Sensors

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Freescale Semiconductor, Inc.

MMA685x

3.8.9

Arming Function

MMA685x 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.

3.8.9.1

Arming Function: Moving Average Mode

In moving average mode, the arming function runs a moving average on the offset cancelled output. The number of samples

used for the moving average (k) is programmable via the AWS_x[1:0] bits in the ARMCFGX register. Reference Section 3.1.8 for

register details.

Where n is the current sample.

The sample rate is determined by the SPI acceleration data sample rate. At the falling edge of CS for an acceleration data SPI

response, the moving average is updated with a new sample. Reference Figure 26. The SPI acceleration data sample rate must

The moving average output is compared against positive and negative 8-bit thresholds that are programmed via the ARMT_x

registers. Reference Section 3.1.9 for register details. If the moving average equals or exceeds either threshold, an arming con-

dition is indicated, the ARM output is asserted, and the pulse stretch counter is set as described in Section 3.8.9.4.

The ARM output is de-asserted only when the pulse stretch counter expires. Figure 26 shows the arming output operation for

different SPI conditions.

Figure 24. Arming Function Block Diagram - Moving Average Mode

Offset Cancellation

AWS_P[1:0]

APS[3:0]

Pulse Stretch

ARM

Gating

I/O

ARMT_N[7:0]

ARMT_P[7:0]

Moving Average

Positive

Moving Average

Negative

AWS_N[1:0]

OffCanc_ARM[10:0]