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CHAPTER 6 USB CONTROLLER
Preliminary User’s Manual S15543EJ1V0UM
367
6.8 Receiving SOF Packet
USB Controller can receive SOF Packets, and check if Frame Number is incremented correctly.
In addition, USB Controller can detect the timing skew of SOF Packet.
6.8.1 Receiving SOF Packet and updating the Frame Number
After USB Controller receives a SOF Packet, FN field in USB Frame Number/Version Register (Address:
1000_1004H) is updated. After FN field is updated, FW bit (Bit 21) in USB General Status Register 2 (Address:
1000_1018H) is set to a ‘1’.
6.8.2 Updating Frame Number automatically
If received SOF Packet has incorrect Frame Number, USB Controller can execute one of two processes shown
below.
- USB Controller reflects the incorrect Frame Number to FN field directly.
- USB Controller increments Frame Number automatically and write the incremented value to FN field.
The policy of updating FN field is shown in following table:
Case
AU bit
Note
FN field USB General Status Register 2
Correct SOF 0 Just load the received Frame Number FW bit is set to 1
1 Just load the received Frame Number FW bit is set to 1
Loss of SOF 0 Not updated SL bit is set to 1
1 Increment the current FN FW bit and SL bit are set to 1
Extra SOF 0 Not updated ES bit is set to 1
1 Not updated ES bit is set to 1
Bit Stuff Error 0 Just load the received Frame Number FW bit is set to 1
1 Increment the current FN FW bit is set to 1
CRC Error 0 Just load the received Frame Number FW bit is set to 1
1 Increment the current FN FW bit is set to 1
Incorrect Frame Number 0 Just load the received Frame Number FW bit is set to 1
1 Increment the current FN FW bit is set to 1
Note AU bit is in the USB General Mode Register (Address: 1000_1000H).
6.8.3 Checking if the skew of SOF arrival time is allowable of not
The allowable SOF skew can be defined by SOFINTVL field in the USB General Mode Register (Address:
1000_1000H).
Figure 6-30. Allowable Skew for SOF
Allowable
skew
Allowable
skew
1 msec
The timing
SOF comes
The timing
SOF comes
SOFINTVL field is set to 18H (24 clocks) at default. This is 0.05 % of 48000 clocks (1 msec).
The value of SOFINTVL should be set before the first SOF packet comes.