. LVDS carries pixels as differential pairs, which allows longer cables and higher resolutions with fewer conductors. MIPI DSI is the mobile-derived serial interface, efficient in pins and power, and common on small and mid-size modules. eDP targets higher resolutions and larger panels. HDMI is a consumer-facing link that suits monitors and kiosks, not embedded modules with no bezel.

The choice is therefore about the constraints of the product: pin budget, cable run, resolution, refresh rate and the host’s existing outputs.

Bandwidth math: pixels, refresh rate and blanking

The first calculation is unavoidable.

Required bandwidth is the active pixel count multiplied by the refresh rate, with an allowance for blanking intervals, multiplied by the bits per pixel, plus any overhead for encoding and control. That number is then compared with what each interface can carry at the intended clock rate. Bar formats make the arithmetic interesting, because a 1920×360 panel has the horizontal resolution of a full-HD panel with a tenth of the vertical, so the bandwidth requirement is modest while the timing is unusual.

Doing this calculation before choosing an interface avoids the common situation where a panel is selected for its size and the interface is then asked to carry more than it can at the required frame rate. CDTech’s notes on interface selection checks before design freeze set out the parameters to verify.

Pin count, PCB layers and connector cost

Fewer conductors usually means cheaper boards and connectors, up to a point.

Interface characteristics compared

Interface	Conductors	Typical use
Parallel RGB	Up to 28 plus timing and power	Small and mid-size panels, short runs, MCU or FPGA hosts
SPI	4 to 6	Small panels, low refresh, simple hosts
MCU (8080/6800)	8 to 20	Small panels with a controller memory, moderate refresh
LVDS	4 to 10 plus clock	Mid to high resolution, longer cables, industrial standard
MIPI DSI	2 to 8 lanes	Small and mid-size, power-sensitive, mobile-derived hosts
eDP	1 to 4 lanes plus AUX	High resolution, larger panels, embedded PCs
HDMI	External cable	Kiosks, monitors, products with a standard external input

Parallel RGB consumes board area and connector pins, which raises PCB layer count and connector cost on small products. Serial interfaces reduce both, at the cost of tighter layout requirements for impedance and length matching.

Cable length limits in real machines

The distance question decides more designs than bandwidth.

Parallel RGB is generally limited to short runs inside a single enclosure, because skew and crosstalk grow quickly with length. LVDS extends the usable distance to metres when the pairs are properly routed and terminated, which is why it remains the industrial default. MIPI DSI sits between the two, with practical limits shorter than LVDS over simple cables. Serialiser-deserialiser links over coax or shielded twisted pair carry video over tens of metres and are the standard answer for long runs in machines and vehicles, as described in device documentation such as TI’s FPD-Link family.

Where the display is more than a metre or two from the host, the interface decision is effectively made by the cable rather than by the panel.

EMI behaviour of parallel vs serial links

Differential wins, but only with correct layout.

Parallel interfaces switch many conductors simultaneously at the pixel clock, which radiates and is also sensitive to incoming noise. Differential serial links carry the same information on fewer, balanced pairs and reject common-mode noise, provided the pairs are routed with consistent impedance and matched lengths and the reference plane is continuous. The failure modes of a poorly routed differential link – split pairs, length mismatch, plane splits – convert common-mode noise into differential noise and defeat the advantage.

In machines with drives and contactors, this is usually the deciding factor in favour of a differential interface, even where bandwidth would allow a parallel one.

RGB parallel: still the default for small industrial panels

Simple, fast to bring up, short-lived in the wrong application.

For panels up to a few inches and hosts with an LCD controller peripheral, parallel RGB is often the fastest route to a working display. The pin count is manageable in a small product, and debugging is straightforward because every signal is visible. The limits appear when the panel grows, the cable lengthens or the resolution climbs: at that point the same design has to move to a serial interface, which may mean a different host or a bridge chip.

CDTech’s notes on when to use the RGB parallel interface describe the boundary conditions.

SPI and MCU: cheap, slow and often enough

For status displays, low pin count beats bandwidth.

A small display showing numbers and simple graphics at a low refresh rate does not need a wide bus. SPI with a display controller, or an MCU-style parallel bus, keeps the pin count low and the firmware simple, and it is often the cheapest total solution. The limitation is refresh: animations and video are impractical, and partial updates require careful handling to avoid tearing. The comparison between SPI and I2C for these small displays is covered in CDTech’s SPI versus I2C notes.

Where the content is a moving graphic or a live value updating many times a second, the interface choice has to move up a class.

LVDS, MIPI DSI and eDP at higher resolutions

Three serial families, three application profiles.

LVDS is the industrial workhorse: well understood, available on many industrial hosts and FPGAs, and tolerant of a few metres of cable with proper routing. MIPI DSI is common where the host is a mobile or embedded processor and pin count and power matter, with shorter practical cable limits. eDP suits larger, higher-resolution panels and hosts with a display output designed for it, and it carries an auxiliary channel for control and EDID. The underlying specifications are published by MIPI for DSI and by VESA for eDP.

Matching the interface to the host’s existing output avoids adding a bridge, which is often the largest single cost in an interface change.

HDMI for kiosks and monitors, not embedded modules

It solves a different problem.

HDMI is designed for a cable between two consumer devices, with hot-plug detection, EDID and audio. That makes it ideal for a kiosk, a signage panel or a monitor where the display is a standalone unit connected to a source. For an embedded module inside a machine, HDMI adds connector cost, EDID management and the need for the host to behave like a source, which is rarely what the product needs. HDMI’s own specification and compliance programme are documented at HDMI.org.

Where the product genuinely has a standard external video input – a monitor or an industrial display used with a PC – HDMI is the right answer and the extra features become useful.

Selection table by resolution, cable length and volume

Use the table as a first filter, then check the host.

Interface selection guide

Situation	Candidate interfaces	Notes
Small panel, status content, low volume	SPI, MCU, RGB	Lowest pin count and firmware effort
Small to mid panel, short cable, FPGA or MCU host	RGB	Fast bring-up; limited length
Mid resolution, cable up to a few metres	LVDS, MIPI DSI	LVDS for industrial hosts; MIPI where the host is mobile-derived
High resolution or large panel	eDP, LVDS dual, HDMI	Check the host output before committing
Long cable, machine or vehicle	SerDes link	Carries video and control over one pair
External video source, kiosk or monitor	HDMI, DisplayPort	Standard connector and EDID handling

Interface design guidance for the electrical side is published for components by vendors such as TI, Analog Devices and STMicroelectronics, and the mechanical consequences of the choice – connector position, FPC and bonding – are covered by CDTech’s TFT module interface selection notes.