On Tue, Jul 17, 2018 at 7:02 AM, Pičugins Arsenijs <[email protected]> wrote: > > Some thoughts: > > > 3. 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 _______________________________________________ arm-netbook mailing list [email protected] http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook Send large attachments to [email protected]
