Universal Serial Bus (USB)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
31-11
31.7.2
SOF transmission for USB Host controller
SOF packets should be transmitted every 1 ms. Since the precision of the time interval between two SOF
packets is strict, a CPM timer is used to assert an external interrupt to the CP. The user should program the
CPM timer 1 to a value that is equal to a 1 ms time interval. Before each expiration, the software should
prepare a value for the frame number and crc5, to be transmitted in the SOF token, and place it in the
parameter RAM (for further details please refer to
Section 31.10.1, “Frame Number (FRAME_N)”
). Upon
timer expiration, the external interrupt is asserted. When the external interrupt is serviced by the CP, a
microcode routine prepares a SOF token and loads it to the host endpoint. Once it is loaded to the FIFO,
it will be transmitted as any other token. The application software should guarantee that the USB host has
completed all pending transactions prior to the 1 ms tick.
Figure 31-6. SOF Generation
DREQ0 should be configured as external interrupt. When there are no hardware originated requests to the
CP, it enters stall state. Configuring DREQ0 as an external interrupt request ensures that only a hardware
interrupt request can wake up the host controller.
31.8
USB Function and Host Parameter RAM Memory Map
The USB controller parameter RAM area, shown in
Table 31-7
, begins at the USB base address, 0x1C00
(offset from RAM_Base). The user must initialize certain parameter RAM values before the USB
controller is enabled. Other values are initialized by the CP.
Start of Frame
(SOF)
SOF is generated every 1 ms. The timing must be exact and is controlled by a CPM timer, programed by
the user. From the host state machine point of view it is a packet to transmit, placed in its FIFO,
transmitted as is.
Preamble
(PRE)
The PRE token signals the hub that a low-speed transaction is about to occur. The PRE token is read
only by the hub. The USB host controller generates a full-speed PRE token before sending a packet to a
low-speed peripheral.
Table 31-3. USB Tokens (Host Mode) (continued)
Token
Description
TIMER1
TOUT1
CPM RISC
cpm_dreq0
dreq0
USMOD.EN & USMOD.SFTE
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