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Universal Serial Bus, OTG Capable Controller
MCF52211 ColdFire® Integrated Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor 15-27
15.4.1.19 SOF Threshold Register (SOF_THLD)
The SOF Threshold Register is used only in Hosts mode (HOST_MODE_EN=1). When in Host mode, the
14-bit SOF counter counts the interval between SOF frames. The SOF must be transmitted every 1msec
so the SOF counter is loaded with a value of 12000. When the SOF counter reaches zero, a Start Of Frame
(SOF) token is transmitted. The SOF threshold register is used to program the number of USB byte times
before the SOF to stop initiating token packet transactions. This register must be set to a value that ensures
that other packets are not actively being transmitted when the SOF time counts to zero. When the SOF
counter reaches the threshold value, no more tokens are transmitted until after the SOF ha been
transmitted. The value programmed into the threshold register must reserve enough time to ensure the
worst case transaction completes. In general the worst case transaction is a IN token followed by a data
packet from the target followed by the response from the host. The actual time required is a function of the
maximum packet size on the bus. Typical values for the SOF threshold are: 64-byte packets=74; 32-byte
packets=42; 16-byte packets=26; 8-byte packets=18. Figure 15-26 shows the SOF_THLD Register.
IPSBAR
Offset:
0x1C_00AC (SOF_THLD) Access: User read/write
76543210
R
CNT7 CNT6 CNT5 CNT4 CNT3 CNT2 CNT1 CNT0
W
Reset:00000000
Figure 15-26. SOF_THLD Register
Table 15-30. SOF_THLD Field Descriptions
Field Description
7 – 0
CNT[7:0]
This 8 bit field represents the SOF count threshold in byte times.