
Universal Serial Bus
Specifications in this manual are tentative and subject to change
Rev. E
MITSUBISHI MICROCOMPUTERS
M30245 Group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
80
USB Special Function Registers
The MCU controls USB operation through the use of special function registers. Some USB-related special
function registers have a mix of read/write, read only, and write only register bits. Additionally, the bits may
be configured to allow the user to write only “0” or “1” to individual bits.
When accessing these registers, writing “0” to a register that can only be set to “1” by the CPU has no
effect on that register bit.
Writing “1” to a register that can only be set to “0” by the CPU has no effect on that register bit.
All USB SFRs, with the exceptions of Endpoint FIFO data registers, USBAD, and USBC can be accessed
by word or by byte at an even or odd address. Endpoint FIFO Data Registers can be accessed by either
word or by byte at even addresses only.
The contents of all USB Special Functions Registers, including USB Attach/Detach and USB Control, are
preserved after a software reset.
USB Attach/Detach Register
The USB Attach / Detach Register is shown in Figure 1.44. The register is used to attach and detach the
USB function from a USB host without physically disconnecting the USB cable. This functionality is enabled
by setting P90_SECOND to a “1”. Doing this forces P90 to operate as a pull-up for D+ (through an external
1.5k ohm resistor). The port driver is tri-stated and a “1” is always read from the port bit in this mode. When
the ATTACH/DETACH bit is a “1” (and P90_SECOND is a “1”), P90 is driven with the voltage on UVcc,
causing D+ to be pulled up and the host to detect an attach. When the ATTACH/DETACH bit is a “0” (and
P90_SECOND is a “1”), P90 is tri-stated, causing D+ to be pulled down (through the cable and 15k ohm
resistor on the host/hub side) and a detach to be registered by the host. A 1.5k ohm pull-up resistor must be
connected externally from P90 to D+ when this functionality is used. When it is not used, the 1.5k ohm
resistor should be placed between UVcc and D+.
See "Vbus Detect" for information on the vbus detect enable bit.
Figure 1.44. USB Attach/Detach register (USBAD)
USB Attach/Detach Register
Symbol
Address
When reset
USBAD
001F16
0016
Bit name
Bit symbol
b7 b6 b5 b4 b3 b2 b1 b0
0 : Normal mode for Port 90
1 : Forces Port 90 to operate as pull up for D+.
P90-second
Function
Reserved
Must always be set to "0"
Port 90-Second
Attach/
Detach
Attach/Detach
0 : Tri-states, P90 causing the host to detect a detach
1 : Drives P90 with voltage on UVcc, causing the host
to detect an attach
W
R
0 0 0 0 0
VBDT
Vbus detect enable
0 : Disabled
1 : Enabled