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
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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
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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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