Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 859

Powerquicc family

Table of Contents

Start of Frame

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.

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.

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

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.

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.

USB Function and Host Parameter RAM Memory Map

The USB controller parameter RAM area, shown in

(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.

Freescale Semiconductor

Table 31-3. USB Tokens (Host Mode) (continued)

Section 31.10.1, "Frame Number

Figure 31-6. SOF Generation

MPC885 PowerQUICC Family Reference Manual, Rev. 2

USMOD.EN & USMOD.SFTE

31-7, begins at the USB base address, 0x1C00

Universal Serial Bus (USB)

(FRAME_N)"). Upon