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ID: 2205
Case Type: faq
Category: Device Programming
Related To: Cables
Family: All Devices

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Why does the USB ispDOWNLOAD cable raise the VCCJ voltage when I connect it to my low power board design?

Situation: A MachXO2 device is powered by a low voltage linear regulator that outputsthe proper voltage when powered up. It is then found that when the USB ispDOWNLOAD cable is connected to the board design to program the MachXO2, that the VCCJ voltage increases. Further testing shows that the design VCCJ voltage is a positive low voltage even when the board is unpowered and the USB ispDOWNLOAD cable is connected. This is not the expected operation in that the design regulator should hold the power supply value constant regardless of the USB ispDOWNLOAD cable being connected. 
Answer: The USB ispDOWNLOAD cables have very good signal isolation between the PC signals at the USB connector and the flywires that connect to your design.The VCCJ flywire does show some low level leakage current when the VCCJ voltage is less than 2.5 volts. You can check this by attaching a DVM to measure the voltage from the VCCJ flywire to the GND flywire when there are no connections to yourboard. You can can also set the DVM to measure current and this will show the maximum amount of leakage current available at the VCCJ flywire (the current setting on the DVM will short VCCJ to GND and this gives the maximum leakagecurrent measurement). Given this, if your board design has a VCCJ power that is both low voltage and low current, and the USB ispDOWNLOAD cable leakage current is higher than the current your design uses, then you might see the VCCJ voltage increase when the USB ispDOWNLOAD cable is connected to the JTAG signals on your board. 
For the vast number of board designs done, the VCCJ leakage current is not an issue, typically because the design's VCCJ voltage and/or current required are not low enough, and in many instances, because the VCCJ voltage is produced bya switching regulator. Switching regulators typically can both source and sinkcurrent to maintain the output voltage, especially when the load currentsare small. Linear regulators are typically very good at sourcing current, butthen tend to become unregulated or let the voltage float if the load current is sinked or attempting to pull the voltage higher than the regulation voltage.
You might be wondering: what is the cause of the VCCJ leakage current? The answer for that is, internal to the USB ispDOWNLOAD cable is a processor that communicates with the PC through the USB connector, and a buffer IC that sends and receives the JTAG flywire signals that connect to your design. The processor is powered by the PC side of the cable, and the buffer IC is powered by your design's VCCJ power. When the VCCJ voltage is 3.3v, there is no leakagecurrent into VCCJ because both the PC side and your design's VCCJ are equal and not connected. When the VCCJ is less than about 2.5v, some signal leakage current begins to show up when the internal cable processor powered by the PC side, sends signals to the buffer IC powered by the VCCJ provided by yourdesign. If the VCCJ is less than the processor 3.3v, the processor signals arriving at the buffer IC are clamped high, and any excess signal current fromthe processor IOs is clamped or shunted to the VCCJ power connection... and this is the leakage current you are measuring with a current meter if you do the DVM current test described just above.
This signal leakage current into VCCJ is small, typically about 1mA when VCCJ is shorted to GND, and much lower current when VCCJ is allowed to float high. Even though the VCCJ leakage current is small, for some very low power designs the leakage current can raise the VCCJ voltage some small amount. If this is determined to be an issue, an easy solution is to add a 500 ohm resistor from VCCJ to GND at the flywire connection point on the board design. Alternately, you can add the 500 ohm resistor to a male/female interposer connector that has the 500 ohm resistor permanently attached between VCCJ and GND. When the modified interposer connector is installed during programming, the USB ispDOWNLOAD cable VCCJ leakage current will be shunted to GND at the connector and the VCCJ voltage will be maintained at the proper voltage level set byyour design.
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