L9678P datasheet(137/203 Pages) STMICROELECTRONICS

137 / 203 page

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L9678P, L9678P-S

DC sensor interface

202

The voltage and current for the selected channel are made available to the main ADC by

selecting the proper channel and enabling the measurement process by dedicated

DIAGCTRLx commands.

The device offers the capability to actively keep all the DCSx lines discharged by means of a

weak pull down. The pull down is active by default on all channels and it comes to be

deactivated in either of the following cases:

when the voltage source is active on the relevant channel

when a voltage measurement is requested on the relevant channel

if SPI bit SWCTRL(DCS_PD_CURR) is set (global pull-down disable for all channels)

In case of Hall-effect sensors, a single current measurement is processed. The current load

needed for regulating the pin is internally reflected to a reference resistance, whose voltage

drop is then measured through the internal ADC converter.

When resistive or switch sensors are used, a more complex measurement is performed. In

a first step the current information as above described is provided. Then, also the

information on the voltage level achieved on the output pin is provided via ADC. By

processing these two values, the micro-controller can understand the resistive value. The

DCSx voltage is internally rescaled by a voltage divider into the ADC converter voltage

range as shown in Figure 38. Additionally a positive voltage offset is internally applied to the

scaled voltage in order to allow voltage measurement capability for DCSx down to -1V.

In order to have accurate resistive information even in case of an external ground voltage

shift on the sensor of up to ±1V, the voltage measurement step actually needs two DCSx

voltage measurements. A first voltage measurement has to be done with selection of 6.25 V

on the output channel and a second one with the regulator switched off. The difference

between the two measurements will cancel out the offsets (both external ground shift and

internal offset).

The DCSx current and voltage can be retrieved from ADC readings according to the

following formulas and related parameters specified in Section 15.19: DC sensor interface

and Section 15.23: Voltage diagnostics (analog Mux):

IDCSx

RREF1_IDCSx

------------------------------------

ADCREF_hi

ADCRES

------------------------------- DIAGCTRLn ADCRESn







@DIAGCTRL(ADCREQn) = $04

VDCSx

RATIOVDCSx

ADCREF_hi

ADCRES

------------------------------- DIAGCTRLn ADCRESn

 V

OFF_DCSx

–













@DIAGCTRL(ADCREQn = $03

The DCSx sensor resistance can be calculated according to the following formula:



Rsensor

VDCSx



IDCSx

----------------------

VDCSx@(SWCTRL(SWOEN)=1 VDCSx@(SWCTRL(SWOEN)=0

–

IDCSx

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



@SWCTRL(CHID) = x

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