I am using a SAM4E-EK board, and the processor is SAM4E. The board is equiped with a ADS7843 touch controller, contected from the processor through a SPI channel.
The chapter of SPI in datasheet of SAM4E said that
While the data in the Shift Register is shifted on the MOSI line, the MISO line is sampled and shifted in the Shift Register. Receiving data cannot occur without transmitting data.
But in an example for ADS7843 from ASF, it just sends data(8 bits) three times at first, and then it can receives data(8 bits) three times! I have test it, and it work fine.
So I think there is a hardware FIFO buffer in SPI receiver. But I can not find any related information in the datasheet and internet.
Am I right? or is there others mechanism making the example runs correctly?
The SAM4E manual says that SPI Receive Data Register has the size of 2 bytes
The ADS7843 manual says:
One complete conversion can be accomplished with three serial
communications, for a total of 24 clock cycles on the DCLK input
The figure 5 shows that byte0 is a request to ADS7843 and bytes1-2 are response.
You should send 1 byte of command and 2 dump bytes to provide SPI clocking while the ADS7843 is responsing (the manual says
Receiving data cannot occur without transmitting data
And when you reading 3 bytes, you get the 2 bytes of answer, stored in the SPI Receiver register
Related
So, what I have learned so far is that CPU programs the source address, dest address, word count and the direction to the DMA controller whenever it needs to transfer the data from say a harddrive. But in this example, the hard drive is just a dumb device, so it makes sense because harddrive can never initiate a data transfer.
But, what if we have connected the serial port where in certain instances we are going to get 8 bits of data. I know the DMA controller is used for large memory transfer, but say I want to do DMA for these 8 bits. But the device driver on the CPU cannot tell when the data is coming and it also can not tell how much data is coming because the serial port may send 8 bits or 16 bits or no data at all. So in this case who fills the DMA controller's count and memory addresses since the device driver is completely unknown when the data is going to come in.
Using DMA serial input is complicated when the incoming data is not a continuous stream or fixed length packets. The exact details will depend on the specific UART and DMA controller, but generally, each character that arrives will be copied to the next location in the provided DMA buffer, and an interrupt will be generated by the DMA controller when the buffer is both half-filled and completely filled.
A single byte DMA buffer serves little purpose over using the UART's data avalable interrupt, and will simply delay byte processing by one character period.
If your DMA buffer were two characters long, you'd then get an interrupt for every character (one for the half transfer, and one for the full transfer), which solves the problem of partially filled buffers not being serviced, but does not reduce the interrupt overhead at all so offers little advantage over direct UART interrupt handling. If your UART includes a FIFO buffer, that would be a better method of dealing with asynchronous serial input when only a small amount of buffering is required.
When a larger DMA buffer is used the interrupt rate is reduced, but when a buffer is incomplete you will not get an interrupt, and the data may wait indefinitely. One solution to that problem is to implement a timeout mechanism whereby if the DMA interrupt does not arrive within a time period determined by the baud rate and buffer length, then the timeout handler retrieves all data currently buffered. Such a mechanism requires care to avoid race conditions between the timeout and the DMA interrupt, and to ensure that data arriving while the timeout is being processed is not lost, or that data retrieved by the timeout is not repeated when the DMA interrupt eventually arrives.
Digging around with/for HID reports, I ran into a strange problem within a USB HID device. I'm implementing an HID class device and have based my program on the HID USB program supplied by Keil. Some codes have been changed in this project and it seems working fine with 32 bytes input and 32 bytes output reports. Somehow, after thousands times data transferring, the Endpoint 1 out would hang and become a bad pipe. Then I searched the google for some tips, a topic remind me that we should write a data length zero packet after sending a length of packet match what you defined in the report description. But it's not working for me. Then I write a data length zero to the control pipe after I receive a out packet and magically, it works! It would never hang after million times transferring!
Here is my question: Why does it works after writing a data length zero to a control pipe. The data transferring in the out pipe should have no relationship with the data in the control pipe. It confuses me!
If you transfer any data that is less than the expected payload size, you must send a Zero Length Packet to indicate that data has transferred.
But it depends heavily on the implementation on the host controller, and not all devices follow the specification to the point and may stall.
Source:
When do USB Hosts require a zero-length IN packet at the end of a Control Read Transfer?
I have to send file byte-by-byte to serially connected AT89s52 from computer (VB.NET).
Every sended byte have some job to do in microcontroller what require some time.
Here is relevant part of my C code to receiving bytes:
SCON = 0x50;
TMOD = 0x20; // timer 1, mode 2, 8-bit reload
TH1 = 0xFD; // reload value for 9600 baud
TR1 = 1;
TI = 1;
again:
while(RI!=0)
{
P1=SBUF; // show data on led's
RI=0;
receivedBytes++;
}
if (key1==0)
{
goto exitreceive; // break receiving
}
show_lcd_received_bytes(receivedBytes);
// here is one more loop
// with different duration for every byte
goto again;
And here is VB.NET code for sending bytes:
For a As Integer = 1 To 10
For t As Integer = 0 To 255
SerialPort1.Write(Chr(t))
Next t
Next a
Problem is that mC have some job to do after every received byte and VB.NET don't know for that and send bytes too fast so in mC finishes just a part of all bytes (about 10%).
