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Understanding USB Error -6911 - (kUSBNotResponding)


Q: I'm getting an occasional USB error -6911 (kUSBNotResponding). According to the documentation, this means that the device is hung. What does this mean in terms of what happened on the bus, and how do I handle this error in my class driver?

A: The kUSBNotRespondingErr results because the host controller on the Macintosh tried three times to transfer data and did not receive an ACK or NAK packet in return. The host internally tries three times before returning this error, which the USB returns to the caller as documented in the USB Specification (revision 1.0, section 4.5.2). From a packet-level perspective, the host controller issued 3 IN or OUT PIDs (Packet ID), and did not receive a response from the USB function.

Usually this happens because the device is hung. Sometimes, transient errors can cause this problem on a request which takes a long time. (Lots of NAKs give lots of opportunity for things to go wrong.)

If this is happening, you can recover from the problem. You first need to clear the stall on the pipe using the USBClearPipeStallByReference function. Second, you need to resynchronize the host and the device (which is usually a Clear_Feature or Endpoint_Stall).

An example of sending the Clear_Feature request is shown below. Note that this code assumes the use of the USB v1.1 USB.h header file.

    pb.pbLength = sizeof(pb);
    pb.usbReference = deviceRef;
    pb.usbCompletion = yourAsnchHandlerRoutine;
    pb.usbStatus = noErr;
    pb.pbVersion = kUSBCurrentPBVersion;
    pb.usb.cntl.BMRequestType = USBMakeBMRequestType(kUSBOut,
         kUSBStandard, kUSBEndpoint);
    pb.usb.cntl.BRequest = kUSBRqClearFeature;
    pb.usb.cntl.WValue = 0;                   // feature selector - Endpoint stall
    pb.usb.cntl.WIndex = endpointNumber;
    pb.usbReqCount = 0;
    pb.usbBuffer = nil;
    pb.usbFlags = 0;
    err = USBDeviceRequest(pb);
 
 Example 1. Clear_Feature request example

When making the Clear_Feature request using USBDeviceRequest, the usbWIndex field must be set to the endpointNumber. The endpointNumber is typically obtained during device configuration. Alternatively, you could obtain the endpointNumber using the USBFindNextAssociatedDescriptor call. Example values are 0x01 for an output endpoint, or 0x81 for an input endpoint.

For USB v1.1 and greater, a minor change was implemented to simplify the requirement that the usbWIndex field be set the to the endpointNumber. Instead of setting the usbWIndex field, set the usbFlags field with the kUSBAddressRequest bit set. The USB Manager v1.1 and greater will check the usbFlags field to see if the kUSBAddressRequest bit is set, and check if the "usb.cntl.BMRequestType" field is for an endpoint, then fill in the endpointNumber into the usbWIndex field corresponding to the endpoint reference passed in the usbReference field. The following code sample demonstrates this modification for USB v1.1 and later. Note that the code in Example 1 also works with USB v1.1 and later. Use the gestalt selector 'usbv' to determine the version of Mac OS USB present.

    pb.pbLength = sizeof(pb);
    pb.usbReference = endpointRef;
    pb.usbCompletion = yourAsnchHandlerRoutine;
    pb.usbStatus = noErr;
    pb.pbVersion = kUSBCurrentPBVersion;
    pb.usb.cntl.BMRequestType =
         USBMakeBMRequestType(kUSBOut, kUSBStandard, kUSBEndpoint);
    pb.usb.cntl.BRequest = kUSBRqClearFeature;
    pb.usb.cntl.WValue = 0;         // feature selector - Endpoint stall
    pb.usbReqCount = 0;
    pb.usbBuffer = nil;
    pb.usbFlags = kUSBAddressRequest;
    err = USBDeviceRequest(pb);
 
 Example 2. Alternate Clear_Feature request for USB 1.1 and later

Note that the kUSBNotRespondingErr can also occur if the USB cable is unplugged while the host is communicating with the device. When this error occurs, make the Clear_Feature request. If the device has been unplugged, the USBDeviceRequest will complete with another -6911 error.

[Nov 16 1998]


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