NRF52820-QDAA-R datasheet(383/437 Pages) NORDIC

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Peripherals

EasyDMA allows transferring ISO data directly from and to memory. EasyDMA transfers must be initiated

by the software, which can synchronize with the SOF (start of frame) events.

Because the timing of the start of frame is very accurate, the SOF event can be used for jobs such as

synchronizing a local timer through the SOF event and PPI. The SOF event gets synchronized to the 16 MHz

clock prior to being made available to the PPI.

Every start of frame increments a free-running counter, which can be read by software through the

FRAMECNTR register.

Each IN or OUT ISO data endpoint has to be explicitly enabled by software through register EPINEN or

EPOUTEN, according to the configuration declared by the device and selected by the host through the

SetConfig command. A disabled ISO IN data endpoint will not respond to any traffic from the host. A

disabled ISO OUT data endpoint will ignore any incoming traffic from the host.

The USBD peripheral has an internal 1 kB buffer associated with ISO endpoints. The user can either

allocate the full amount to the IN or the OUT endpoint, or split the buffer allocation between the two

using register ISOSPLIT.

The internal buffer also sets the maximum size of the ISO OUT and ISO IN transfers: 1023 bytes when the

full buffer is dedicated to either ISO OUT or ISO IN, and half when the buffer is split between the two.

6.26.11.1 Isochronous IN transaction

When the host wants to receive isochronous (ISO) data, it issues an IN token on the isochronous endpoint.

After the data has been transferred using the EasyDMA, the USB controller on the isochronous IN

endpoint responds to the IN token with the transferred data using the ISOIN.MAXCNT for the size of the

packet.

The ISO IN data endpoint has to be explicitly enabled by software through the ISOIN0 bit in register

EPINEN.

When an ISO IN endpoint is enabled and no data transferred with EasyDMA, the response of the USBD

depends on the setting of the RESPONSE field in register ISOINCONFIG. It can either provide no response

to an IN token or respond with a zero-length data.

If the EasyDMA transfer on the isochronous endpoint is not completed before the next SOF event, the

result of the transfer is undefined.

The maximum size of an ISO IN transfer in USB full-speed is 1023 bytes. The data buffer has to be a

multiple of 4 bytes 32-bit aligned in memory. However, the amount of bytes transferred on the USB data

endpoint can be of any size (up to 1023 bytes, if not shared with an OUT ISO endpoint).

USB host

USB device

DATA(0)

Start of frame

DATA n

Start of frame

response depends on the RESPONSE field in the

ISOINCONFIG register

Events & tasks

Software

Figure 145: Isochronous IN transfer

4463_156 v1.0

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