PDA application

adrian · Joined: 08 Sep 2003 Posts: 92 Location: Glasgow, UK

I have been asked to look into a new design and want to 'bounce' a few ideas around before I get started.

I want to connect a PIC to the RS232 connection of a PDA. The PIC has to be powered via the RS232 lines and be able to use its internal USART to communicate with the PDA. Anybody know what the output voltage swing of an RS232 port on a typical PDA is? I don't even have one to measure.

I was thinking of using the 16F627A/628A/648A or 16F688, as these are the smallest PIC's I can find with a USART. My first thought was to put it to sleep and wake it through the RDA interrupt. Is it possible to do this? I have hunted throught the data sheet and found the following - but I need clarification.

From section 14.5 shows that the USART has an interrupt capability.
From section 14.8.1 shows that the device can wake up from sleep through ...... any peripheral interrupt which is active in sleep.
But
From section 12.2 the baudrate generator is used to derive baud rate frequencies from the oscillator.
From section 14.8 the #TO bit is set and the oscillator driver is turned off.

So it looks to me as if when in sleep, the oscillator is off, hence the BRG is off. So if an RDA interrupt occurs, the device will have to wakeup, restart the internal oscillator - which takes time - so I have possibly missed some if not all the incomming data.

Anybody got any thoughts?

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Can you arrange for the PDA to send a "wake-up" signal of a few "U"'s or "^C"'s then pause before sending a real message? This is fairly common in oceanographic equipment.
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adrian · Joined: 08 Sep 2003 Posts: 92 Location: Glasgow, UK

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

If you make it as far as needing a simple Palm app to test your hardware, B&B has a freebie starting point PalmTerm for download.

http://www.bb-elec.com/product.asp?sku=palmterm

I'm not sure if I would try to derive my power from the PDA. I guess it would depend on what else you're going to have to power besides the PIC. You may want to think about a powered cable?

Good luck,

John

adrian · Joined: 08 Sep 2003 Posts: 92 Location: Glasgow, UK

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

Use the IRQ line tied to the receive pin (in addition to it going to the RX pin of your UART). Just before you go to sleep, enable the wake-on-interrupt feature. Then when you wake up, turn off the IRQ until the next time you go to sleep.

Good chance you will miss the first word though. You might have to set up the PDA to ping the PIC a few times, waiting for an ACK back before proceeding.
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jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Guest · Posted: Mon Nov 28, 2005 12:38 pm

adrian · Joined: 08 Sep 2003 Posts: 92 Location: Glasgow, UK

Tut · Guest

A word of caution ...

I can't speak for the Pocket PC, but the serial ports for Palm-based PDAs are not all alike. Some applications that we had using Palm Pilots worked fine plugging into our 3-wire RS232 port to a PIC, but when I tried the serial cable for my Palm-based Treo 600 phone, I found out that the RS232 xcvr IC is in the serial cable and draws its power from the RS232 control lines.

Tut

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

This comment is specific to Palm but may apply to other PDA's.
The good news is that a used palm PDA can be obtained for trash metal prices. The bad news is Palm is paranoid and provides almost no hardware documentation which is perhaps one reason why a used Palm is as valuable as a week old hamburger. I joined Palm's developer program thinking I might get a schematic but no luck. PDA's are useful graphic interfaces..you get a touchscreen 8K or more of ram/rom RS232 ( well sort of) since they only care to have it talk to Palm cradles some of which are passive and some powered even providing an ethernet interface and all for about $20 used. You'll find you'll need your DVM and scope to tease out the details of interfacing. There is little on the WEB about the internals of Palm PDA's ..Palm could be supressing it with patent and copyright enforcement I don't know but be careful about publishing what you find.

arunb · Joined: 08 Sep 2003 Posts: 492 Location: India

Hi,

RS 232 voltage levels are usually between 5 to 12 Volts, my desktop PC has 12 Volts and my Laptop has +5 V. Your application must cater for the lowest possible voltage.

thanks
arunb

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Adrian,

Sorry I didn't get back to you. I got busy and missed several of the posts in the thread. I just saw that it came back up.

I just put the scope on a PALM IIIxE and this is for the data coming out of the palm: It idles at -5.5v and swings to +6v during the transmission.

I would heed the advice to verify whatever hardware you plan on using.

Good luck,

John

adrian · Joined: 08 Sep 2003 Posts: 92 Location: Glasgow, UK

Thanks guys I appreciate the replies.

We have used the HP iPAQ devices in the past, and the Engineering PDA is out on loan - so got another one on order. In the meantime I have been trying to get info out of HP - nightmare. After getting very dizzy going round their web site, I gave up and spoke to their tech support. It would seem that I will have to sign up to their developers network at a cost of $199 - and I only get to ask ONE question! They have got it sewn up like the Palm stories Douglas mentioned.

OK it looks like +/- 5V should be a starting point. Tap it off through an LDO and run the PIC at 3V3 - which will also cut the current requirment. Time to start sharpening my pencil until the iPAQ arrives.