Datasheet

1/ODR
1/ODR
CH0 CH1
SDO
LSB
D6 = drdyb
Reading
ADC_DOUT of CH0
MSB
MSB
LSB
Reading
ADC_DOUT of CH1
Reading
SPI_CRC_DAT
MSB
MSB
LSB
LSB
Reading
SPI_CRC_DAT
1/ODR
1/ODR
Sampling CH0 Sampling CH1
SDO
LSB
D6 = drdyb
Reading
ADC_DOUT of CH0
Reading
SPI_CRC_DAT
LSB
MSB
MSB
LSB
Reading
ADC_DOUT of CH1
Reading
SPI_CRC_DAT
LSB
MSB
MSB
LMP90077, LMP90078, LMP90079, LMP90080
SNAS521F –JULY 2011–REVISED MARCH 2013 www.ti.com
Cyclic Redundancy Check (CRC)
CRC can be used to ensure integrity of data read from LMP900xx. To enable CRC, set EN_CRC high. Once
CRC is enabled, the CRC value is calculated and stored in SPI_CRC_DAT so that the master device can
periodically read for data comparison. The CRC is automatically reset when CSB or DRDYB is deasserted.
The CRC polynomial is x
8
+ x
5
+ x
4
+ 1. The reset value of the SPI_CRC_DAT register is zero, and the final
value is ones-complemented before it is sent out. Note that CRC computation only includes the bits sent out on
SDO and does not include the bits of the SPI_CRC_DAT itself; thus it is okay to read SPI_CRC_DAT repeatedly.
The drdyb signal normally deasserts (active high) every 1/ODR second. However, this behavior can be changed
so that drdyb deassertion can occur after SPI_CRC_DAT is read, but not later than normal DRDYB deassertion
which occurs at every 1/ODR seconds. This is done by setting bit DRDYB_AFT_CRC high.
The timing protocol for CRC can be found in Figure 55.
Figure 55. Timing Protocol for Reading SPI_CRC_DAT
If SPI_CRC_DAT read extends beyond the normal DRDYB deassertion at every 1/ODR seconds, then
CRC_RST has to be set in the SPI Data Ready Bar Control Register. This is done to avoid a CRC reset at the
DRDYB deassertion.Timing protocol for reading CRC with CRC_RST set is shown in Figure 56.
Figure 56. Timing Protocol for Reading SPI_CRC_DAT beyond normal DRDYB deassertion at every
1/ODR seconds
Follow the steps below to enable CRC:
1. Set SPI_CRC_CN = 1 (register 0x13, bit 4) to enable CRC.
2. Set DRDYB_AFT_CRC = 1 (register 0x13, bit 2) to dessert the DRDYB after CRC.
3. Compute the CRC externally, which should include CH_STS, ADC_DOUTH, ADC_DOUTM , and
ADC_DOUTL.
4. Collect the data and verify the reported CRC matches with the computed CRC (step above).
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