SH7721 datasheet(913/1524 Pages) RENESAS

913 / 1524 page

Section 25

USB Function Controller (USBF)

Rev. 3.00 Jan. 18, 2008 Page 851 of 1458

REJ09B0033-0300

Figure 25.14 shows the normal operation of the USB function and firmware in isochronous-out

transfer.

EP4 has two up to 64-byte FIFOs, but the user can perform data transmission and read receive

data without being aware of this dual-FIFO configuration.

In isochronous transfer, transfer occurs only once per one frame (1 ms). So, when SOF is received,

the FIFO buffer is switched automatically with hardware.

FIFO buffers are switched over by the SOF reception. Therefore, the FIFO buffer in which the

USB function receives the data from the host and the FIFO buffer in which the firmware reads the

receive data have different buffers, and a read and write of FIFO buffer are not competed.

Accordingly, the data read by the firmware is the data received in one frame before. The buffers of

FIFOs are switched over automatically by the SOF reception, so reading of data must be

completed within the frame.

The USB function receives data from the host after an out-token is received. If there is an error in

the data, set the internal TF flag to 1. If there is no error in the data, set the internal TS flag to 1.

In firmware, first, the processing routine of the isochronous transfer is called by SOF interrupt to

check the time stamp. Then data is read from the FIFO buffer. The flag information (TS, TF) is

read and decided if the data has an error. The flag information at this time represents the status of

the currently readable FIFO buffer.

SOF happens to be broken because of external cause during transmission from the host. In this

case, an operation flow is different from that in figure 25.14. As an example, figure 25.14 shows

the operation flow of a broken frame and a subsequent frame when SOF is broken once. When

SOF is broken, the FIFO buffer is not switched in current frame, and a time out interrupt occurs

after time set by user has been elapsed. The USB function controller discards the data which has

been transmitted to the frame from the host.

The firmware detects the SOF break by the time out interrupt. In this case, the FIFO buffer

connected to the CPU does not read data since data has already been read. When the SOF interrupt

occurs in the subsequent frame, the processing routine of the isochronous transfer is called and the

time stamps are compared. The time stamps do not much since the SOF break occurred in the

previous frame. Data is not read since the data in FIFO is not current one.

SH7721 Datasheet(PDF) 913 Page - Renesas Technology Corp