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Current output capability of the CycloneIII I/O cell


I am not an FPGA developer, so please bear with me. I am designing a microprocessor circuit to emulate a 1-wire device connected to a 3.3-V LVTTL output of a Cyclone-III FPGA. 1-wire is a Maxim communication protocol which delivers power to, and allows bidirectional communication with, a sensor over a single pair of wires (signal and ground).


My question is about the output capability of the FPGA I/O Cell that is driving my circuit. The lowest duty cycle of the 1-wire line is ~10% and occurs when sending zeroes from the FPGA to my microcontroller (MCU). Since my MCU draws an average current of 0.7mA, it needs to draw 7 mA during the on-time at the 10% duty cycle to keep it's supply up. Could this damage the FPGA output?


The FPGA powering my device is a EP3C25F256C8N. In the datasheet, I see two relevant specifications regarding output current.

  • Absolute maximum ratings (Table 1-1, pg 1-2) list Iout as -25 to 40 mA.
  •  Single-Ended I/O Standard Specifications (Table 1-13, pg 1-10) shows a "current drive characteristic" of ±4 mA.

The former makes it sound like 7 mA is very unlikely to cause damage. But the latter makes me uncertain that there is not a longer term issue. How should I interpret that latter spec?

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Hi THerr9,


Yes you're right. The 7mA is very unlikely to cause the damage as Absolute maximum ratings for DC output current are min -25mA ,max 40mA. Absolute maximum ratings are conditions that you should never be exceeded as it may cause damage to your pins.


However, for I/O standard 3.3V LVTTL, the current characteristic of IOL and IOH are 4mA and -4mA respectively. If you exceed our recommended value, it may not damage your pin but the functionality of this pin we can't guarantee. It's may or may not working properly. You can try on your own risk.



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