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3.8.10
Arming Function
The device 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.
3.8.10.1 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.
ARM_MA n = (OC n + OC n-1 + ... + OC n+1-k )/k
Where n is the current sample.
The sample rate is determined by the SPI acceleration data sample rate. At the rising edge of CS for an acceleration data SPI
request, the moving average is updated with a new sample. Reference Figure 28 . The SPI acceleration data sample rate must
meet the minimum time between requests (t ACC_REQ_x ) specified in Section 2.5 .
The moving average output is compared against positive and negative 8-bit thresholds that are programmed via the ARMT_x
registers. Reference Section 3.1.10 for register details. If the moving average equals or exceeds either threshold, an arming con-
dition is indicated, the ARM output is asserted for the associated axis, and the pulse stretch counter is set as described in
Section 3.8.10.4 .
The ARM output is de-asserted only when the pulse stretch counter expires. Figure 28 shows the arming output operation for
different SPI conditions.
ARMT_P[7:0]
AWS_P[1:0]
Positive
Moving Average
Offset Cancellation
OffCanc_ARM[10:0]
Pulse Stretch
Gating
I/O
ARM
Negative
AWS_N[1:0]
Moving Average
ARMT_N[7:0]
APS[3:0]
Figure 26. Arming Function Block Diagram - Moving Average Mode
The moving average window size must be set prior to setting the arming function to moving average mode, or prior to request-
ing acceleration data via the SPI. If the moving average window size is changed after enabling moving average mode, the arming
function must first be disabled by setting the A_CFG bits to “000”. Once the desired moving average window size is set, the mov-
ing average mode can be re-enabled.
MMA655x
Sensor
36
Freescale Semiconductor, Inc.
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