
EZ-USB FX2 Technical Reference Manual
Page 3-8 EZ-USB FX2 Technical Reference Manual v2.1
3.5 EEPROM Configuration Byte
The configuration byte is valid for both EEPROM load formats (C0 and C2) and has the following
format:
Figure 3-1. EEPROM Configuration Byte
Bit
6: DISCON
USB Disconnect
A USB hub or host detects attachment of a full-speed device by sensing a high level on the D+
wire. A USB device provides this high level using a 1500-ohm resistor between D+ and 3.3V (the
D+ line is normally low, pulled down by a 15 K-ohm resistor in the hub or host). The 1500-ohm
resistor is internal to the FX2.
The FX2 accomplishes ReNumeration by selectively driving or floating the 3.3V supply to its inter-
nal 1500-ohm resistor. When the supply is floated, the host no longer “sees” the FX2; it appears to
have been disconnected from the USB. When the supply is then driven, the FX2 appears to have
been newly-connected to the USB. From the host’s point of view, the FX2 can be disconnected
and re-connected to the USB, without ever physically disconnecting.
The “connect state” of FX2 is controlled by a register bit called DISCON (USBCS.3), which
defaults to 0, or “connected”. This default may be overridden by setting the DISCON bit in the
EEPROM configuration byte to 1. This bit is copied into the USBCS.3 bit before the CPU is taken
out of reset. Once the CPU is running, firmware can modify this bit.
Bit
0: 400KHz I²C-compatible bus speed
0: 100 KHz
1: 400 KHz
If 400KHZ=0, the I²C-compatible bus operates at approximately 100 KHz. If 400KHZ=1, the
I²C-compatible bus operates at approximately 400 KHz. This bit is copied to I²CCTL.0, whose
default value is 0, or “100 KHz”. Once the CPU is running, firmware can modify this bit.
Configuration
b7 b6 b5 b4 b3 b2 b1 b0
0 DISCON 0 0 0 0 0 400KHz
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