Rev 1.00

March 22, 2010

18

HT45B0K

SPI to USB Bridge

USB Module Suspend Mode and Wake-up

If there is no signal on the USB bus for over 3ms, the devices will go into a suspend mode. The

Suspend indication bit SUSP, bit 0 of the USC register, will be set to 1 and a USB interrupt will

be generated to indicate that the device should jump to the suspend state to meet the 500μA USB

suspend current specification. In order to meet the 500μA suspend current, the firmware should

disable the USB clock by clearing the USB clock enable control bit USBCKEN in the UCC register

to 0. Also the USB PLL and PHY circuitry control bits known as PLL and SUSPC should be set to

1 to disable the USB PLL and PHY function. The suspend current is about 400μA.

USB Suspend Mode

When the resume signal is asserted by the USB host, the USB device will be woken up with a USB

interrupt generated on the INT line and the Resume indication bit RESUME in the USC register

will be set. In order to make the device function properly, the application program must set the

USBCKEN bit in the UCC register to 1 and clear the PLL and SUSPC bits in the USC register

and PIPE register respectively to 0. When the resume signal is de-asserted by the USB host, the

USB device actually leaves the suspend mode and the USB host will start to communicate with the

USB device. The SUSP bit will be cleared as well as the RESUME bit when the USB device really

leaves the suspend mode. So when the MCU is detecting the Suspend bit, the Resume bit should

be stored and taken into consideration. The following diagram shows the relationship between the

SUSP and RESUME bits and INT signal.

USB Host Wake-up

S U S P

R E S U M E

I N T

Suspend and Host Wake-up

As the device has a remote wake-up function, it can wake up the USB Host by sending a wake-up

pulse by setting the RMWK bit in the USC register to 1 for 2μs and then setting the RMWK bit to 0.

Once the USB Host receives a wake-up signal from the device, it will send a Resume signal to the

device. The timing is as follows:

USB Remote Wake-up

Suspend and Remote Wake-up

S U S P

R E S U M E

I N T

R M W K

2 . 5 m s ( M i n . )

2μs (Min.)