Atmel SAM4S-XPLD Atmel ATSAM4S-XPLD ATSAM4S-XPLD Hoja De Datos

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SAM4S [DATASHEET]

11100E–ATARM–24-Jul-13

Data OUT transactions are used in control, isochronous, bulk and interrupt transfers and conduct the transfer of data
from the host to the device. Data OUT transactions in isochronous transfers must be done using endpoints with ping-
pong attributes.

To perform a Data OUT transaction, using a non ping-pong endpoint:

1.

The host generates a Data OUT packet.

2.

This packet is received by the USB device endpoint. While the FIFO associated to this endpoint is being used by 
the microcontroller, a NAK PID is returned to the host. Once the FIFO is available, data are written to the FIFO by 
the USB device and an ACK is automatically carried out to the host.

3.

The microcontroller is notified that the USB device has received a data payload polling RX_DATA_BK0 in the end-
point’s UDP_CSRx register. An interrupt is pending for this endpoint while RX_DATA_BK0 is set.

4.

The number of bytes available in the FIFO is made available by reading RXBYTECNT in the endpoint’s 
UDP_CSRx register.

5.

The microcontroller carries out data received from the endpoint’s memory to its memory. Data received is available 
by reading the endpoint’s UDP_FDRx register.

6.

The microcontroller notifies the USB device that it has finished the transfer by clearing RX_DATA_BK0 in the end-
point’s UDP_CSRx register.

7.

A new Data OUT packet can be accepted by the USB device.

An interrupt is pending while the flag RX_DATA_BK0 is set. Memory transfer between the USB device, the FIFO and
microcontroller memory can not be done after RX_DATA_BK0 has been cleared. Otherwise, the USB device would
accept the next Data OUT transfer and overwrite the current Data OUT packet in the FIFO.

During isochronous transfer, using an endpoint with ping-pong attributes is obligatory. To be able to guarantee a
constant bandwidth, the microcontroller must read the previous data payload sent by the host, while the current data
payload is received by the USB device. Thus two banks of memory are used. While one is available for the
microcontroller, the other one is locked by the USB device.

ACK

PID

Data OUT

NAK

PID

PID

PID

PID

PID

Data OUT2

ACK

Data OUT

Data OUT 1

USB Bus
Packets

RX_DATA_BK0

Set by USB Device

Cleared by Firmware,
Data Payload Written in FIFO

FIFO (DPR)
Content

Written by USB Device

Microcontroller Read

Data OUT 1

Data OUT 1

Data OUT 2

Host Resends the Next Data Payload

Microcontroller Transfers Data

Host Sends Data Payload

Data OUT2

Data OUT2

Host Sends the Next Data Payload

Written by USB Device

(UDP_CSRx)

Interrupt Pending