M68HC08 datasheet(174/332 Pages) FREESCALE

174 / 332 page

Universal Serial Bus Module (USB)

Technical Data

MC68HC908JB16 — Rev. 1.1

174

Universal Serial Bus Module (USB)

Freescale Semiconductor

The reset flag bit (RSTF) in the USB interrupt register 1 (UIR1) also will

be set after the internal reset is removed. Refer to 11.8.3 USB Interrupt

After a reset is removed, the device will be in the default, but not yet

addressed or configured state (refer to Section 9.1 USB Device States

of the Universal Serial Bus Specification Rev. 2.0). The device must be

able to accept a device address via a SET_ADDRESS command (refer

to Section 9.4 Standard Device Request in the Universal Serial Bus

Specification Rev. 2.0) no later than 10ms after the reset is removed.

Reset can wake a device from the suspended mode.

NOTE:

USB reset can be configured not to generate a reset signal to the CPU

by setting the URSTD bit of the configuration register (see Section 5.

Configuration Register (CONFIG)). When a USB reset is detected, the

CPU generates an USB interrupt.

11.5.3 Suspend

The MCU supports suspend mode for low power. Suspend mode should

be entered when the USB data lines are in the idle state for more than

3ms. Entry into suspend mode is controlled by the SUSPND bit in the

USB interrupt register. Any low-speed bus activity should keep the

device out of the suspend state. Low-speed devices are kept awake by

periodic low-speed EOP signals from the host. This is referred to as low

speed keep alive (refer to Section 11.8.4.1 Low-speed Keep-alive in the

Universal Serial Bus Specification Rev. 2.0).

Firmware should monitor the EOPF flag and enter suspend mode by

setting the SUSPND bit if an EOP is not detected for 3ms.

Per the USB specification, the bus powered USB system is required to

draw less than 500

µA from the V

DD supply when in the suspend state.

This includes the current supplied by the voltage regulator to the 1.5k

Ω

to ground termination resistors placed at the host end of the USB bus.

This low-current requirement means that firmware is responsible for

entering stop mode once the USB module has been placed in the

suspend state.

M68HC08 Datasheet(PDF) 174 Page - Freescale Semiconductor, Inc