The Video Critter Hi-Res is based on a single ARM7TDMI microcontroller, the Philips LPC2138. It is a powerful 32 bit processor with a good amount of flash (program) memory and RAM. It is a fast controller, capable of up to 60 MIPS. It has many peripherals, including analog to digital and digital to analog converters. The LPC2138 users manual is a great source of information on the microcontroller.
In addition to the passive componets to support the LPC2138, the Video Critter Hi-Res also contains a AD725 RGB to NTSC encoder IC from Analog Devices. 8 pins on PORT1 of the LPC2138 are connected to 3 resistor based DACs to produce a red, green, and blue voltage level for the AD725. Three bits are devoted to green, three to red, and two to blue, giving 256 possible colors.
The AD725 also requires a sync signal which is provided by one of the LPC2138's PWM channels. All of these signals are controlled in software, so for more information on producing video with the Video Critter Hi-Res, check out the tutorials.
In order to produce color video signals with as few components as possible, we have clocked the microcontroller at a multiple of the color-burst frequency. For NTSC the color-burst frequency is 3.579545 MHz, and we use a 14.31818 MHz crystal (4x color-burst). Sockets are provided so different crystals may be plugged in for PAL operation.
The Video Critter has a 3.3 volt and 5 volt regulator to power the board. The microcontroller runs at 3.3 volts while the AD725 requires 5. There is a header (labled 6 on the picture) with four pins each for ground and Vcc (3.3 volts). Power input must be in the range of 5 - 9 volts. A 9 volt wall wart is a good choice. Anything higher, and the voltage regulators will start to get quite hot.
RCA jack for video output. The video signal is scaled for a 75 Ohm load (composite video input standard).
Line level audio output on RCA jack.
7.5 - 9 volt input jack. Standard 9 volt wall wart works well.
Power connection to Vin.
The Video Critter comes with a 14.31818 MHz crystal for NTSC operation. This may be replaced with a PAL compatible frequency.
Power connections. Useful for connecting external circuitry (buttons, pots, LEDs).
Pressing this button resets the processor.
A 16 pin header for interfacing switches, LEDs , or other sensors. The pinout:
1: P0.2
2: P0.3
3: P0.4
4: P0.5
5: P0.6
6: P0.7
7: P0.8
8: P0.9
9: P0.10
10: P0.11
11: P0.12
12: P0.13
13: P0.15
14: P0.16
15: P0.18
16: P0.19
Many of these pins connect to the many peripherals of the LPC2138, such as analog to digital converters. Check the LPC2138 datasheet for more info.
This is where the USB programmer attaches.
Use this header to select for NTSC or PAL operation of the AD725 video encoder. When the center pin is jumped to the pin nearest the RCA jacks, NTSC is selected (as in the picture). When the center pin is jumped to the pin nearest the Programming Header, PAL is selected. Note that a different crystal frequency is also required for PAL and the video routines may have to be updated (currently they only support NTSC).
This LED may be controlled in software. It connects to pin P0.31 on the microcontroller.
On when the board is powered up.