MMA685X datasheet(37/52 Pages) FREESCALE

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Sensors

Freescale Semiconductor, Inc.

MMA685x

3.8.9.2

Arming Function: Count Mode

In count mode, the arming function compares each input sample against positive and negative thresholds that are pro-

grammed via the ARMT_x registers. Reference Section 3.1.9 for register details. If the sample equals or exceeds either thresh-

old, a sample counter is incremented. If the sample does not exceed either threshold, the sample counter is reset to zero.

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

response, a new sample is compared against the thresholds. Reference Figure 26. The SPI acceleration data sample rate must

meet the minimum time between requests (tACC_REQ_x) specified in Section 2.5.

A sample count limit is programmable via the AWS_x[1:0] bits in the ARMCFG register. If the sample count reaches the pro-

grammable sample count limit, an arming condition 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 25. Arming Function Block Diagram - Count Mode

Figure 26. MMA685x Arming Condition, Moving Average and Count Mode

ARMT_N[7:0]

Offset Cancellation

AWS_P[1:0]

APS[1:0]

Pulse Stretch

ARM

ARMT_P[7:0]

Gating

I/O

1-4 Sample

Counter

OffCanc_ARM[10:0]

X-Axis Arm Condition

Not Present

X-Axis Data Latched for

Arm Function and SPI

SCLK

MOSI

MISO

Request X-Axis

X-Axis Response

Request X-Axis

X-Axis Response

ARM

X-Axis Response

X-Axis Arm Condition

Not Present

X-Axis Arm Condition

Not Present

X-Axis Arm Condition

Present

tARM

Pulse Stretch Time

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