User manual

Display Expansion Board - User’s Guide

Page 13

SPI interface, miso signal (output from the

board)

Currently not used.

SPI interface, ssel signal when

communicating with resistive touch screen

controller (TSC2046).

Interrupt output signal from resistive touch

screen controller (TSC2046).

Currently not used.

4.4 Pixel Clocks

There are a number of oscillators on the Display Expansion Board that can supply the pixel clock to the

display controller on the LPC MCU. Only one at a time is active, via output enable control, which in turn

is controlled via U20, which in turn is controlled via the I2C-GPIO U7.

Three fixed oscillators are mounted with 25.175MHz, 27.000MHz and 36.000MHz frequency. These

oscillators are spread spectrum oscillators with +2% center frequency variations. The reason for using

spread spectrum oscillators is to comply with EMI emission levels. Although Embedded Artists classify

the Display Expansion Board as a component, it is typically connected to an OEM Base Board.

Together they form a product that typically shall comply with EMI emission levels.

The spread spectrum oscillators works well for the digital transmission interfaces (HDMI/DVI/LVDS)

but not for analog transmission (VGA). An oscillator with fixed frequency must be used for the VGA

interface to work properly. For example a 25.175MHz or 36MHz oscillator mounted in U4 position.

Note that having a fixed frequency oscillator will likely compromise EMI emission levels. It is the user’s

responsibility to make sure such a setup stays in a laboratory environment.

The board is also prepared for mounting an I2C programmable clock generator, DS1086LU+ from

Maxim. The chip has an input for enabling/disabling spread spectrum. The DS1086LU+ is not mounted

but can easily be so and the output from the chip can be enabled by having U20 pulling output Y3 high.

4.5 Things to Note

4.5.1 VGA output does not work without fixed frequency oscillator

An oscillator with fixed frequency must be used for the VGA interface to work properly. For example a

25.175MHz or 36MHz oscillator mounted in U4 position. The spread spectrum oscillators mounted on

the Display Expansion Board makes the VGA output picture blurry since the pixel frequency is not

fixed.

Note that having a fixed frequency oscillator will likely compromise EMI emission levels. It is the user’s

responsibility to make sure such a setup stays in a laboratory environment.

4.5.2 36MHz Pixel Clock Limitations

Note that when using a 36MHz pixel clock, the performance of the MCU systems are pushed to the

limit.

 When using the LPC3250 Developer’s Kit together with the Display Expansion Board and

when running SVGA resolution (800x600) at 36MHz, there will be no picture on the

monitor/tv. This is true when running the sample application from internal SRAM. When

running from external SDRAM, the system will hang.