mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 976

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Memory Map/Register Definitions
39.4.20 SOF Threshold Register (USBx_SOFTHLD)
The SOF Threshold Register is used only in Hosts mode (HOSTMODEEN=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.
Addresses: USB0_SOFTHLD is FFFF_9000h base + ACh offset = FFFF_90ACh
39.4.21 BDT Page Register 2 (USBx_BDTPAGE2)
The Buffer Descriptor Table Page Register 2 contains an 8-bit value used to compute the
address where the current Buffer Descriptor Table (BDT) resides in system memory.
Addresses: USB0_BDTPAGE2 is FFFF_9000h base + B0h offset = FFFF_90B0h
976
Reset
Field
CNT
Reset
Read
7–0
Read
Write
Write
Bit
Bit
This 8-bit field represents the SOF count threshold in byte times.
7
0
7
0
0
0
6
6
MCF51JF128 Reference Manual, Rev. 2, 03/2011
USBx_SOFTHLD field descriptions
0
0
5
5
Preliminary
0
0
4
4
BDTBA
Description
CNT
0
0
3
3
0
0
2
2
Freescale Semiconductor, Inc.
0
0
1
1
0
0
0
0