Hi, on my AD Spartan3 board I have 3 jumpers selecting the boot mode M0-2. They let you connect each pin to GND, so if you place no jumper on a ceretain pin think you get a high signal ('1') on that mode pin. Why? I was supposed to have internal pullups only on the I/Os, enabled during the configuration stage by the HSWAP_EN and not also on those M0-2 pins. In my board should I do it the same way or shoud I consider a connector in which jumpers could link those pins to Vcc for '1' or to GND for '0'? Thanks, Marco
All Xilinx devices have weak (10kohm+)internal pullups on the configuration mode select pins. The HSWAP_EN pin only enables/disable the pullup on the non-dedicated pins during configuration.
If you do not connect this pin to anything else on the PCB that might cause a current drain (like an LED) you do not need to add a dedicate pullup resistor.
Funny a Xilinx rep would make such a mistake. Yes, these parts have an internal pullup on the Mode pins, but it is nowhere near 10K ohms. The data sheet specs it at between 1 and 3 kohms and on our board it is measuring about 1 kohms. This is significant if you use pulldown resistors thinking the pullup is very light. Using a 1 kohm pulldown is often used when the internal pullup is 10k or more. But with the Spartan 3 parts you need a resistor of 330 ohms or less to be sure you pull down hard enough. Of course using jumpers will do the job. I just wanted to clarify the 10 kohm mistake since this has caused grief with engineers who did not read the data sheet in enough detail themselves and relied on advice like Ed (and my FAE) gave.
Also, the details of the configuration signals IO characteristics are much more complex that you have described. Some pins have pullups at all times like the Mode pins, at least until configuration is done. Other signals can have pullup, no pullup, or totempole outputs depending on configuration parameters. The data sheet does a poor job of putting all this in one place and you will have to hunt around to figure out what has pullups and what is programmable. Basically, you need to read all 206 pages of the data sheet very carefully to make sure you get all the info on how to configure the part and the details on the IOs.
Rick is correct and I made a mistake. My statement of 10K+ was too generic and is not accurate for the Spartan-III families. For Virtex-4 and Virtex-5 the minimum value is 16.5K ohm with a typical of 60K ohm. However for the Spartan-III family our spec has a low of 1.2K ohm and 2.4K ohm for the Spartan-IIIe family.
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