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LOOK MA, NO PC!

A $55 Webcam
by Steve Freyder, David Helland, & Bruce LIghtner

Start • Hardware Design • Picking a Camera • Cheap CMOS Cameras • PicoWeb Server Hardware • Firmware Functions • Software Design • PicoWeb Firmware • Java Applet • Smile • Source and PDF

PicoWeb Server Hardware

The PicoWeb server uses the Atmel AT90S8515 microprocessor because the architecture is quite sophisticated for a processor of this size and cost. All of the registers are directly available (not mapped, as in the 8051) and the memory address space is linear (not segmented into pages, as in the PIC).

The AT90S8515 is a low-power RISC processor with 8 KB of flash program memory, 512 bytes of EEPROM, 512 bytes of RAM, 32 I/O lines, and a built-in UART. With an execution rate of one instruction per clock and a clock rate of 8 MHz, the AT90S8515 can drive the PicoWebıs 10BaseT Ethernet controllerıs I/O bus at 1 MBps. The PicoWeb server includes a 16-KB serial EEPROM chip to hold things like GIF and JPEG images as well as things like HTML, text files, and Java byte-codes. You can see a photo of the commercial version of the PicoWeb server in our "$25 Web Server" article. The schematic for this version of the PicoWeb server can be found in Figure 3.

The PicoWebıs Ethernet controller is a Realtek RTL8019AS, a single chip NE2000-compatible device with 16 KB of on-chip packet buffer RAM. This chip only needs a transformer, a single resistor and a few capacitors to implement a complete 10BaseT Ethernet network connection. The PicoWebıs DB-25 connector has up to 16 free general purpose digital I/O lines, an RS-232 serial port, and an in-circuit flash-memory programming port. An onboard voltage regulator accepts either AC or DC power in the range of 7 to 25 V . Typical current consumption is under 30 mA from the 5-V DC supply.

An NE2000 Ethernet chip is optimal because the Atmel processor memory is limited. The NE2000 controller has 16 KB of onboard SRAM that functions as a ring buffer to allow unattended reception of back-to-back Ethernet packets. (Because the commercial version of the PicoWeb server operates the Realtek chip in 8-bit mode, the available buffer RAM is reduced to 8 KB.) The same onboard Ethernet controller SRAM can be used to assemble transmitted Ethernet packets. The result is that the Atmel microcontrollerıs meager 512 bytes of on-chip SRAM is not needed to send or receive the maximum-sized 1500-byte Ethernet packets.

Connecting the camera to the PicoWeb server is simple, as Figure 4 illustrates. The data connection to the camera is a mini-stereo jack. The cable that comes with the camera (not used in our application) has this jack on one end with three wires (TX, RX, GND) that connect to a PC-compatible DE-9S serial connector on the other end.

Figure 4ıA simple 5-wire cable connects the PicoWeb server to the Mattel digital camera. Adding a power connector to the cameraıs body means the camera can be powered from the same 9-VDC supply as the PicoWeb server.

A 9-V battery normally powers the camera, supplying a 5-VDC regulator chip inside the camera. We disassembled our camera and drilled a hole to add a power plug so we could power it off of the same unregulated 9-VDC supply as the PicoWeb. The PicoWebıs unregulated DC is available on a pin in its DB-25 connector. The camera only draws 70 mA from this connection. (We tried powering the cameraıs logic board directly from the PicoWebıs regulated 5-VDC power supply, but the camera kept warning us about its (missing) "low" 9-V battery!)

The images stored in the camera are located in its RAM, so removing the battery or disconnecting the DC cable from the PicoWeb will result in the loss of any stored images. To keep the camera from turning itself off to save its (now missing) 9-V battery, we programmed the PicoWeb to probe the camera over the serial port, about once per second. Modifying the standard PicoWeb clock frequency (from 8 MHz to 7.372 MHz) derived the 57.6 kbps rate needed by the camera.

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