I can incorporate "Sleep(20)" in VB loop ant then thing will work but I have many of wasted time because every byte need different time to process and that would be unacceptable slow communication.
Now, my question is if 8051 can set some busy status on UART which VB can read before sending to decide to send byte or not.
Or how otherwise to setup such communication as described?
I also try to receive bytes with serial interrupt on mC side with same results.
Hardware is surely OK because I can send data to computer well (as expected).
Your problem is architectural. Don't try to do processing on the received data in the interrupt that handles byte Rx. Have your byte Rx interrupt only copy the received byte to a separate Rx data buffer, and have a background task that does the actual processing of the incoming data without blocking the Rx interrupt handler. If you can't keep up due to overall throughput issue, then RTS/CTS flow control is the appropriate mechanism. For example, when your Rx buffer gets 90% full, deassert the flow control signal to pause the transmit side.
As #TJD mentions hardware flow control can be used to stop the PC from sending characters while the microcomputer is processing received bytes. In the past I have implemented hardware flow by using an available port line as an output. The output needs to be connected to an TTL to RS-232 driver(if you are currently using a RS-232 you may have and extra driver available). If you are using a USB virtual serial port or RS-422/485 you will need to implement software flow control. Typically a control-S is sent to tell the PC to stop sending and a control-Q to continue. In order to take full advantage of flow control you most likely will need to also implement a fully interrupt driven FIFO to receive/send characters.
If you would like additional information concerning hardware flow control, check out http://electronics.stackexchange.com.
Blast from the past, I remember using break out boxes to serial line tracers debugging this kind of stuff.
With serial communication, if you have all the pins/wires utililzed then there is flow control via the RTS (Ready To Send) and DTR (Data Terminal Ready) that are used to signal when it is OK to send more data. Do you have control over that in the device you are coding via C? IN VB.NET, there are events used to receive these signals, or they can be queried using properties on the SerialPort object.
A lot of these answers are suggesting hardware flow control, but you also have the option of enhancing your transmission to be more robust by using software flow control. Currently, your communication is strong, but if you start running a higher baud rate or a longer distance or even just have a noisy connection, characters could be received that are incorrect, or characters could be dropped.
You could add a simple two-byte ACK sequence upon completion of whatever action is set to happen. It could look something like this:
Host sends command byte: <0x00>
Device echoes command byte: <0x00>
Device executes whatever action is needed
Device sends ACK/NAK byte (based on result):
This would allow you to see on the host side if communication is breaking down. The echoed character may mismatch what was sent which would alert you to an issue. Additionally, if a character is not received by the host within some timeout, the host can try retransmitting. Finally, the ACK/NAK gives you the option of returning a status, but most importantly it will let the host know that you've completed the operation and that it can send another command.
This can be extended to include a checksum to give the device a way to verify that the command received was valid (A simple logical inverse sent alongside the command byte would be sufficient).
The advantage to this solution is that it does not require extra lines or UART support on either end for hardware flow control.
I am writing code for a USB device. Suppose the USB host starts a control read transfer to read some data from the device, and the amount of data requested (wLength in the Setup Packet) is a multiple of the Endpoint 0 max packet size. Then after the host has received all the data (in the form of several IN transactions with maximum-sized data packets), will it initiate another IN transaction to see if there is more data even though there can't be more?
Here's an example sequence of events that I am wondering about:
USB enumeration process: max packet size on endpoint 0 is reported to be 64.
SETUP-DATA-ACK transaction starts a control read transfer, wLength = 128.
IN-DATA-ACK transaction delivers first 64 bytes of data to host.
IN-DATA-ACK transaction delivers last 64 bytes of data to host.
IN-DATA-ACK with zero-length DATA packet? Does this transaction ever happen?
OUT-DATA-ACK transaction completes Status Phase of the transfer; transfer is over.
I tested this on my computer (Windows Vista, if it matters) and the answer was no: the host was smart enough to know that no more data can be received from the device, even though all the packets sent by the device were full (maximum size allowed on Endpoint 0). I'm wondering if there are any hosts that are not smart enough, and will try to perform another IN transaction and expect to receive a zero-length data packet.
I think I read the relevant parts of the USB 2.0 and USB 3.0 specifications from usb.org but I did not find this issue addressed. I would appreciate it if someone can point me to the right section in either of those documents.
I know that a zero-length packet can be necessary if the device chooses to send less data than the host requested in wLength.
I know that I could make my code flexible enough to handle either case, but I'm hoping I don't have to.
Thanks to anyone who can answer this question!
