Wednesday, April 2, 2014

Doing USB right

AN11392 - Guidelines for full-speed USB on NXP's LPC microcontrollers (19 Feb 2014). This finally answers all my questions about what all the USB parts are. Based on it, let's review the Loginator USB/charge/power supply section.

We are now using a Micro USB connector with through-hole posts for better mechanical security and easier alignment during soldering. Micro USB takes up less board space and is compatible with the cords used for Android phone charging.

The first thing the app note says is that there must be a 33 ohm resistor on each of D+ and D-. This, plus the internal resistance of the LPC itself, add up to the 90 ohm total impedance required. It implies that there is 12 ohms on each of the pins inside the LPC. This is what I have been carrying all along from the Logomatic design.

Secondly, USB_Softconnect is required if the device is self-powered, or if it takes longer than a certain amount of time to boot up. Since my device can be self-powered and might never boot up and connect, I intend to use this as intended. However, I still like a PMOS rather than PNP transistor. The 1.5k resistor required for softconnect to work is also well-sized for the LED, so I use an array.

The signal lines have capacitors to ground, for exactly the purpose I deduced - shorting high-frequency signals to ground. The app note says that they are not strictly necessary but that it has been reported to improve certain noise issues. I have always built circuits with these included, so I shall continue. These are one of the rare cases where it makes sense to use a capacitor array.

Next, we have R011 and R015, which I completely whiffed on. In my defense, I am not the only one. My design came from the Logomatic, which came from the LPC2148 Reference Design which appears to be a mistranscription of the Keil MCB2140 schematic. Even then, the Keil board does not seem to be what was intended.

The idea is that P0.23, USB_ON, is 5V tolerant if and only if the rest of the circuit is powered. So, if it is not possible for the MCU to be off while the USB voltage is present, then you can just plug VBUS into P0.23. However, if the MCU is not powered, the voltage on that pin is supposed to be limited to less than 3.6V. The app note recommends a voltage divider, with 24k on the high side and 39k on the low side, resulting in about 3.09V on the input.

The way both the microbuilder circuit and Logomatic circuits are arranged, that isn't a voltage divider at all, and the pin eats a full 5V. Since the MCU can be turned off (the power switch disables the 3.3V VCC line), this is technically out of spec.

R011 should be connected to the right pin of R015B, and should be closer to 20k. This divider will draw three times as much current as the recommended value, but that is still less than 1mA.

There is supposed to be between 1 and 10 uF between VBUS and ground, visible through regulators and other parts. The real spec is that the inrush current should be limited, but I don't intend to ever submit my device to USB compliance testing, so as long as it works for me, it's fine. There is 4.7uF on the input to the voltage regulator, so I do not inculde any intentional capacitance in this section.

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