gbulmer
Moderator
I'm reading the maple-rev3 schematic, trying to understand it.
What is the circuitry using Q2 & Q3, with R7-R10, and R12 doing?
I can see that the base of Q3 is (presumably) driven by PC12 (pin 53), and is labelled DISC. The emitter of Q2 is connected to USBDP.
But what does it do? (is there some documentation explaining this that I've missed?)
That circuitry is used for signalling in the USB protocol. For example, it's used to select the bandwidth of the device. From Wikipedia:
"A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". For USB 1.x, the choice of data line indicates a device's bandwidth support; full-bandwidth devices pull D+ high, while low-bandwidth devices pull D− high."
anton
Member
gbulmer, D+ pin can be pulled up to 3.3V
http://www.beyondlogic.org/usbnutshell/usb2.shtml
gbulmer
Moderator
pagercam -
Why not just pull up to USB +5V then???
The host determines what speed the USB device wants to run at by looking at which of the two USB data lines, D+ or D-, is pulled up with the 1.5K resistor. This lets the device say it is a low-speed, 1.2Mbits, or full-speed 12Mbits (High-speed is different).
Every time a USB device is plugged in, the host also needs to figure out exactly what type of USB device it is. The process the host goes through to determine that is called 'enumeration'. Once the host knows what speed the USB device is running at, the host and device have a chat so the host knows which device driver to use to communicate with the device.
Currently Maple changes its USB device from a USB DFU (which is used by the bootloader when a program is being uploaded to it from the host) to a USB serial the rest of the time.
Maple needs to force the USB host to changes its mind about what type of device it is by forcing the host to query its type of USB device when it needs to transition between those two USB devices. It does that by disconnecting the pull-up resistor, so it looks like the Maple has been unplugged, then raising it again to trigger the host going through USB 'enumeration'.
1. The resistor can't just be tied to a voltage, or Maple couldn't force the host to 'enumerate' it.
Even if LeafLabs made the Maple be a single USB device, IMHO it is better for Maple to be able to force USB re-enumeration because then folks can more easily make the Maple look like different USB devices.
By allowing Maple firmware/software to decide when to activate the pull-up resistor, it can go through a pile of internal stuff, getting ready, which again makes it easier to program the Maple to take on different USB device persona's.
The USB 2.0 specification is at http://www.usb.org/developers/docs/
The bundle of documents is currently http://www.usb.org/developers/docs/usb_20_021411.zip but it changes name with revisions.
usb_20.pdf defines the electrical characteristics of USB in Table 7-7 in section 7.3.2 "Bus Timing/Electrical Characteristics"
The maximum (signalling) voltage that can be applied to the USB D+/D- lines is called Voh (V subscript OH).
For low-speed and full-speed USB Voh maximum is 3.6V, with a minimum of 2.8V (this is the voltage used for signalling data)
The 1.5K +/-5% pull-up resistor is used to signal the speed of the USB device to the host USB controller for low-speed or full-speed. The resistor is attached to USB D+ or D- depending on whether it wants to be treated as low-speed or full-speed.
The pull-up resistor is 'terminated' by connecting it to a voltage called Vterm (V subscript TERM), and Vterm's value is minimum 3V maximum 3.6V. (I think there is an engineering change notice in that set of documents which recommends that devices should be able to cope with a voltage of up to 4.1V, but I can't see it now)
2. The pull-up resistor should not be pulled up to 5V.
So, if you really wanted to connect the pull-up to 5V USB, you could get a value in the 3V to 3.6V range using a couple of diodes in series which will drop about 0.7V each.
(full disclosure: I am not a member of LeafLabs staff)
gbulmer
Moderator
I am trying out a much simpler USB Rup (pull-up resistor) circuit.
It uses a single P-Channel MOSFET with a pull-up resistor to hold it off by default.
The MOSFET source is tied to 3.3V, and the drain to the USB pull-up resistor.
Two components vs six. A significant saving in board area and assembly cost.
(I have other changes too, which require a new PCB)
I have seen the Maple USB Rup circuitry on boards from ST, ETT, and maybe Olimex.
(full disclosure: I am not a member of LeafLabs staff.)
siy
Member
I've successfully tried even more simple circuit, which replaces both, USB input network and disconnect circuit with only two components: STF202 USB upstream protection IC and PNP-driver with built-in resistors. Emitter of the transistor is connected to VCC, base to DISC pin and collector to pin 1 of the IC (VBUS). Resulting equivalent circuit is almost exact copy of the original (except it uses single PNP transistor instead of two NPN). I've also tried P-channel MOSFET as switch and resulting circuit also works just fine, but it requires two additional resistors (probably only one is actually necessary - between gate and VCC).
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