NXP Semiconductors MKL25Z128VLK4 - SOF Threshold Register (Usbx_Softhld)

USBx_TOKEN field descriptions (continued)
Field Description
0001 OUT Token. USB Module performs an OUT (TX) transaction.
1001 IN Token. USB Module performs an In (RX) transaction.
1101 SETUP Token. USB Module performs a SETUP (TX) transaction
3–0
TOKENENDPT
Holds the Endpoint address for the token command. The four bit value written must be a valid endpoint.
35.4.20 SOF Threshold Register (USBx_SOFTHLD)
The SOF Threshold Register is used only in Host mode (HOSTMODEEN=1). When in
Host mode, the 14-bit SOF counter counts the interval between SOF frames. The SOF
must be transmitted every 1ms so therefore 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 has 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 an 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.
Address: 4007_2000h base + ACh offset = 4007_20ACh
Bit 7 6 5 4 3 2 1 0
Read
CNT
Write
Reset
0 0 0 0 0 0 0 0
USBx_SOFTHLD field descriptions
Field Description
7–0
CNT
Represents the SOF count threshold in byte times.
Memory map/Register definitions
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
638 Freescale Semiconductor, Inc.