I am a bit confused about USB Softconnect.
Is the circuitry to turn off the USB+ against 3.3 V resistor mandatory ? Id like to simply tie a fixed resistor instead. Is that possible? Are there any disadvantages to expect?
What impedance is correct, I found values between 800 and 1500 Ohm, so is 1200 the best choice?
many thanx in advance (Tsuneo ? ;), as usual Uli
"Is the circuitry to turn off the USB+ against 3.3 V resistor mandatory ? Id like to simply tie a fixed resistor instead."
For final product, the pull-up resistor to USB D+ (or D-) line can be a fixed one. But while development, the resistor should be switched by a MCU port - soft attach/detach.
a) Power-up delay Host PC (or hub) detects USB plug in by the D+ (or D-) line voltage change. This pull-up resistor on the device side makes the voltage change. With 100 ms (min) delay after plug in detection, host starts enumeration sequence. Therefore within this period, your MCU should finish its power-up initialization. If the initialization takes more than 100 ms, soft attach should be used even for the final product.
b) Disconnection by device Soft attach/detach is used to disconnect/re-connect USB by the device firmware, too. This function is often used for firmware update over USB.
If your final product use neither power-up delay nor firmware update, fixed pull-up will do.
While development, soft attach/detach is indispensable. Otherwise, you have to plug off USB cable every time you reset the firmware. "What impedance is correct, I found values between 800 and 1500 Ohm, so is 1200 the best choice?"
1k5 +/- 5%
New resistor_ecn.pdf has widened the tolerance. But to specify resistor value for production, this old spec is better.
Tsuneo
Ah, power-up delay is applied just for bus-powered device - supplied power from USB VBUS. Self-powered device doesn't care of this delay.
Thanks Tsuneo, you've been such a help, as usual ;)
So I will fix the 1k5.
be well Uli