[Arm-netbook] Microdesktop v1.7

[Richard Wilbur]

Addendum

Why?
1.  Make VESA DDC lines conform to VESA DDC spec. (VDD=5.0V, I2C
signalling)[1]
2.  Respect VREFTTL in the range of 3.3-5.0V on EOMA side of VESA DDC lines.
3.  Respect EOMA requirement that all lines except EOMA I2C be tri-stated
at power on.

How?
1.  Connect pull-up resistors for VESA_SDA(R8) and VESA_SCL(R12) to
VESA_PWR (+5.0V) instead of VREFTTL.  Maximum current requirement on
VESA_PWR is ~670uA.
2.  Implement circuit in attached
diagram[EOMA_I2C_circuit_sketch_small.png] on each of SDA and SCL.  I have
attached a spreadsheet[EOMA_I2C_VESA_calc.pdf] showing how this
accommodates the I2C signalling for VDD=3.3V=VREFTTL and VDD=5.0V.[2]  It
also accommodates the EOMA68 VREFTTL input levels by not allowing the EOMA
side of the signal to exceed one Schottky diode drop(0.3V) above VDD=3.3V.
3.  Enable the current limiting regulator (SY6280) for VESA_PWR (+5.0V on
VGA pin 9) with latch of LCDDE when it first goes active (line coming from
EOMA68 connector).

References:

[1] https://en.wikipedia.org/wiki/Display_Data_Channel
[2] http://www.nxp.com/documents/user_manual/UM10204.pdf