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MB90335 Series
11.4.7
NULL Transfer Function
If data sent from the USB Function is the last packet and a maximum number of
packets, it is possible to automatically transfer 0-byte data in the next packet transfer.
The NULL transfer function requires that DMAE is enabled and is a function that is only
valid for IN transfer.
■ NULL Transfer Mode
●
This is the mode where if the automatic buffer transfer mode is set (DMAE=1) and IN-direction data
transfer request arrives, and a maximum number of packets are written via DMA and the last data write
decrements the number of DMA count data to "0", it automatically sets 0-byte data transfer and will
send 0-byte data for the next IN-direction data transfer request when the last IN-direction data transfer
request from the HOST has been received. The DRQ interrupt flag is not set until 0-byte data is read
from the HOST after the last data was written into a buffer via DMA. Timing by which the buffer is
accessed is shown as follows.
●
Only IN direction (device → host PC) forwarding
Host PC
Device
DRQ flag
Device
Automatic
Host PC
clear
DMAE
DRQIE
DRQ
NULE
DER(ENx)
For the devices, the following processing is required:
DMAE, DRQIE, and NULE are permitting set.
CM44-10137-6E
Figure 11.4-19 NULL Data Transfer Operation
Data before last
IN
DATA0
Last data
DATA1
DMA sending buffer
MAX packet write
FUJITSU MICROELECTRONICS LIMITED
11.4 Operation Explanation of USB Function
Last data
ACK
IN
ACK
DATA1
Interrupt factor is
not set.
CHAPTER 11 USB FUNCTION
0 byte data
ACK
IN
DATA0
241
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