On Tue, Jul 17, 2018 at 7:02 AM, Pičugins Arsenijs crimier@yandex.ru wrote:
Some thoughts:
- Add another SY6280 current limiter for +5V and connect to VGA pin
9 (VESA power). Seems to be some difference of opinion on current requirements: 50mA[2], 300mA-1000mA[1]. If we were to limit at 300mA, it should easily supply the needs of I2C serial EEPROM and probably not over-tax our power supply.
I've also seen the VGA/HDMI +5V used to power various converters (most often, HDMI->VGA, but I imagine the inverse is sometimes used as well).
I agree. I designed a converter that was powered by VGA DDC pin 9 power or, in the absence of that, the VESA DDC SDA and SCL pull-up, or the VGA horizontal and vertical synchronization signals. I don't remember exactly how much power it used but it was relatively efficient as I used low-power CMOS PAL (Programmable Array Logic, precursor to FPGA's). Basically its largest-current operating mode was driving to TTL loads across the video cable.
For VESA_SDA and VESA_SCL, add diode limiters connected to ground similar to ESD117-ESD123 on the SD bus lines provided the diode-limiting voltage is greater than VREFTTL nominal range. Otherwise use BAT54S connected between ground and VREFTTL.
Is it not possible that VESA_SDA and VESA_SCL are 5V? If so, they could require level shifting from 5V, am I wrong here?
It turns out VESA DDC uses I2C signalling.[1] I2C signalling is bi-directional open drain or open collector so the high state is due to a pull-up resistor. The I2C bus voltage can be +5 V or +3.3 V, although other voltages are permitted.[2] All the devices on the bus have to tolerate the bus voltage.
In my experience, the VESA DDC reference design used a pull-up supply of +5 V. The EOMA68 card doesn't have to drive it that high, simply go hi-Z and the pull-up resistor in the microdesktop will pull it that high. But the EOMA68 card will have to tolerate +5 V on those two lines (VESA_SDA, VESA_SCL) or we jump into the tricky area of logic-level translation for bi-directional open-drain signals.
Good observations. Thanks Arsenijs!
References: [1] https://en.wikipedia.org/wiki/Display_Data_Channel [2] https://en.wikipedia.org/wiki/I%C2%B2C