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3.8.9.4
Arming Pulse Stretch Function
A pulse stretch function can be applied to the arming outputs in moving average mode, or count mode.
If the pulse stretch function is not used (APS_X[1:0] = ’00’ or APS_Y[1:0] = ’00’), the arming output is asserted if and only if
an arming condition exists for the associated axis after the most recent evaluated sample. The arming output is de-asserted if
and only if an arming condition does not exist for the associated axis after the most recent evaluated sample. If the pulse stretch
function is used, (APS_X[1:0] not equal ’00’ or APS_Y[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 for the associated axis
after the most recent evaluated sample. Reference Figure 25 .
The desired pulse stretch time is individually programmable for each axis via the APS_X[1:0] and APS_Y[1:0] bits in the ARM-
CFG 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 28 .
Open Drain, Active High
V CC
Open Drain, Active Low
V CC
Arm Function
Gating
ARM
Arm Function
Gating
ARM
Figure 28. Arming Function - Pin Output Structure
3.8.10 PCM Output Function
MMA68xx 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 proportional
to the respective acceleration onto the PCM_X and PCM_Y pins. A block diagram of the PCM output is shown in Figure 29 .
Exception conditions affect the PCM output as listed in Section 4.5 .
Output Scaling
9
A
CARRY
ARM/PCM
OC[9:1]
9 Bit ADDER
Sample updated every 8 μ S
B
SUM
9
D
D
Q
Q
D
Q
D FF
Q FF
D Q
D FF
Q FF
Q FF
CLK Q
CLK
Q
9
f CLK = 8 MHz
D FF Q
D FF Q
D FF Q
CLK
CLK
CLK
CLK
FF
Q
Q
Q
CLK
CLK
CLK
Q
Q
Q
Figure 29. PCM Output Function Block Diagram
MMA68xx
Sensors
40
Freescale Semiconductor, Inc.
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