VDD/VSS connections and general questions

Jonathan

Hi All,

Finally getting back to working on my Prop based bot as the school year winds down. What took it out was blowing the beta Prop, cause unknown, and then the Prop was back ordered, then the school year took over...

A project re-cap: The robot is for a class I want to develop based on a mars rover type semi-autonomous robot. The robot has a V2XE compass and an·odometer to·provide dead reckoning navigation. I have been working on and having sucess with an ultra-sonic based navigation system. The idea is simple; three ultra sonic TX beacons are placed in the room in an equilateral triangle. The 'bot has a RF link to each beacon and can request a ping from each in turn. The 'bot has and ultra-sonic RX and begins running a timer when the request for a ping is sent. Do a little math, and we have the distance from the 'bot to each beacon. I call this my CPS ( Classroom·Positioning System)

The raw distances are sent to a laptop via and EB500 Bluetooth unit. The laptop calculates the position and displays it on a map. You can click anywhere on the map, and the program calculates the direction and distance required to reach the desired postion. This information is relayed to the bot, which uses the dead reckoning to attempt to reach the location, and can then verify using the CPS.

So far, the dead reckoning sytem works suprisingly well on a linoluem floor.·The 'bot·can do a triangle 10' on each side and come back to within a few CMs of it's starting position. I was frankly shocked. I have built·the ultra-sonic reciever and one beacon, and get pretty good and consistant distance readings over about 20'. I have some details to work on like RF interference getting into the ultra-sonic recievr and a few other issues, but I feel confident I can get it to work. The software on the PC side is essentially working, thanks to help from a friend. Once I got the CPS done, I plan to add object mapping to the PC side and ping and IR sensors on the 'bot.

Anyway, this time I'm trying to do it right and not blow more props. The schematic only shows the VDD and VSS connected on one side. I'm assuming that I should use both sides for even current flow. I have seen posts regarding ground loops blowing the PLL, so I'll give both sides a home run to the VDD/VSS bus. Should I put a decoupling caps between both VDD/VSS pins? Sorry if this is covered elsewhere, but looking through the data sheet I found nothing. I also don't see a maximum current for banks of pins or the chip in total.

Is there any difference or benifit to using a particular frequency resonator? I can use a 5,10 or 20mhZ resonator and adjust the PLL accordingly. Is there any reason to prefer one over the other?

Im planning on putting 220 ohm resistors on all I/O, except those that see TTL levels where I will use 1k resistors. Any other protections or anything else I can do?

Thanks for the help.

Jonathan



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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

Paul Baker

Yes the Vss and Vdd should be connected and should at the chip, a leading contribution to ground loops is the resistance on the ground, so running each side seperately won't help. Route them wide and local, also dont place the ground return of major loads downstream of the Propeller, branch them. Place a bypass on both sides, not nessesary but recommended. There is no "bank limit", the chip limit is in the Absolute Maximum section of the datasheet. 5 MHz is the best choice of the 3 resontators, the 20MHz is too high for the PLL to work with and the 10MHz is right at the upper limit of what the PLL can handle and in temperature extremes may not work.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Jonathan

Thanks Paul! The Prop was recognized and downloaded a program to the EEPROM on the first shot. Now it's time to solder away!





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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot