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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 |
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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).
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... |
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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).
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..
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). |
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Figure 37: example of VCCIO0 assembly not dependent on power rail 2.5 V power rail5V. The other way is also possible.
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