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When S2 is turned on (closed, FX2 PON), the power rails 1.2 V and 2.5 V power rails are controlled by the USB (EZ-USB FX2LP USB FX2) microcontroller. At start-up, the USB microcontroller switches off the 1.2 V and 2.5 V power rails and starts up the module in low-power mode. After enumeration, the USB microcontroller firmware switches the 1enables (switches on) power rails 1.2 V and 2.5 V power rails on, if enough current is available from the USB bus.
When S2 is turned off (open, PON), the 1.2 V and 2.5 V power rails are always enabled (switched on).
S2 position | Default position | Effect on 1.2 V and 2.5 V rails |
|---|---|---|
FX2 PON (on, closed) |  | Power rails 1.2 V and 2.5 V rails controlled by USB FX2 microcontroller (signal FX2_PS_EN) PS_EN = FX2_PS_EN = 1 or 0 |
PON (off, open) |  | Power rails 1.2 V and 2.5 V rails always enabled (PS_EN = 1) PS_EN ≠ FX_PS_EN = 1 or 0 |
Table 14: Slide switch S2 settings overview (power rails 1.2 V and 2.5 V rails only).
Signal FX2_PS_EN
To command signal FX2_PS_EN, read the reference firmware code.
IOD = 0x03; // Enable PS_EN and disable PROG_B
OED = 0x03; // Configure PS_EN and PROG as outputs
| Port D Pin | Alternate Function | Alternate Function is Selected By... | Alternate Function is Described in... |
|---|---|---|---|
| PD.7:0 | FD[15:8] | IFCFG1 = 1 and any WORDWIIDE bit = 1 | Slave FIFOs chapter 9 on page 99 |
Table from EZ-USB(R) Technical Reference Manual (EZ-USB_TRM.pdf).
Signal PS_EN
- Signal PS_EN enables (1) or disables (0) power rails 1.2 V and 2.5 V.
Figure 36: Power rails 1.2 V and 2.5 V could be enabled/disabled by signal PS_EN. Power-rail 3.3V is not controlled by signal PS_EN and is unconditionally enabled.
Figure 37: Power rails rail 3.3 V could not be enabled/disabled by signal PS_EN..
VCCIO0 assembly options
According to the corresponding assembly option, power rail VCCCIO0 can depend or not on the power rail 2.5V power rail.
Voltage VccIO for bank B0 shall span from 2.5 V to 3.3 V. VccIO can be supplied either externally or internally to the micromodule.
 | Warning! Spartan-3 I/Os are not 5 V tolerant. Applying more than the recommended operating voltages at any pin, results in a damaged FPGA (see Xilinx Answer AR#19146). |
|---|
Figure 37: example of VCCIO0 assembly not dependent on power rail 2.5 V power rail5V. The other way is also possible.
Slide Switch S2 = FX2 PON
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