Hi dear friends, I am using P89LPC936 of philips for my project and i use keil MCB900 evaluationboard. When i test my projects on the MCB900 everything works perfect when sending data trough Rx and Tx pin on the P89LPC936(not after rs-232 tranciever). My GSM phone recives AT-commands perfectly but when i want to use the single chip on a sperate wrapboard using Vdd and Vss only UART does not seem to work properly. The bytes I read from the Rx and Tx seem to be correct but my GSM does not respond to it. I am totally stuck. Tried to change the output mode of the pins but no results. Any clue or help is really appreciated Thank you, Sarsam
thank you for the reply Neil, as I can trace the dataflow from the Tx or Rx pin trough a rs-232 datacable connected to my PC the data i send is working good and the phone is responding also correctly. the voltage over the Tx pin is around 3.27V. but even the GSM module uses strange voltages, why does i work great on the MCB900, it is not by luck after i tested it more than 50 times and using different AT commands, even ATDT"telephone number" was working great so i am totally lost. Are there any other pins on the LPC936 which i have to take care of, connecting to logic '0' or logic '1'? regards sarsam
"trough a rs-232 datacable connected to my PC the data i send is working good and the phone is responding also correctly." You've been told all this before: A PC COM: port is a proper RS232 port - it includes an RS232 transceiver. You say it works with a PC COM: port. You say it doesn't work direct to your microcontroller with no RS232 transceiver. Surely the answer is obvious - You need to use an RS232 transceiver!
"the voltage over the Tx pin is around 3.27V." That is meaningless. You need to use an oscilloscope, and verify that both the positive & negative voltages are correct during a transmitted character. Take a look at this article: http://www.maxim-ic.com/appnotes.cfm/appnote_number/527 Note that the RS232 receiver threshold for a high level is 3V - so, as I suggested originally, your 3.27 is right on the margin. You are just barely giving it sufficient to be recognised as a high level. Obviously, it is quite likely that just a small change somewhere in your circuit could reduce this sufficiently to fall below the "high" threshold - it would only take 0.28V to do it... "it is not by luck after i tested it more than 50 times and using different AT commands" That is irrelevant unless you took care to test at the extremes of supply tolerance, different temperatures, etc - to ensure that the necessary voltage is maintained under All conditions. Simply repeating the test without varying any test conditions adds no information!