Dual power supplies on the Propeller chip?

Nightblade

Hello,

Can someone tell me why there are VSS and VDD pins on both sides of the Propeller? When I remove the wires from the right side, my code seems to still be working fine. Do these duplicate pins serve any purpose or is it just for convenience?

Thanks
Matt

Mike Green

You must provide Vss and Vdd to both sides of the chip. It may work normally, but, without both sets of connections, the Propeller is vulnerable to damage from noise on Vdd and Vss. Usually the system clock PLL selector fails first. In that case, the Propeller will work off its internal clocks, but not the crystal-based clock.

The problem is that the power connections across the chip are just deposited films of aluminum. At the scale involved, these connections have some significant resistance that can cause a significant voltage drop across the chip in an environment where the voltages are already low. The multiple connections to the power supply help reduce the maximum voltage drops involved.

Similarly, with other large chips with multiple power supply connections, you have to provide short connections to all the power pins and there needs to be a bypass capacitor (typically 0.1uF) between Vdd and Vss at each set of power connections. On most of the Propeller boards, these power connections run under the chip, so they're short. Sometimes the bypass capacitors are also under the chip on the bottom of the board.

Nightblade

Interesting... the capacitor was going to be my next question, but you already answered it.

I have built the circuit from the diagram at http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PEKit_40PinBoxInsert-v1.2.pdf and it works great. I am just using the breadboard from the education kit and another breadboard I already have. I don't have anything premade for me except the propeller plug itself.

I am now setting up my other two propeller chips on two other breadboards and that is why is was trying to find any shortcuts I could in the wiring (to save time). I have two of them working now, and tonight I will wire the last one.

Ultimately, I am going to have all three Propellers connected to XBees and talking to each other wirelessly.

Mike Green

I think you'll be pleased with the Propeller and how easy it is to use. It does work well with the xBees. The chip itself is pretty robust as long as you wire it up properly and include the bypass capacitors. One other thing it doesn't tolerate is connecting the power supply backwards or to voltages exceeding its Absolute Maximum rating (about 4V). This is true of the xBees and many other chips and devices as well.