Robotics with the Boe-Bot: Vdd vs. Vin

ToddH

I'm working through the Robotics with the Boe-Bot book. I have a question about page 61. It has me set my servo jumper to Vin since I'm using the battery pack that came with the Boe-Bot.

What is really going on here? What is the difference between the jumper set to Vin and it set to Vdd? I also have noticed that the BoE has a "Vin" sandwiched in between Vdd and Vss? What is this mysterious Vin?

Thanks for the newbie help!

Todd

allanlane5

Vin is the 'raw' voltage coming from the batteries (or the wall-wart, if you're using it). This is 'un-regulated', meaning there's no 2940 or 7805 regulator making sure the voltage is exactly 5 volts. If it's a wall-wart, a 6-volt un-regulated wall-wart can produce 9 volts with no load.

Now, Parallax lets you connect your Servo directly to the battery voltage (using the Vin jumper) so the Servo gets the full battery voltage -- you get more power this way into the Servo. But, the Servo will only work with 5 to 6 volts. 9 volts will burn it out.

OK, so Vdd is the output of the regulator on the BOE board. It produces 5 volts. If you are using a wall-wart, then you want to connect Vdd to your Servos, since Vin would reduce their life or destory them.

Now, the BS2 ALSO has a small regulator on-module. Thus you CAN power the BS2 directly from a 9-volt battery wired across Vin and Vss. The on-module regulator will give the on-module PIC a regulated 5 volts from this. But the on-module regulator is limited to about 50 mA -- so you don't want to drive much besides the BS2.

A note of caution here -- if you wire a 9-volt across the BS2 VDD and VSS, it WILL destroy the BS2. You've just bypassed the BS2 on-module regulator when you do that, and the on-module PIC really can't handle 9-volts.

On the one hand, it may seem that all these power options are a pain in the butt. However, all these power options give the BS2 its enormous flexibility. No other microprocessor IC will let you connect a battery directly to it. With the BS2, you just have to be careful how you do it.

To summarize:

Vin is the 'raw', un-regulated voltage input to the board. If 6 volts or less, it can be used to drive Servos.

Vdd is the regulated voltage output from the 2940 or 7805 regulator. It is 5 volts. This voltage is safe for everything, servo's, BS2, other IC's -- but can 'waste' power if you're running from battery.

If your Vin is 7 to 18 volts, you MUST use Vdd to drive your servo.

The BS2 DOES have a regulator on-module, so you CAN drive Vin to use it. You can also merely put a regulated 5 volts on the BS2 Vdd line, which is how the BOE does it.

ToddH

wow - thanks so much for the great answer!

Nav

allanlane5 said...
.....if you wire a 9-volt across the BS2 VDD and VSS.....

Don't you mean Vcc on the BS2 ?· Although the BoE is marked with Vdd, Vin and Vss, the markings on the BoE that are printed beside the BS2 pins are Vcc, Vin and Vss.· I take that to mean that Vdd is called Vcc on the BS2 itself.· Is that correct ?

dandreae

Hello,

Yes, you are correct.

Dave

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Dave Andreae

Tech Support
dandreae@parallax.com
www.parallax.com

·

RDL2004

Just out of curiosity, where do these odd subscript abbreviations come from? Why not use Vreg or something like that? What does the ss and dd refer to?

Beau Schwabe

I believe it refers to the technology or process used...

A CMOS transistor has a Source and Drain connection thus Vss corresponds to the Source and Vdd corresponds to the Drain.
A Bipolar transistor has a Collector and an Emitter thus Vcc corresponds to the Collector and Vee corresponds to the Emitter.

I am not exactly sure why there is a double stamp representation unless it represents a hierarchical nomenclature.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

---

Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

---