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UART TX/RX Error as defined in Users Guide Table 15-4 / 15-5

Tom Smith
Expert 2930 points


According to the Tables 15-4 + 15-5 of the MSP430Fx2xx Users Guide Max TX error and Max RX error is defined as a negative and a %, please clarify.

For example in Table 15-5, for BRCLK of 16,000,000 / Baude Rate of 460800 / Max. TXError  −2.1 (?) / Max TX Error[%]  4.8 (?) / Max. RX Error −2.5 (?) / Max RX Error [%] 7.3 (?). Please clarify, what is a negative error and define Error %.

-Tom-

  • 0
    Guru 12160 points

    Hi Tom,

    I once duplicated these calculations to figure out which bit gave the listed worst-case error.  Because my calculations gave matching results, I think I understand the meaning.

    The negative errors indicate that the middle sampling time (for RX) or the end of the transmitted bit (for TX) came too early by the given percentage.  The percentage is the percentage of a single bit of perfect length.

    So an RX error of -50% means that the USCI is taking the middle sample at the same instant a perfect transmitter is just starting that bit.  An RX error of +50% means that the USCI is taking the middle sample at the same instant a perfect transmitter is starting the next bit.  An RX error of 0% means that the middle sample is exactly in the middle of the bit from a perfect transmitter.

    (As a side note, the "middle" sample doesn't really exist in 16x oversampling mode.  In that case, the error percentage refers to the time halfway between samples 8 and 9.)

    A TX error of -50% means that the transmitter ended the bit early by half a bit length.  A TX error of +50% means that the transmitter ended the bit late by half a bit length.  The errors are cumulative from the beginning of the start bit, using a perfect transmitter as reference.

    Hope that helps.

    Jeff

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