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ChipVid project



General description

The Iosoft ChipVid project is a demonstration of high-speed low-cost networking; video images are captured using a 'frame grabber' added to the Microchip PICDEM.net board, as shown above. The raw image data is transmitted in UDP format over Ethernet at approx. 2 images/sec, the net data throughput being approx. 2 Mbit/s.

The frame grabber is compatible with both European (625-line) and US (525-line) composite video signals, and produces good-quality monochrome images from low-cost monochrome or colour cameras.

The UDP network data is received and displayed by a Windows utility written in Visual C v6. The above picture is an unretouched screen-grab from that Windows utility, and shows the frame grabber capturing a picture of itself.

The frame grabber hardware was designed by Iosoft Ltd. The PCB artwork was produced by Microchip Technology Inc., and has been released for general non-profit use.

Hardware construction

Important note. It is feasible for an experienced hobbyist or student constructor to undertake this project, though it does demand a significant level of electronics knowledge, skill and equipment. Although the design has been kept as simple as possible, it does include a mix of high-speed analog and digital circuitry, and both through-hole and surface-mount components. A faulty ChipVid board could damage the PICDEM.net board as well, so take great care when constructing and testing the board. Always use a current-limited supply, and remove power immediately if there is any sign of overcurrent.

Iosoft offers this design 'as-is', with no warranties of any kind, and regrettably can not provide assistance in the event of problems.

Click here for a PDF of the circuit diagram.
Click here for a PDF of the PCB.
Click here for a zipfile with the PCB artwork (Gerber files) and parts list (CSV file).

ChipVid assembly

Assemble all the components on the PCB; U4, U5 and Y1 may be socketed. Omit C10 if using 625-composite (PAL) video. JP1 enables 75-ohm termination of the video input, and should normally be made. U2 and U6 may be HC or HCT devices. All the PCB interconnect sockets (to the PICDEM.net board) are mounted on the underside of the PCB.

ChipVid testing

Before attaching the ChipVid board to the PICDEM.net, apply a 9 volt DC supply to it and check that the idle current is about 90 mA. Connect a video source to the board (ideally a video pattern generator with a grayscale or colour bar output) and check with an oscilloscope
  • The input video signal is approx 1V p-p (if larger, check termination resistor R5 and JP1)
  • U5 is detecting the horizontal & vertical syncs correctly (pins 1 and 3)
  • U3 pin 19 (AOUT test point) has a buffered version of the video signal with DC offset. The amplitude should remain stable if the video signal is attenuated (AGC operation)
  • U3 pin 1 (D7 test point) switches cleanly on when video level is above half-way (m.s. bit of ADC)
  • With a colour video signal, adjust C4 to minimise the amplitude of the colour subcarrier, as measured on AOUT test point.

PICDEM.net modifications

Four 0.1"" pitch sockets are mounted on the underside of the ChipVid frame grabber board, so 0.1" pins (Berg pins or equivalent) have to be soldered to the PICDEM.net PCB at the corresponding locations.
  • J1 (2 pins) is only used for mechanical support
  • J3 (3 pins) must be connected to 9-volt power and 2 x ground - keep these connections as short as possible.
  • J4 (27 pins) The I/O connections to the PICmicro - no additional connections
  • OSC1 (1 pin, to the left of U2, linked to its pin 4) It is necessary to replace the existing PICmicro clock circuitry (crystal and capacitors) with the clock signal from the ChipVid board (U2 pin 4). This is achieved by cutting the two top-side tracks between the PICmicro pins 13 & 14 and the crystal, then wiring the OSC1 pin to pin 13
Once the clock modification is in place, the PICDEM.net board will not work if the ChipVid module is removed. This can be inconvenient if the PICDEM.net is to be used for other purposes, in which case a socket for an additional oscillator module can be mounted in the prototyping area of the PICDEM.net board, with a link to switch between clock sources.
With the ChipVid board fitted to the PICDEM.net, the total current consumption should be approx. 150 mA.

If the same supply is used to power the video source and the PICDEM.net assembly, then there is the possibility that the return current for the PICDEM.net will flow through the video cable screen, rather than the negative supply connection. This is due to the bridge rectifier on the PICDEM.net, which effectively introduces a single diode drop between the supply -ve terminal and the PICDEM.net ground. To cure this problem, the PICDEM.net bridge rectifier should be replaced with a single series diode in the +ve supply connection.

Modifying the design

If you are considering any changes to the circuit diagram or PCB layout, please bear in mind the following points:
  • PCB layout. The PCB incorporates a large ground-plane area, and this is very important to maintain video quality. Any noise on the analogue circuitry will generally show up as 'wallpapering' (a background pattern on the video image) which can only be eliminated by careful layout and a good ground plane.
  • Power supply. The ChipVid board has its own voltage regulator, which helps to eliminate noise carry-over from the microcontroller supply.
  • Clock. A single clock source must used to drive both the ADC and microcontroller. The buffering of the clock line to the microcontroller is important; I have experienced ADC stability problems when using a simpler buffering scheme.
  • Devices. Both 74HC04 and 74HCT04 devices have been used with success. Two devices have been used for convenience of layout; they could probably be combined into one.
  • Filters. There are two LC filters in the design. The circuitry around L3 is supposed to eliminate high-frequency video components from the ADC input; it has been included as a precautionary measure, but probably isn't needed when using a low-cost solid-state camera as the video source. The filter around L1 removes the PAL/NTSC colour subcarrier, and may be omitted if using a monochrome video source.

Software

The PICmicro software is described in chapter 14 of TCP/IP Lean 2nd edition, which also includes the source code on CD-ROM (see file P16CAP.C)

There is a Win32 network client which receives and displays the images, and a Linux network client which creates an uncompressed GIF file. Click here for a zipfile containing the source code. The client software may only be used in non-commercial applications, and the Iosoft copyright notices must be retained. Contact Iosoft for commercial licensing information.


© Iosoft Ltd. 2002 All rights reserved.