Sensors

Freescale Semiconductor

10

MMA7456L

LEVEL DETECTION

When in Level or Pulse detection mode, it is not advisable to read the XYZ measurements because this can conflict with timing.

The interrupts for level and pulse detection are at 600 Hz, while measurement mode is at 125 Hz. It is best to exit the pulse/level

mode before taking a measurement on the XYZ.

Both the Level Detection and Pulse Detection modes can trigger an interrupt. Typically one interrupt is assigned to either pulse

detection or level detection. To detect both at the same time 2 interrupts are required. The level detection mechanism has no

timers associated with it. Once a set acceleration level is reached the interrupt pin will go high and remain high until the interrupt

pin is cleared (See Assigning, Clearing & Detecting Interrupts).

By default all three axes are enabled and the detection range is 8g only. X and/or Y and/or Z can be disabled. There is a choice

between detecting an Absolute signal or a Positive or Negative only signal on the enabled axes. There is also a choice between

doing a detection for Motion where X or Y or Z > Threshold vs. doing a detection for Freefall where X& Y & Z < Threshold.

$18: Control 1 (Read/Write) Setting the Detection Axes for X, Y and Z

This allows the user to define how many axes to use for detection. All axes are enabled by default. To disable write 1.

XDA: Disable X

YDA: Disable Y

ZDA: Disable Z

$19: Control 2 (Read/Write) Motion Detection (OR Condition) or Freefall Detection (AND Condition

)

LDPL = 0: Level detection polarity is positive and detecting condition is OR for all 3 axes.

X or Y or Z > Threshold

||X|| or ||Y|| or ||Z|| > Threshold

LDPL = 1: Level detection polarity is negative detecting condition is AND for all 3 axes.

X and Y and Z < Threshold

||X|| and ||Y|| and ||Z|| < Threshold

$18: Control 1 (Read/Write): Setting the threshold to be an integer value or an absolute value

This allows the user to set the threshold to be absolute, or to be based on the threshold value as positive or negative.

THOPT = 0 Absolute; THOPT = 1 Positive Negative

$1A: Level Detection Threshold Limit Value (Read/Write)

When an event is detected the interrupt pin (either INT1 or INT2) will go high. The interrupt pin assignment is set up in Register

$18, discussed in the Assigning, Clearing & Detecting Interrupts section. The detection status is monitored by the Detection

Source Register $0A.

LDTH[7:0]: Level detection threshold value. If THOPT bit in Detection Control Register is “0”, it is unsigned 7 bits value and

LDTH[7] should be “0”. If THOPT bit is “1”, it is signed 8 bits value.

D7

D6

D5

D4

D3

D2

D1

D0

Reg $18

DFBW

THOPT

ZDA

YDA

XDA

INTREG[1]

INTREG[0]

INTPIN

Function

0

000

0000

Default

D7

D6

D5

D4

D3

D2

D1

D0

Reg $19

--

--

--

--

--

DRVO

PDPL

LDPL

Function

00000000

Default

D7

D6

D5

D4

D3

D2

D1

D0

Reg $18

DFBW

THOPT

ZDA

YDA

XDA

INTREG[1]

INTREG[0]

INTPIN

Function

00000000

Default

D7

D6

D5

D4

D3

D2

D1

D0

Reg $1A

LDTH[7]

LDTH[6]

LDTH[5]

LDTH[4]

LDTH[3]

LDTH[2]

LDTH[1]

LDTH[0]

Function

00

000

000

Default