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  1. #1
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    Problem with understanding a datasheet

    Hi

    CC2500 datasheet
    I am a student and I am now working on a project with CC2500, I have some questions regarding the basics of the CC2500.

    1. Frequency range 2400 - 2483.5 MHz, what does it really mean? does it mean that the carrier frequency is in between this range please?

    2. Programmable data rate from 1.2 to 500kBaud, what is this data rate please?

    3. The data which the CC2500 transmits out in wireless is coming from the SPI interface, which has a limit max frequency, so does it mean that my input data rate is limited by this frequency? does this data rate have any relationship to the questions I asked above please?

    4. Let say now I have some information wants to be transmitted, and I know this data rate is 200kbps, is this chip able to transmit it? what parameters are needed to be consider in order to derive the max data rate of my information?

    Thank you in advance first, and sorry for my silly questions

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  2. #2
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    Re: Problem with understanding a datasheet

    Luckyvictor,

    First of to clear something up eith regards "baud" and "data rate" they are two seperate measurments.

    Baud is "symbols per second" on/in the communications channel.

    Data rate is the number of bits or Bytes of information that goes through the communications system.

    Frequently a communications channel symbol represents just a single bit of data (in which case baud = bits per second) which causes confusion of terms. However in more complicated signaling systems the communications symbol could be two, four or more data bits (think QAM etc) And in systems using data compression before transmission the relationship between Baud and data rate can be complex.

    Thus Baud is a measure of communications symbol rate not information rate. And data rate is a measure of information rate in bits per second not symbol rate in the channel. Say it ten times a day before breakfast untill it sticks 8)

    Without going through the near 100page data sheet I'll try to answer your questions.

    (1) The device included a Direct Digital Syntesiser which produces an RF carrier. The frequency of the carrier is dependant on the "allowable" numbers and base clock rate of the synth. It might also depend on the charecteristics of some of the internal components but you can probably pull it outside of the "ISM Band" coverage it's been designed for (It might well work in the adjacent Amateur radio band).

    (2) Like the RF carrier the chip can be programed to send using different modulation methods and baud rates, and thus data rates. It's upto the system designer to chose.

    (3) the SPI data rate is probably going to limited by the functionality of the microcontroler you use more than this device. When you know what the microcontroler is you can look up it's maximum data rate in it's data sheet. But... bear in mind there is no free lunch the higher the data rate you use the shorter the communications range, the faster te interupt rate and the harder to get working the system will be. So pick the lowest "easy data rate" that will meet your projects needs.

    (4) As I said earlier your data rate is the number of bits or bytes a second you send or recive, you devide this by the modulation method bits per symbol to get the baud rate. You then programe the chip as appropriate.

    A word of caution sometimes a Byte is not what you thing it is in terms of data rate. In the protocol used in RS232 systems, you have 1 8bit data byte that gets wrapped in start and stop bits and sometimes even an aditional parity bit. Thus you see "8n1" which means 8 data bits no parity bit and one stop bit for a total of 10 communications symbols sent...



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  3. #3
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    Re: Problem with understanding a datasheet

    Thank you for your detail explanation.

    Just to verify my understanding:
    1) For the carrier frequency, I think basically you are saying it is in the frequency band stated, but it does mean I can use any frequency for the carrier, there are only some 'allowable' frequency can be used as carrier frequency, and this is depend on the device characteristic.

    2) the programmable data rate from 1.2 to 500kBaud is referring to the data rate at the antenna, either in receiver mode (receiver baud rate) or transmitter mode (transmit baud rate).

    3) the SPI data rate is going to be limited by the slowest clock of either this transceiver or the MCU, and this data rate is fsclk (since SPI clock in a bit of data every clock edge).

    4) continues from 3), fsclk is indeed the input data rate of this transceiver, by choosing the modulation method, the corresponding baud rate can be calculated, and this baud is going to be limited by the send data rate 1.2 to 500kBaud.

    Please correct me if I am still misunderstanding.



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