Datasheet

SAM9G20
DS60001516A-page 666  2017 Microchip Technology Inc.
Figure 36-9: Data OUT Transfer for Non Ping-pong Endpoints
An interrupt is pending while the flag RX_DATA_BK0 is set. Memory transfer between the USB device, the FIFO and microcontroller mem-
ory 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.
• Using Endpoints With Ping-pong Attributes
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.
Figure 36-10: Bank Swapping in Data OUT Transfers for Ping-pong Endpoints
When using a ping-pong endpoint, the following procedures are required to perform Data OUT transactions:
1. The host generates a Data OUT packet.
2. This packet is received by the USB device endpoint. It is written in the endpoint’s FIFO Bank 0.
3. The USB device sends an ACK PID packet to the host. The host can immediately send a second Data OUT packet. It is accepted
by the device and copied to FIFO Bank 1.
4. The microcontroller is notified that the USB device has received a data payload, polling RX_DATA_BK0 in the endpoint’s
UDP_CSRx register. An interrupt is pending for this endpoint while RX_DATA_BK0 is set.
5. The number of bytes available in the FIFO is made available by reading RXBYTECNT in the endpoint’s UDP_CSRx register.
ACK
PID
Data OUTNAK
PIDPIDPIDPIDPID
Data OUT2ACKData 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
USB Device USB Bus
Read
Write
Write and Read at the Same Time
1
st
Data Payload
2
nd
Data Payload
3
rd
Data Payload
3
rd
Data Payload
2
nd
Data Payload
1
st
Data Payload
Data IN Packet
Data IN Packet
Data IN Packet
Microcontroller
Endpoint 1
Bank 0
Endpoint 1
Bank 1
Endpoint 1
Bank 0
Endpoint 1
Bank 0
Endpoint 1
Bank 0
Endpoint 1
Bank 1