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Display and Backlight

This module brings up a display panel and its backlight. The panel is connected while the board is unpowered, and the bring-up app drives it from a known test pattern.

Before the panel lights​

  • The backlight is off at power-up. The dimming signal reads below its logic-low threshold and the panel stays dark until firmware turns it on. A backlight that flashes on at power-up shows the dimming input is floating or wrongly pulled.
  • The panel's power sequence passes. Its rail order, the delay between its logic and analog supplies, and its reset timing are checked in Power sequence and supply range before any picture is expected.
  • The panel answers its identity read. A MIPI DSI panel returns its driver IC's identity bytes to a short read before video starts. A correct identity proves the control path, the lane wiring of the low-power link and the panel's supplies.

Picture​

The app shows a test pattern, and the step passes when the pattern appears with no corruption on the expected number of lanes. A panel that answers its identity read and takes its initialisation table but shows a black screen with the backlight on is receiving a video stream it cannot display. The fault is then in the video settings, not the hardware. Each setting is found by changing one variable at a time, working through the lane count, the pixel format, the pixel clock and the vertical and horizontal timing. Each result is recorded, including the ones that failed.

A pixel clock request is checked against the clock the driver actually produces, because a clock divider can only reach certain frequencies, and a driver that compensates for the difference may cut the line short.

Backlight current​

The backlight current is measured at full brightness against the window the design gives, either with a meter in series or across a sense resistor. A reading taken indirectly carries its uncertainty, and a reading at the edge of the window is recorded as "at the limit", not as a clean pass.

Dimming​

The backlight is swept across its full dimming range while it is filmed on a phone at 24, 25, 30 and 60 frames per second, and listened to in a quiet room. The step passes when the film shows no flicker or banding at any frame rate, and when no whine is audible. Pulsed backlight current can make a panel's flex and ceramic capacitors sing at the dimming frequency. A dimming frequency above the audible range at normal brightness, with a lower frequency kept only for the lowest levels, removes the whine without a visible step at the changeover.

Supply behaviour​

The backlight is often the board's largest load, so its behaviour on a weak supply is part of this module. A backlight boost converter that cycles on and off at a supply's current limit shows as a flashing screen. The supply limit is set well above the board's full-load current before this module runs, and the board's behaviour on a sagging input is tested separately in Power sequence and supply range.