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Forum Index : Microcontroller and PC projects : The mystery of the 5K1 USB pull down resistors
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Mixtel90 Guru  Joined: 05/10/2019 Location: United KingdomPosts: 8088 |
All is (apparently) solved. They tell the load how much current is available from the power supply. This can change dynamically and, should it fall, the load is supposed to react within 60ms to reduce the current that it is drawing. If you can't reduce the load you are supposed to power off. The load monitors the voltage across the resistors on CC1 and CC2. One of them will change and the other won't, this is used to detect the polarity of the connector. It means that you have to use independent resistors (unlike Raspberry Pi who screwed it up) and monitor both signals. Below 0.2V means that no supply is present. Between 0.2V and 0.66V you can draw up to 500mA (a basic supply) Between 0.66V and 1.23V you can draw up to 1.5A Above 1.23V you can draw up to 3A This is why systems work without the resistors. CC1 and CC2 simply aren't being monitored, the assumption being that there will be adequate current available at all times so they don't need to be. You can actually draw up to 1.5A without the resistors but the voltage might be out of regulation and you may not get 5V at the load end of the lead. You should *NOT* draw more than 500mA (5x 100mA unit loads) if a USB-A to USB-C lead is being used due to the restriction of USB-A. It looks like 500mA is the limit for battery charging on USB1 and USB2 (900mA on USB3). You are supposed to stay well below this for normal powering operation. A bus powered hub takes 5 unit loads, one for itself and one for each output. This is why you can't cascade them. The USB3 spec states that a device must be capable of operating at 4V. This doesn't give a lot of headroom for a Pico clone that uses a linear regulator (virtually all of them) as the minimum input voltage to meet the 3V3 spec is typically around 3.46V even without any series diodes. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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