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Wearwulf

The Display Interface

Wearwulf will use the Kopin 320M CyberDisplay as a head mounted display, placing graphical information directly in the user's field of view. (The displays are available from Kopin themselves, and have been known to be sold on Ebay.) The CyberDisplays have a proprietory driving mechanism which make them non-trivial to connect to a computer. The drive circuitry can be broken down into three separate components:

• Power supply
• Timing logic
• Video generation

To reduce costs, and simplify the design, a 1-bit colour depth is used, even though the Kopin 320M CyberDisplay is capable of displaying 8-bits (256-levels) of grey.

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Power Supply

To minimize power consumption, the display should be switched off when not in use. To satisfy the absolute maximum ratings of the display, the 9V & 2V power supplies should be switched off, and all inputs into the display should be less than 0.5V, i.e. logic low. This is accomplished using regulators with a shutdown input, and having an enable control for the programmable logic's outputs.

To prevent the display being damaged it will be vital to shutdown the display in the correct order:

1. Bring the PDR^* signal low for at least 25ms.
   After the power down request signal is brought low, the Kopin 320M CyberDisplay must assert RENE^* or RENO^* twice. If this is not done, the lifetime of the display may be reduced.
2. Disable all outputs, bringing them low.
   All signals to the CyberDisplay must be brought to less than 0.5V, including the outputs from the processor and the programmable logic.
3. Switch off both power supplies.
   The power supplies can only be switched off after all other inputs to the CyberDisplay are less than 0.5V (i.e. logic low).

(The last two steps can be done simultaneously.)

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Timing Logic

It is remarkably simple to generate the majority of the logic for driving the Cyberdisplay due to the Philips LPC2106's extensive pulse-width-modulation and matching support:

• Matching Support:
  The LPC2106 has two timers, one with three match registers, the other with four match registers. When the value in the timer equals the value in the match register the state of the assoicated pin can be changed. The processor will optionally reset the timer, and-or generate an interrupt.
• Pulse Width Modulation Support:
  The LPC2106 has an additional counter for pulse width modulation support. PWM differs in that two registers are used for matching. For example, a signal could be set at the 10th count, reset at the 25th count, and the counter could be restarted at the 100th count.

Counters have been largely eliminated from the programmable logic, having replaced them with signals from the LPC2106. The signals HPL^*, FRAME, FRAMEM3, FRAMEP6, ODD^*, MARK3 and MARK6 are generated within the LPC2106. The signals HODL, VPL^*, VCK^*, RENE^* and RENO^* are generated from logical combinations of the other signals. HCK^* is derived from the system clock.

Timing Logic to Drive Kopin CyberDisplay
Note: Signal names ending with an asterix are active low.

For more detailed expanations of why the signals change state at given times, consult the »Kopin 320M CyberDisplay datasheet, and the »Vidi VHDL source code.

The framebuffer describing the video data will be stored within the LPC2106 processor. The CoolRunner II logic contains a 4-deep, 8-bit wide, buffer that empties into an 8-bit shift register once every 1.33µs. While the last byte of data is being written out (once every 5.33µs), the logic causes an interrupt to be generated, and the LPC2106 has 1.33µs to refill the buffer.

The LPC2106 code to handle the video display must be carefully considered. At 60MHz, the LPC2106 has 80 instructions to refill the buffer, and is interrupted every 320 instructions. The ISR will be located in RAM, ensuring the fastest operation, and will have a worst case latency of 12 instruction cycles before execution.

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Video Generation

The CyberDisplay's VIDH and VIDL video input signals vary between 5.5V-9.0V and 2.0V-5.5V respectively at 6MHz. To achive this, the »Linear Technology LT1719 split-voltage comparator was used.

Video Generation using LTLT1719 Split-Supply Comparator
Note: +5.5V supply is generated from the 2.0V & 9.0V supplies.

The LT1719 comparator has the useful property that its inputs and outputs are powered by different supplies, and do not have to share a common ground. While there are restrictions on the relative voltages, the voltages used by the Kopin CyberDisplay are within its specifications.

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