Ensuring data integrity in a noisy world

     A CRC is generated through a calculation that’s done on the data and then transmitted with the data. The receiver can do the same calculation and then compare the CRCs. If the CRCs match, the data is most likely good. Many communication schemes use CRC because it’s easy to implement on the transmitting side and analyze on the receiving side.

     In this example, we’ll use the LMP90100 to show how the CRC is generated. Data is sent out of the ADC on a serial bus in 24 bit pieces for each conversion of the ADC. An 8-bit CRC can be sent with the data when the LMP90100 is used in a noisy environment. The CRC transmission can also be turned off when it is not needed.

     A generator polynomial is used to calculate the CRC. This polynomial is one bit longer than the resulting CRC. The LMP90100 uses a polynomial of x⁸ + x⁵ + x⁴ + 1. It has a degree of 8, which means the polynomial has 9 terms and the CRC will have 8 bits. Commonly used polynomials range from a degree of 1, (x + 1) which will give you a parity bit, to a degree of 64. [USB uses a polynomial with a degree of 5 (x⁵ + x² + 1), Bluetooth uses a polynomial with a degree of 16 (x¹⁶ + x¹² + x⁵ + 1), and Ethernet uses a 32 degree polynomial. ]

     The polynomial can be represented in binary terms for the CRC calculation by listing its coefficients. If this is done to the LMP90100 polynomial we get 100110001 (the coefficient of x⁸ is 1, the coefficient of x⁷ is 0, and so on). The CRC is generated using the following method.

1. The data is left-shifted by the same amount of bits as the size of the CRC, and 0s are added on the right side. For example, if the 24-bit ADC data from the LMP90100 is 010101011000100101001011

and the CRC from the LMP90100 is 8 bits, the data is shifted left 8 bits and 8 zeros are added on the right side. The data for the CRC calculation would  be: 01010101100010010100101100000000.

1. The polynomial in binary form is put underneath the modified data. The MSB of the polynomial is put under the left-most 1 of the data.                                                               

01010101100010010100101100000000                                                                                                                                                0
  100110001

1. A XOR (exclusive-OR) function is applied to the data and the polynomial. The remaining data to the right of the polynomial is brought down. The polynomial is once again put underneath the left-most 1 and the data is again XORed. This continues until the left-most 1 of the remainder is in the 8th or less bit position. Figure 1 shows the entire process for the three bytes of data we are using.

Figure 1: Generation of a CRC

(click here to see enlarged image).

     Once the CRC has been calculated, it can be sent with the conversion data. The LMP90100, however, does not send the actual CRC, it sends the one’s complement of the calculated CRC. This is done because if the data is 0x000000, the CRC will be 0x00. The resulting data thatm is sent is 0x00000000 (three bytes of data and CRC).

     This could be cause for concern (especially if this data set of all zeros was repeatedly sent for a length of time) as perhaps something happened to the data line, such as a short to ground. The one’s complement of 0x00 is 0xFF, which means that the four bytes sent has become 0x000000FF. This lets the microcontroller know that the data line is good.

     The ADC data in the LMP90100 is located in registers 0x1A, 0x1B and 0x1C. The CRC, in ones complement form, is located in register 0x1D. This makes it easy for the microcontroller to read this data because it starts at register 0x1A and simply reads a block of four bytes.

     Once the microcontroller has the data and CRC, it can then do the same calculation on the data. If the CRCs match the data, it’s most likely good and can be used. If the CRCs do not match, the data can be discarded and the microcontroller can ask for the data again, or just wait for the next set of data.

Conclusion

     When used in an industrial sensing application the integrity of the data is critically important. The LMP90100 sensor AFE gives the designer a proven method of checking that the microcontroller is receiving good data by using a CRC. This approach ensures data integrity and a more robust system, even when used in a noisy environment.

About the Author

Mike Stout is an applications manager for Texas Instruments Precision Systems group, focused on precision signal chain products. Previously, Stout worked as an applications engineer in National Semiconductor’s CRT and Digital Television Products group. Stout received his Bachelor of Science degree in Electrical Engineering from Brigham Young University.

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