Programmer Design for AVR USB

The project uses a USB hub with 4 ports that can be enabled and disabled from PC software in order to create pulses on each hub output for USB interface.

A device reset state is defined by the USB where both data lines to ground are shorted by a hub for 10 milliseconds. Both data lines are kept at high impedance where no data is routed to the disabled port since USB hubs also implement a disable feature. To detect device arrival at a disabled port, the devices contain a pull-up resistor. Each port can be used as a single bit input and/or a momentary active-low open collector pulsable output by taking control of a hub and keeping its ports disabled.

Cutting and preparing the USB cables is probably the most tedious but simplest step because the wires inside are really delicate although the plastic jacket was really tough. It is preferred to cut the red and black lines at slightly different lengths in order to avoid short to each other. Decoupling capacitors should be placed close to the power pins of 74HC00s since power can be obtained through several inches of USB.

Rest of the project