3 ms·

...and I now see your response[1] to an equivalent question. [1] https://news.ycombinator.com/item?id=24359220 https://news.ycombinator.com/item?id=24359220 L

...and I now see your response[1] to an equivalent question. [1] https://news.ycombinator.com/item?id=24359220 https://news.ycombinator.com/item?id=24359220 Looking at the normative language of the early DP v1.1a § 3.3 Hot Plug/Unplug Detect Circuitry: > The HPD signal is asserted by the DisplayPort Sink whenever the Sink is connected to either its main power supply or “trickle” power. HPD signal specification is shown in Table 3-2. To be sure, HPD assertion on "trickle" power is the monitor's requirement. Looking closer at Table 3-2 (empty Nom col removed, Comments col separated for clarity): | Parameter | Min | Max | Units | Comments | | ---------------------------------- | ---- | --- | ----- | -------- | | HPD Voltage | 2.25 | 3.6 | Volt | (a) | | Hot Plug Detection Threshold | 2.0 | | Volt | (b) | | Hot Unplug Detection Threshold | 0.8 | | Volt | (b) | | HPD source termination | 100 | | kΩ | (c) | | HPD sink termination | 100 | | kΩ | (d) | | IRQ HPD Pulse Width Driven by Sink | 0.5 | 1.0 | ms | (e) | | IRQ HPD Pulse Detection Threshold | 2.0 | | ms | (f) | > Comments: > (a) HPD signal to be driven by the Sink Device > (b) HPD signal to be detected by the Source Device > (c) Source Device must pull down its HPD input with a ≥ 100kΩ resistor. > (d) When a Sink Device is off, it must pull down its HPD output with ≥ 100kΩ resistor. > (e) Sink generates a low going pulse within this range for IRQ (interrupt request) to the Source > (f) When the pulse width is narrower than this threshold, the Source must read the link / sink status field of the DPCD first and take corrective action. When the pulse width is wider than this threshold, it is likely to be actual cable unplug / re-plug event. Upon detecting HPD high, the Source must read the link / sink status field, and if the link is unstable, read the link / sink capability field of the DPCD before initiating Link Training. Then just below Table 3-2: > The voltage level of the HPD pin is monitored by the Source Device. TTL levels must be used for the detection. > The Sink Device may detect the presence of the Source Device by monitoring the DC voltage level of the AUX CH lines. Source detection is an optional feature of a Sink Device. Based on this language and description of noted workarounds, color me unconvinced that DP spec ambiguity was the root cause of this issue.

formerly_proven 6y ago
Where is "trickle power" supposed to come from? DisplayPort also mandates that DP_PWR not be connected by cables (only intended for adapters / dongles, see § 4.2.1.1 and Figure 4-23). Note that EC demands that electronics put in stand-by / "off-mode" may not consume more than 0.5 W of wall power. So I think what might be going on behind the curtains is that 1) DP requiring handling of hot plug 2) DP forbidding "trickle power" being sourced from the DP port 3) monitors perhaps not being able to meet 0.5 W standby if they'd entertain the DP interface 4) Operating systems handling display hotplug disgracefully combine into one heck of a bad user experience. Things actually start to align now, because this issue started to happen en-masse around 2013-2014. And that's precisely when the more stringent standby power requirements came into force. For example, the Dell U2713H doesn't do this, while the later U2715H does. Both of these are DP 1.2 monitors, so any new mandates in the non-publicly available DP 1.3/1.4 specs shouldn't matter.
- metaphor 6y ago
  > Where is "trickle power" supposed to come from? DisplayPort also mandates that DP_PWR not be connected by cables (only intended for adapters / dongles, see § 4.2.1.1 and Figure 4-23). The answer seems to me unambiguous when considering the normative language in § 4.2.1.1: > The standard external cable connector assembly must not have a wire on pin 20, DP_PWR. ...in conjunction with the normative definition of trickle power per § 1.4 (my emphasis): > Power for Sink Device that is sufficient to let the Source Device read EDID via the AUX CH, but insufficient to enable Main Link and other sink functions. For sink to drive the HPD signal high, at least the trickle power must be present. The amount of power needed for the trickle power is sink implementation specific. (To be sure, Figure 4-23 per DP v1.1a is single-slot PCI "Panel Cut Out Reference Dimensions", so unsure what was meant to be conveyed there). > Note that EC demands that electronics put in stand-by / "off-mode" may not consume more than 0.5 W of wall power. ...which is ~150 mA at 3.3 V nominal. Even generously derating 30% to account for ballpark efficiency losses, we're still talking > 100 mA budget to support an explicitly constrained requirement. I mean asserting HPD in the worst case is apparently a 36 uA affair. Anecdotally, I haven't encountered the issue despite a bias towards DP (on advise from a friend in compliance test over a random beer discussion on radiated emissions performance); all my setups discriminate to either dual U2415 or dual U2412M as a matter of personal preference. Just thought I'd remark in passing because VESA specs never really struck me as being technically egregious in its ambiguity.

formerly_proven 6y ago

Where is "trickle power" supposed to come from? DisplayPort also mandates that DP_PWR not be connected by cables (only intended for adapters / dongles, see § 4.2.1.1 and Figure 4-23). Note that EC demands that electronics put in stand-by / "off-mode" may not consume more than 0.5 W of wall power. So I think what might be going on behind the curtains is that 1) DP requiring handling of hot plug 2) DP forbidding "trickle power" being sourced from the DP port 3) monitors perhaps not being able to meet 0.5 W standby if they'd entertain the DP interface 4) Operating systems handling display hotplug disgracefully combine into one heck of a bad user experience. Things actually start to align now, because this issue started to happen en-masse around 2013-2014. And that's precisely when the more stringent standby power requirements came into force. For example, the Dell U2713H doesn't do this, while the later U2715H does. Both of these are DP 1.2 monitors, so any new mandates in the non-publicly available DP 1.3/1.4 specs shouldn't matter.

metaphor 6y ago
> Where is "trickle power" supposed to come from? DisplayPort also mandates that DP_PWR not be connected by cables (only intended for adapters / dongles, see § 4.2.1.1 and Figure 4-23). The answer seems to me unambiguous when considering the normative language in § 4.2.1.1: > The standard external cable connector assembly must not have a wire on pin 20, DP_PWR. ...in conjunction with the normative definition of trickle power per § 1.4 (my emphasis): > Power for Sink Device that is sufficient to let the Source Device read EDID via the AUX CH, but insufficient to enable Main Link and other sink functions. For sink to drive the HPD signal high, at least the trickle power must be present. The amount of power needed for the trickle power is sink implementation specific. (To be sure, Figure 4-23 per DP v1.1a is single-slot PCI "Panel Cut Out Reference Dimensions", so unsure what was meant to be conveyed there). > Note that EC demands that electronics put in stand-by / "off-mode" may not consume more than 0.5 W of wall power. ...which is ~150 mA at 3.3 V nominal. Even generously derating 30% to account for ballpark efficiency losses, we're still talking > 100 mA budget to support an explicitly constrained requirement. I mean asserting HPD in the worst case is apparently a 36 uA affair. Anecdotally, I haven't encountered the issue despite a bias towards DP (on advise from a friend in compliance test over a random beer discussion on radiated emissions performance); all my setups discriminate to either dual U2415 or dual U2412M as a matter of personal preference. Just thought I'd remark in passing because VESA specs never really struck me as being technically egregious in its ambiguity.