I'm writing code to learn about the USB peripheral on a Freescale Kinetis microcontroller. I've managed to get through enumeration on a Linux host, and I can send & receive packets using vendor-custom codes on EP0, interacting with a libusb test program.
It looks like I can configure additional control endpoints (non-zero endpoint numbers) on the microcontroller, but I don't see a way to make libusb send / receive control transfers to those endpoints. (libusb_control_transfer doesn't require an endpoint number, though libusb_bulk_transfer and libusb_interrupt_transfer do.)
Are non-zero control endpoints so uncommon or unnecessary that it's not worth bothering with them? Is there some way to get libusb to execute control transactions to non-zero endpoints?
Is there some way to get libusb to execute control transactions to non-zero endpoints?
You can try to modify the endpoint field in the libusb_transfer structure of the asynchronous I/O API.
But it would surprise me if your microcontroller could actually support non-zero control endpoint(s) - not that many do.
In practise you would rather use either interrupt or bulk endpoints. Both have less overhead - allowing higher throughput with bulk transfers (see for example USB 2.0 SPEC Table 5-2 vs. Table 5-9).
Related
I'm developing a USB device driver for a microcontroller (Atmel/Microchip SAMD21, but I think the question is a general one). I need multiple endpoints for control & data, and the USB hardware uses per-endpoint descriptors to (among other things) locate buffers for input and output data.
Since IN data is polled at the host's discretion it makes sense that each endpoint has its own IN buffer, so that any endpoint's data (if it has any to send) is immediately available when polled.
But as far as incoming data from SETUP & OUT transactions is concerned, it occurs to me that I can save memory by configuring all endpoints to use a shared buffer. It seems wasteful for each endpoint to have its own buffer when, given the nature of USB transactions, only one such transaction can occur at a time.
Obviously this approach requires that transaction interrupts are handled sufficiently quickly that the shared buffer is freed and prepared for a new transaction in time for whatever the next transaction might be - but this is already a requirement for the control endpoint, where some SETUP transactions are immediately followed by an OUT.
So, assuming the timing is feasible, is there any other reason why such an approach wouldn't work?
Probably not.
Normally, the USB module on a microcontroller handles OUT packets by keeping track of which packet buffers it has written data to, and it waits for your firmware to say it is done processing the buffer before accepting more data from the computer and overwriting the buffer. If an endpoint has no buffers available to receive more data, but the computer sends an OUT packet to the endpoint, the USB module typically responds to the computer with a NAK packet, which tells the computer it should retry later. In this situation, your firmware can take pretty much as long as it wants to handle the OUT packets.
By having multiple endpoints configured to use the same buffer, you mess up this system. When you receive an OUT packet on any of your endpoints, the USB module would (probably) not know that multiple endpoints use the same buffer, so it would not issue NAK packets on your other OUT endpoints. If it receives another OUT packet right away, it would write it to the same buffer, overwriting the previous packet. Therefore, whenever you receive a packet, your code would have to rush as fast as it can to do something like copying the data out of that buffer, disabling other OUT endpoints, or reassigning buffers.
Even if you can actually get this to work, it means that your scheme to save a little bit of memory turns the servicing of USB events into a real-time task (i.e. a task that requires responses from your code in a few microseconds). If you want to add another real-time task to your system later, it will be very difficult, because you always have to be ready to be interrupted by your USB-handling code.
The SAMD21 has tons of memory (32 KB) so you probably don't need to worry about optimizing this part of it.
I agree with David's Response. You didn't mention the speed of the device you are creating. A low-speed would need just a few 8-byte buffers. A full-speed, a few 64-byte buffers. High-speed, maybe eight 64-byte buffers, depending on your use. A super-speed device, your still only talking a few 512-byte buffers.
I would create a ring buffer for each endpoint. This way you are not moving data around. You are simply using a pointer that points to an entry within a memory ring. A full-speed device with a control endpoint, an interrupt endpoint, and two bulk endpoints, each endpoint having sixteen 64-byte entries per ring, is still only a total of 4k RAM, 1/8th of the total RAM.
However, I am not familiar with the SAMD21, so please check the specification to be sure this will work.
I'm developing an embedded app, written in C, using a M16C/28 uC from Renesas.
The app manages two simple task:
RFID for detection and reading MIFARE tags. ( Using HW: Mf500 from NXP ). The uC handles whole FW implementation.
To deal with a RS485 frame protocol as slave. ( This app, have to be able to process RS485 frames every 10ms ).
The RFID implementation contains blocking code and the time response to detect a RFID tag is about 15ms. This causes RX reception buffer overflows on the RS485 processing.
My questions are as follows:
Is it normal to deal with such time responses in the RFID world?
Should I use a RTOS to preempt RFID task to meet RS485 frames requirements?
Should I use an external uC acting as host controller to release the load of the RFID manager uC?
Thanks in advance
To answer your questions:
Depends
You could use a RTOS.
You could use an additional uC.
Better options would be to:
Use DMA on serial communications.
Make the RFID code non blocking.
Do more in your serial interrupt.
