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interfacing microcontroller with PSTN

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Sameh Yassin

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fsk server pstn

Dear all,
my project is to make biomedical hand held unit that is used in house. Simply, I need to read data from different biomedical sensors via USB port and send it to a microcontroller (Task1: I need to choose asuitable controller), then the data is sent via telephone lines to a remote server (task2: I need to decide whether to use DTMF to send data or use Modem Moule such as Ceremetek ch1788).
Concerning Task1: I found the chip "pic16F455" which has internal usb port, also I found many usb microcontroller from ATMEL such as AT89C5131A-L. could you please help me choose a suitable controller ?
Concerning Task 2 : As we can see we are sending low rate data, so U think I can use DTMf to send the required data to the remote server. Am I forced to use a modem module such as ceremetek ch1788 or the dtmf module such as tp5088 is enough. anyhelp is appreciated.
Thanks in advance
Sameh Yassin
 

betwixt

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Be careful in your choice of USB interface, there are many microprocessors with USB but most will only work as slaves. You can connect them to a host, for example like a mouse or webcam, but they can not act as hosts themselves.

For sending the information, any signalling that uses audio will work over a telephone network as long as you check that your tones are 'safe' in the country it will be used in. Some telephone companies use tone signalling between switching centers or to tell the equipment when a call has finished. If you use the same tones you might get unexpected line drop or find data is filtered out between source and destination.

DTMF is a good and reliable method of sending data and you can also use it to dial the call in the first place. Most countries need at least 40mS of both tones for the number to be recognized, some are much longer so it would be wise to aim for say 75mS to be sure. The data rate will therefore be quite low.

If you need faster rates, consider using a 1200 or 2400 bauds FSK modem system. It is fairly easy to create the data directly in a microprocessor but you may need a PLL tone detector at the receiving end to recover the information.

Brian.
 

elrayes

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Hi concerning task 1: i prefer PIC18F4550, also you will find on microchip website "Microchip USB framework", these are ready made USB firmwares customized for microchip microcontrollers, but you will need MPLAB 8.2 or higher and C18 compiler, one of these projects you can make PIC mcu act as a host which mean you can connect other USB peripherals as slaves to it using a hub.
concerning task 2: should you use PSTN or you can use mobile phone network, for mobile phone solutions search about "GSM AT commands", the DTMF solution sounds good for me, you can use IC 8880 DTMF transceiver (you are from Egypt then you will find it at "RAM" store at bab el-louq, 35 L.E.).
good luck :)
 

Sameh Yassin

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Hello all,

First thanks for you Brian and ELrayes for replying, your replies are very helpful. Now, I know an extra requirment for my project, I have to send data from the microcontroller to the PSTN with the data rate of 56bps, so I have to use a modem. So, now I guess I need to use a modem chip such as the one from cermetek.
Concerning what you said rayes, I'm enforced to use the PSTN , it is one of a system specs, so I can't use popular wireless techniques which are easier to implement!
I have another question, I can't find many topics regarding the interfacing of a microcontroller with the PSTN, shall I use DSP chip or I just need to use a modem chip along with the microcontroller...

Sameh Yassin
 

betwixt

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Unless you need to complete the whole design yourself, the simplest way to talk to PSTN is to buy an analog modem and interface it to your microprocessor. The modem will appear as a standard 'AT' command serial interface and it will also already include the hardware PSTN interface circuitry. Remember that PSTN is a powered line and you have to control the DC load across it as well as matching the line impedance and provide high voltage isolation to satisfy telephone company regulations. A ready made modem will do everything for you and most are programmable to work on lines in different countries as well.

Brian.
 

elrayes

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Hi
betwixt inspired me with an idea
Why don't you build a computer interface between PC and the microcontroller and use PC modem AT commands to communicate over PSTN network, i think this may ease and reduce costs and hardware needed, you just need a serial or USB interface, then a software to read these data and re-send it to the modem in order to be sent over the PSTN to any where, just get the standard fax modem AT commands.
what do you think?
 

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