Read carefully USB specification:
The Data stage of a control transfer from an endpoint to the host is complete when the endpoint does one of
the following:
Has transferred exactly the amount of data specified during the Setup stage
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
So, in your case, when wLength == transfer size, answer is NO, you don't need ZLP.
In case wLength > transfer size, and (transfer size % ep0 size) == 0 answer is YES, you need ZLP.
In general, USB uses a less-than-max-length packet to demarcate an end-of-transfer. So in the case of a transfer which is an integer multiple of max-packet-length, a ZLP is used for demarcation.
You see this in bulk pipes a lot. For example, if you have a 4096 byte transfer, that will be broken down into an integer number of max-length packets plus one zero-length-packet. If the SW driver has a big enough receive buffer set up, higher-level SW receives the entire transfer at once, when the ZLP occurs.
Control transfers are a special case because they have the wLength field, so ZLP isn't strictly necessary.
But I'd strongly suggest SW be flexible to both, as you may see variations with different USB host silicon or low-level HCD drivers.
I would like to expand on MBR's answer. The USB specification 2.0, in section 5.5.3, says:
The Data stage of a control transfer from an endpoint to the host is
complete when the endpoint does one of the following:
Has transferred exactly the amount of data specified during the Setup stage
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When a Data stage is complete, the Host Controller advances to the
Status stage instead of continuing on with another data transaction.
If the Host Controller does not advance to the Status stage when the
Data stage is complete, the endpoint halts the pipe as was outlined in
Section 5.3.2. If a larger-than-expected data payload is received from
the endpoint, the IRP for the control transfer will be
aborted/retired.
I added emphasis to one of the sentences in that quote because it seems to specifically say what the device should do: it should "halt" the pipe if the host tries to continue the data phase after it was done, and it is done if all the requested data has been transmitted (i.e. the number of bytes transferred is greater than or equal to wLength). I think halting refers to sending a STALL packet.
In other words, the device does not need a zero-length packet in this situation and in fact the USB specification says it should not provide one.
You don't have to. (*)
The whole point of wLength is to tell the host the maximum number of bytes it should attempt to read (but it might read less !)
(*) I have seen devices that crash when IN/OUT requests were made at incorrect time during control transfers (when debugging our host solution). So any host doing what you are worried about, would of killed those devices and is hopefully not in the market.
I'm implementing USB on a PIC 18F2550 using a generic HID interface. I've set up the HID profile configuation to have a single 64 byte message for both inputs and outputs.
Now it's basically working. The device registers OK with windows. I can find it in my program on the PC and can send and receive data to it. The problem is this though - messages from the PC to the PIC are truncated to the size of the EP0 endpoint buffer.
Before I go debugging too much further I want to try to clarify my understanding of the USB protocols here and check I got it right.
Assume that the EP0 input buffer is 8 bytes. It's my understanding that the PC end will send a control packet which is 8 bytes. In there is the length in bytes of the data to follow. And then it will send a sequence of 8 byte data packets and the PIC end has to acknowledge each one.
It's my understanding that the PC end knows how big each packet may be by looking in the maximum packet size field in the device descriptor and will divide up the message accordingly into multiple data packets.
Before I go looking for more hours at the code, can anyone confirm that this is basically correct? That if the EP0 buffer size is 8 bytes then the PC should know this because of the configuration field I mentioned above and send multiple data packets?
If I make my receive buffer on the PIC 64 bytes then I get 64 bytes of the message which is sufficient for my needs, but I don't like not understanding why it doesn't work with small buffers, and one day I'll probably need them anyway.
Any advice or information would be welcome.
There is something called Endpoint Descriptor, which, among other things, defines the wMaxPacketSize - which is what the Host Controller Interface drivers use to subdivide a large USB transfer into smaller packets.
This is entirely different from the EP0 buffer size - which however, is always required to be larger than the wMaxPacketSize. My guess is (try posting your usb_config.h and usb_descriptors.c, if you use Microchip USB stack), that you're either trying to use 8-byte long EP0 with 64-byte long wMaxPacketSize, which is truncating the transfer.
Also, be aware that in USB 1.1 Low Speed, the wMaxPacketSize cannot exceed 8, and in USB 1.1 Full Speed it cannot exceed 64.
0x07,/*sizeof(USB_EP_DSC)*/
USB_DESCRIPTOR_ENDPOINT, //Endpoint Descriptor
HID_EP | _EP_IN, //EndpointAddress
_INTERRUPT, //Attributes
DESC_CONFIG_WORD(9), //size
0x01, //Interval
/* Endpoint Descriptor */
0x07,/*sizeof(USB_EP_DSC)*/
USB_DESCRIPTOR_ENDPOINT, //Endpoint Descriptor
HID_EP | _EP_OUT, //EndpointAddress
_INTERRUPT, //Attributes
DESC_CONFIG_WORD(9), //size
0x01 //Interval