L9679ETR datasheet(87/127 Pages) STMICROELECTRONICS

87 / 127 page

8.1.2

Sensor data integrity: LCID and CRC

Each RSDRx data register contains a Logical Channel ID which is a 4/2-bit field for remote sensors used to link

the received data to the corresponding logical channel number.

In case of asynchronous mode, the two LSB of the LCID field are stuck to “00”.

Each RSDRx register contains also a CRC bit field computed on the data packet for data integrity check. To

satisfy functional safety requirements, the LCID, DATA and CRC bit fields propagate through the same data path

as a single item to the SPI output.

The polynomial calculation implemented for PSI-5 data is described as in PSI-5 specification

g(x)=1+x+x^3 with initialization value equal to "111".

These are the equations to calculate the CRC in combinatorial way:

CRC[2] = CRCext[0]+D[0]+D[1]+D[3]+D[6]+D[7]+D[8]+D[10]+D[13]+D[14]+D[15]

CRC[1] = CRCext[2]+D[0]+D[1]+D[2]+D[4]+D[7]+D[8]+D[9]+D[11]+D[14]+D[15]+D[16]

CRC[0] = CRCext[1]+CRCext[0]+D[0]+D[2]+D[5]+D[6]+D[7]+D[9]+D[12]+D[13]+D[14]+D[16]

Where D[16:0]= RSDR[16:0] and CRCext[n] are the starting seed values (all '1').

8.1.3

Detailed description

Manchester decoding

The Manchester decoder will support remote sensor communication as per PSI specification rev 1.3 for the

modes configurable via the STS bits in the RSCRx registers. The Manchester decoder checks the duty-cycle and

period of the start bits to determine their validity, depending on the configuration of the PERIOD_MEAS_DISABLE

bit in the RSCRx registers. The expected time windows for the mid bit transitions of each subsequent bit within

the received frame are determined by means of the internal oscillator time base. Glitches shorter than 25% of the

minimum bit time duration are rejected.

A Manchester decoder error occurs if one or more of the following are true:

•

Two valid start bits are detected, and at least one of the expected 13 mid-bit transitions are not detected;

•

Two valid start bits are detected, and more than 13 mid-bit transitions are detected;

•

When the number of bits decoded is incorrect (either too many or too few), a bit error is indicated. When

any bit error is detected (bit time, too many bits, too few bits), the decoder will revert to the minimum bit

time of the selected range and the message is discarded.

All errors are readable through the Sensor fault status register and the RSFLT bit in the Global status word

When a valid message is correctly decoded, the 10/8 data bits are stored into the appropriate RSDRx register

together with the related LCID. The RSDRx register contains the 10/8 bits data as they are received from the

sensor (no data range check/mask is done at this stage). The 8-bit data word is right-justified inside the 10-bit

data field in the RSDRx registers.

Current sensor w/ auto-adjust trip current

The current sensor is responsible for translating the current drawn by the sensor into a digital state. Each remote

sensor channel has a dedicated current sensor.

The current flowing through the RSU power stage is internally downscaled by a factor 100, sent to a 10 bits A/D

converter and digitally processed to extract both the sensor quiescent and delta currents.

The delta current threshold for signal detection is auto-adjusted to the actual calculated sensor delta current.

The current trip point is dynamically determined by adding the delta current threshold (auto-adjusted) to the

quiescent current (auto-adjusted). The RSU current is compared against the current trip point to determine the

current demodulator digital output. A logic ‘1’ represents the sensor current above the current trip point. The

current demodulator output is fed into the Manchester decoder.

Thanks to the quiescent and delta current tracking features the receiver is capable of automatically adapting to

different nominal sensor currents and/or of being tolerant to sensor current drifts over lifetime.

Compared with other products of the same family (L9679 and L9680), L9679E follows only one approach to

adjust base and delta current. This approach can be selected also in L9679 and L9680 by setting high the AVG/

SSDIS bit of RSCRx.

L9679E

Remote sensor interface

DS12485 - Rev 8

page 87/127

L9679ETR Datasheet(PDF) 87 Page - STMicroelectronics

L9679ETR Datasheet(HTML) 87 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL