MMA6525KW datasheet(38/56 Pages) FREESCALE

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Sensor

Freescale Semiconductor, Inc.

MMA65xx

3.8.10.2

Arming Function: Count Mode

In count mode, the arming function compares each offset cancelled sample against positive and negative 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 sample equals or exceeds either threshold, a sample counter is incremented. If the sample does not exceed either thresh-

old, the sample counter is reset to zero.

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, a new sample for the associated axis is compared against the thresholds. Reference Figure 28. The

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

A sample count limit is programmable via the AWS_Xx[1:0] and AWS_Yx[1:0] bits in the ARMCFGX and ARMCFGY registers.

If the sample count reaches the programmable sample count limit, 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 27. Arming Function Block Diagram - Count Mode

Figure 28. X and Y Axis Arming Conditions, Moving Average and Count Mode

ARMT_xN[7:0]

Offset Cancellation

AWS_xP[1:0]

APS_x[1:0]

Pulse Stretch

ARM_x

ARMT_xP[7:0]

Gating

I/O

1-4 Sample

Counter

OffCanc_ARM_x[10:0]

Y-Axis Arm Condition

Present

SCLK

MOSI

MISO

Request X-Axis

Request Y-Axis

X-Axis Response

Y-Axis Response

Request X-Axis

Request Y-Axis

X-Axis Response

ARM_X

ARM_Y

Y-Axis Response

Y-Axis Arm Condition

Not Present

X-Axis Arm Condition

Not Present

X-Axis Arm Condition

Present

Y-Axis Data

Latched for

tARM

Y-Axis Pulse Stretch

X-Axis Pulse Stretch

Arm & SPI

X-Axis Data

Latched for

Arm & SPI

X-Axis Data

Latched for

Arm & SPI

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