NRF52820-QDAA-R datasheet(382/437 Pages) NORDIC

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Peripherals

It is possible (and in some situations it is required) to respond to an IN token with a zero-length data

packet.

Note: On many USB hosts, not responding (DATA+ACK or NAK) to three IN tokens on an interrupt

endpoint would have the host disable that endpoint as a consequence. Re-enumerating the

device (unplug-replug) may be required to restore functionality. Make sure that the relevant data

endpoints are enabled for normal operation as soon as the device gets configured through a

SetConfig request.

6.26.10.2 Bulk and interrupt OUT transaction

When the host wants to transmit bulk/interrupt data, it issues an OUT token (packet) followed by a DATA

packet on a given endpoint n (n=1..7).

A NAK is returned until the software writes any value to register SIZE.EPOUT[n], indicating that the content

of the local buffer can be overwritten. Upon receiving the next OUT+DATA transaction, an ACK is returned

to the host while an EPDATA event is generated (and the EPDATASTATUS register flags are set to indicate

on which endpoint this happened). Once the EasyDMA is prepared and enabled, by writing the EPOUT[n]

registers and triggering the STARTEPOUT[n] task, the incoming data will be transferred to memory. Until

that transfer is finished, the hardware will automatically NAK any other incoming OUT+DATA packets. Only

when the EasyDMA transfer is done (signalled by the ENDEPOUT[n] event), or as soon as any values are

written by the software in register SIZE.EPOUT[n], the endpoint n will accept incoming OUT+DATA again.

It is allowed for the host to send zero-length data packets.

Bulk/interrupt OUT endpoints are enabled or disabled through their respective OUTn bit (n=1..7) in

the EPOUTEN register. It is also possible to stall or resume communication on an endpoint through the

EPSTALL register.

USB host

USB device

OUT

DATA (n)

NAK

OUT

DATA (n)

ACK

OUT

DATA (n+1)

NAK

OUT

DATA (n+1)

ACK

Software

Events & tasks

Figure 144: Bulk/interrupt OUT transaction

6.26.11 Isochronous transactions

The USBD peripheral implements isochronous (ISO) endpoints.

The ISO endpoints have a fixed USB endpoint number, summarized in the following table.

ISO endpoint #

USB IN endpoint

USB OUT endpoint

[0]

0x88

0x08

Table 109: Isochronous endpoint numbering

An isochronous transaction consists of a single, non-acknowledged data stage. The host sends out a start

of frame at a regular interval (1 ms), and data follows IN or OUT tokens within each frame.

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