MMA685X datasheet(39/52 Pages) FREESCALE

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Freescale Semiconductor, Inc.

MMA685x

3.8.9.4

Arming Pulse Stretch Function

A pulse stretch function can be applied to the arming output in moving average mode, or count mode.

If the pulse stretch function is not used (APS[1:0] = ‘00’), the arming output is asserted if and only if an arming condition exists

after the most recent evaluated sample. The arming output is de-asserted if and only if an arming condition does not exist after

the most recent evaluated sample.

If the pulse stretch function is used, (APS[1:0] not equal ‘00’), the arming output is controlled only by the value of the pulse

stretch timer value. If the pulse stretch timer value is non-zero, the arming output is asserted. If the pulse stretch timer is zero,

the arming output is de-asserted. The pulse stretch counter continuously decrements until it reaches zero. The pulse stretch

counter is reset to the programmed pulse stretch value if and only if an arming condition exists after the most recent evaluated

sample. Reference Figure 26

The desired pulse stretch time is programmable for via the APS[1:0] bits in the ARMCFG register.

Exception conditions listed in Section 4.5 do not impact prior arming function responses. If an exception occurs after an arming

activation, the corresponding pulse stretch for the existing arming condition will continue. However, new acceleration reads will

not reset the pulse stretch counter regardless of the acceleration value.

3.8.9.5

Arming Pin Output Structure

The arming output pin structure can be set to active high, or active low with the A_CFG bits in the DEVCFG register as de-

scribed in Section 3.1.6.5. The active high and active low pin output structures are shown in Figure 29.

Figure 29. Arming Function - Pin Output Structure

3.8.10 PCM Output Function

MMA685x provides the option for a PCM output function. The PCM output is enabled by setting the A_CFG bits in the

DEVCFG register to the appropriate state as described in Section 3.1.6.5. When the PCM function is enabled, the upper 9 bits

of the 10-bit, offset cancelled, output scaled acceleration values are used to generate 8 MHz Pulse Code Modulated signals pro-

portional to the acceleration onto the PCM pin. A block diagram of the PCM output is shown in Figure 30.

Exception conditions affect the PCM output as listed in Section 4.5.

Figure 30. PCM Output Function Block Diagram

Arm Function

ARM

Gating

VCC

Arm Function

ARM

Gating

VCC

Open Drain, Active High

Open Drain, Active Low

Output Scaling

OC[9:1]

9 Bit ADDER

ARM/PCM

CARRY

SUM

fCLK = 8 MHz

Sample updated every 8

μS

CLK

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