Start • Simple Video Signal • Video with the LPC2138 • Video Algorithm • Tricks Used • Vertical Sync & Resolution • Video for other Apps • Oscilloscope • Implementation • Construction & Results • Generate Video • Sources and PDF

VIDEO WITH THE LPC2138

In order to obtain high-resolution video, I had to generate a TV signal via the hardware as much as possible. The LPC2138 microcontroller has a PWM block and SPI communication module. The former can be used for generating sync pulses. The latter can be used to generate the video stream! Want to know a secret? I already tried this with Cypress Semiconductor PSoC chips. Unfortunately, the capabilities of the PSoC SPI module did not allow me to obtain a horizontal resolution of more than 256 pixels.

The useful feature of the LPC2138’s (also called the synchronous serial port (SSP)) SPI1 block is the fact that it can operate in a continuous SSI mode. It has an eight-frame FIFO buffer and can send 16-bit frames. This means that it can process a 128-bit block without participation from the software. Of course, to send more bits, the software must reload the FIFO buffer with additional data.

An external DAC is required to mix both the video and sync pulses. It can be built using a resistor matrix like the one shown in Figure 2. With a 3.3-V supply and a 75-W load (a TV), this matrix produces the sync and white levels of approximately 0.3 and 1 V.  The output impedance of this simple DAC is approximately 75 W for compatibility with 75-W coaxial cable. In reality, the level of black is a bit shifted up after the last and before the first video bit inside the TV line because the SSP block has a high output impedance in Idle mode.