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What type of IO to use

I bought the Digilent XC2 board (Xilinx CoolRunner II XC2C256 TQ144) a while back for electronics hobby stuff and I was wondering anyone could help with ...

I want to expand/add to the board some LEDs and slide switches so I can experiment with more stuff. I want to add a buffer in between the switches and LEDs but I am not sure types to use.

1.) I see in the manual that the XC2C256 on the board has different types of IOs (LVTTL, LVCMOS33, etc.). Am I correct in assuming that I can pick a buffer (like say one that is LVTTL) and then when I program the chip just select that type for the IO for each pin in the WebPack ISEs "Assign Package Pins" and that will be OK?

2.) Do you have recommendation for what is best IO type to use (I think the board has power supply for 3.3 volts - -I use the batteries)?

3.) Is putting buffers between LEDs and switches correct thing to do in order to ensure safety for the main XC2C256 chip or does the chip have protection and I can put an LED directly to the IO of the chip (with a resistor -- size?)?

Any recomendations or ideas and any hobby level infor fo rthe XC2 board would be greatly appretiated!

Reply to
GT
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You can use the FPGA I/O directly, if you apply some common sense. The sink or source current should be limited to 20 mA ( this may be overly conservative, but I like to play it safe), so LEDs must have a current-limiting resistor. Avoid pulling the pins higher than Vcc or lower than Ground, but there are also protective diodes that prevent excessive voltages. Use reasonable care against electrostatic discharge (walking over a nylon carpet in low-humidity environments can easily generate 10 000 V or more.) Do not discharge this directly into the pins. Peter Alfke, Xilinx Applications.

Reply to
Peter Alfke

Thanks for the info. What is correct current limiting resistor size to use? Thank you

Reply to
GT

Let's say that the voltage drop on LED is 1V, the FPGA/CPLD output voltage is 0.3V, and the LED current is 5 mA. The resistor would be (3.3-1-0.3)V/5 mA=0.4 kOhm

vax, 9000

Reply to
vax, 9000

its sometimes also ok to use no resistor by selecting proper IO drive level and using approp PWM to control the LED overage current.

A suitable 4Bit PWM ip core consumes 1 Xilinx LUT and 1 Flip Flop

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source code is available from the above project downloads

Antti

Reply to
Antti Lukats

What about using a mismatched IOSTANDARD constraint (ex. using IOSTANDARD=LVCMOS25 and vcco=3.3v) ?

Regards Sandro

Reply to
Sandro

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