Freescale Semiconductor HCS08 Series, Reference Manual

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Universal Serial Bus Device Controller (S08USBV1)
MC9S08JS16 MCU Series Reference Manual, Rev. 4
Freescale Semiconductor 263
Figure 15-18. USBDP/USBDN Pullup Resistor Configuration for USB module
15.4.1.7 USB Powering and USBDP Pullup Enable Options
The USB module provides a single-chip solution for USB device applications that are self-powered or
bus-powered. The USB device needs to know when it has a valid USB connection in order to enable or
disable the pullup resistor on the USBDP line. For the USB module on this device, the pullup on USBDP
is only applied when a valid VBUS connection is sensed, as required by the USB specification.
In bus-powered applications, system power must be derived from VBUS. Because VBUS is only available
when a valid USB connection from host to device is made, the VBUS sensing is built-in, and the USBDP
pullup can be enabled accordingly.
With self-powered applications, determining when a valid USB connection is made is different from that
of bus-powered applications. In self-powered applications, VBUS sensing must be built into the
application. For instance, a KBI pin interrupt can be utilized (if available). When a valid VBUS connection
is made, the KBI interrupt can notify the application that a valid USB connection is available, and the
internal pullup resistor can be enabled using the USBPU bit. If an external pullup resistor is used instead
of the internal one, the VBUS sensing mechanism must be included in the system design.
Table 15-19 summarizes the differences in enabling the USBDP pullup for different USB power modes.
Table 15-19. USBDP Pullup Enable for Different USB Power Modes
Power USBDP Pullup Pullup Enable
Bus Power
(Built-in VBUS sense) Internal Set USBPU bit
External Build into application
Self Power
(Build VBUS sense into application) Internal Set USBPU bit
External Build into application
USBDP
USBDN
USB DEVICE
V
USB33
3.3 V
R
DPPU