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3.6.4.2 USB Rate Control

The Audio core must consume data from USB and provide data to USB at the correct

rate for the selected sample frequency. The USB 2.0 Speciﬁcation states that the

maximum variation on USB packets can be +/- 1 sample per USB frame. USB frames

are sent at 8kHz, so on average for 48kHz each packet contains six samples per

channel. The device uses Asynchronous mode, so the audio clock may drift and run

faster or slower than the host. Hence, if the audio clock is slightly fast, the device

may occasionally input/output seven samples rather than six. Alternatively, it may

be slightly slow and input/output ﬁve samples rather than six. Figure 9 shows the

allowed number of samples per packet for each example audio frequency.

See USB Device Class Deﬁnition for Audio Data Formats v2.0 section 2.3.1.1 for

full details.

Frequency (kHz) Min Packet Max Packet

44.1 5 6

48 5 7

88.2 10 11

96 11 13

176.4 20 21

192 23 25

Figure 9:

Allowed

samples per

packet

To implement this control, the decoupler core uses the feedback value calculated

in the buﬀering core. This value is used to work out the size of the next packet it

will insert into the audio FIFO.

3.7 Device Firmware Upgrade (DFU)

The DFU interface handles updates to the boot image of the device. The interface

links USB to the XMOS ﬂash user library (see XM-000953-PC). In Application mode

the DFU can accept commands to reset the device into DFU mode. There are two

ways to do this:

The host can send a

DETACH

request and then reset the device. If the device is

reset by the host within a speciﬁed timeout, it will start in DFU mode (this is

initially set to one second and is conﬁgurable from the host).

The host can send a custom user request

XMOS_DFU_RESETDEVICE

to the DFU

interface that resets the device immediately into DFU mode.

Once the device is in DFU mode. The DFU interface can accept commands deﬁned

by the DFU 1.1 class speciﬁcation

. In addition the interface accepts the custom

command

XMOS_DFU_REVERTFACTORY

which reverts the active boot image to the

factory image. Note that the XMOS speciﬁc command request identiﬁers are

deﬁned in dfu_types.h within module_dfu.

http://www.usb.org/developers/devclass_docs/DFU_1.1.pdf*USB

REV 6.1

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