MMA6519KW datasheet(37/56 Pages) FREESCALE

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Sensor

Freescale Semiconductor, Inc.

MMA65xx

3.8.10

ARMING FUNCTION

The device provides the option for an arming function with 3 modes of operation. The operation of the arming function is se-

lected by the state of the A_CFG bits in the DEVCFG register.

Reference Section 4.5 for the operation of the Arming function with exception conditions. Error conditions do not impact prior

arming function responses. If an error occurs after an arming activation, the corresponding pulse stretch for the existing arming

condition will continue. However, new acceleration reads will not update the arming function regardless of the acceleration value.

3.8.10.1

Arming Function: Moving Average Mode

In moving average mode, the arming function runs a moving average on the offset cancelled output of each acceleration axis.

The number of samples used for the moving average (k) is programmable via the AWS_Xx[1:0] and ARM_Yx[1:0] bits in the

ARMCFGX and ARMCFGY registers. Reference Section 3.1.8 for register details.

ARM_MAn = (OCn + OCn-1 + ... + OCn+1-k)/k

Where n is the current sample.

The sample rate for each axis is determined by the SPI acceleration data sample rate. At the rising edge of CS for an accel-

eration data SPI request, the moving average for the associated axis is updated with a new sample. Reference Figure 28. The

SPI acceleration data sample rate must meet the minimum time between requests (tACC_REQ_x) specified in Section 2.6.

The moving average output is compared against positive and negative 8-bit thresholds that are individually programmed for

each axis via the ARMT_Xx and ARMT_Yx registers. Reference Section 3.1.10 for register details. If the moving average equals

or exceeds either threshold, an arming condition is indicated, the ARM_X or ARM_Y output is asserted for the associated axis,

and the pulse stretch counter is set as described in Section 3.8.10.4.

The ARM_X or ARM_Y output is de-asserted only when the pulse stretch counter expires. Figure 28 shows the arming output

operation for different SPI conditions.

Figure 26. Arming Function Block Diagram - Moving Average Mode

The moving average window size must be set prior to setting the arming function to moving average mode, or prior to request-

ing acceleration data via the SPI. If the moving average window size is changed after enabling moving average mode, the arming

function must first be disabled by setting the A_CFG bits to “000”. Once the desired moving average window size is set, the mov-

ing average mode can be re-enabled.

Offset Cancellation

AWS_xP[1:0]

APS_x[1:0]

Pulse Stretch

ARM_x

Gating

I/O

ARMT_xN[7:0]

ARMT_xP[7:0]

Moving Average

Positive

Moving Average

Negative

AWS_xN[1:0]

OffCanc_ARM_x[10:0]

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