The response time varies depending on the type of card/rfid that your are communicating with. I don't know the timings of Mifare RFIDs but 15 ms does not seem to be bad.
In your situation, you may have more requests coming from RS485 than you can handle on the RFID part. You can use queues or FIFOs to store the input requests so that you can treat them later on, according to the physical limitations of your system.
Using an RTOS can help but usually, they are not free. Plus, you may have to port it to your platform if it is not already supported. If all your firmware does is handling RS485 requests and communicating with the RFID, you should sort this out with interruptions to store the incoming commands and a loop to process them separately.
And for the second uC, it's like the RTOS. It can help but might not be the right solution in this scenario (you will have to manage 2 firmwares, a communication protocol or a FIFO between uCs, it will cost twice the price, ...).
I learned on OSDev wiki that Endpoint 0 is the default control pipe, allowing for bi-directional control transfers. This is used for device configuration, e.g. to retrieve device descriptors. The USB 2.0 spec explains this more thorougly in section 5.5 Control Transfers.
There are also a limited amount of endpoints available (2 for low-speed, 15 for full- and high-speed devices). Somewhere in the USB 2.0 spec, I have read that there must be at least one control pipe. This implies that there may be multiple control endpoints, but what is the use of it? Do you know any particular USB device or class that has an EP configured as control pipe?
Later, I found this in the spec, section 10.1.2 Control Mechanisms:
A particular USB device may allow the use of additional message pipes
to transfer device-specific control information. These pipes use the
same communications protocol as the default pipe, but the information
transferred is specific to the USB device and is not standardized by
the USB Specification.
If I understand it correctly, this means that non-EP0 cannot be used to configure the device (say, a standard request such as GET_DESCRIPTOR). But the setup/data/status stages seem still to be available ("[..] use the same communications protocol [..]"). Is this correct? Or is the use of standard/class requests forbidden for non-EP0?
Background: while working on an emulated USB device in QEMU, the need for a USB monitor for debugging purposes appeared. During inspection of the QEMU core USB code, I noticed that it only processed control commands for EP0. Other endpoints would be treated as data. There are some virtual devices (host-libusb) that always reject control transfers for those other endpoints. Hence the question whether this is the correct behavior or not (and if valid, whether there exist devices that really implement this).
As far as I can tell, there is no use for a non-EP0 control endpoint. I have developed several products that use custom control transfers on endpoint 0 as the main way to send device-specific requests and I have not encountered any fundamental problems with doing that.
If you did make a non-EP0 control endpoint I think your understanding is correct; you wouldn't be able to use it for standard requests but you would be able to use it for custom requests and the transaction sequences would be the same as on EP0.
I have an embedded system that controls a motor using pwm and some other things, I send commands through a serial connection, which is connected to a Fastrack Wavecom Supreme GSM Module. However, the module connected to the embedded system (the client), fails to send the message to the server module.
I have been able to send messages back and forth between the two wavecom modules, however, when I try and send from my PIC18F45k22 to the wavecom module, it fails.
Any ideas of what could be going wrong?
You did not specify what type of serial communication you are using. For instance, if you are using the PIC's SPI module you may be sampling on the wrong edge of the clock. There are at least 2 common SPI modes widely used and 4 all together. If you are using the PIC's UART there are "a whole bucket full" of setting that may be off. Speed, number of bits, in band signaling, out of band signaling, parity, ect.
I have embedded firmware which have terminal over serial transmission. I am doing command from terminal which waits data (text file) which it should save to flash chip. However, writing flash is much slower than terminal transmission.
Text file may be pretty big (many kB), so in small embedded environment I cannot simply dump it to RAM. I though if it possible to communicate with standard terminal emulator (which have drag/dop support for files) to pause transmission every time when write buffer is full and tell continue again after write is done? I haven't find anything which may help me throught this.
Well, offcourse I can make PC frontend which understands this trick, but in basic level it should be nice if all function can be used through normal terminal if needed.
For a basic serial connection you could see if the hardware supports flow control. This would be the CTS, RTS lines (clear to send, request to send).
http://en.wikipedia.org/wiki/RS-232_RTS/CTS#RTS.2FCTS_handshaking
However many simple embedded systems do not implement this type of flow control.
If the hardware does not support flow control, then you will have to look at using some form of software flow control. You maybe able to implement the Xon/Xoff flow control ( http://en.wikipedia.org/wiki/XON/XOFF ) or could implement a simple file transfer protocol, like XMODEM, or ZMODEM, or even tftp. This depends on what your terminal can support.
I always use XMODEM when programming data into FLASH via a serial link from a PC. When using XMODEM it only sends one data packet at a time and waits for you to acknowledge the packet before sending the next one.
This means we control the flow via software on the receiving side:
Get packet ->
Write packet ->
Ack packet ->
Repeat util done...
XMODEM can be implemented on the smallest of devices (less than 1K RAM) and the code is very simple. All serial terminals support XMODEM (upto windows XP ship with an XMODEM capable terminal). XMODEM requires no special hardware.
Here is the spec.
Here is an example implementation.