MMA685X datasheet(22/52 Pages) FREESCALE

22 / 52 page

Sensors

Freescale Semiconductor, Inc.

MMA685x

3.3.1

CREG Failure Detection

The digital supply voltage regulator is designed to be unstable with low capacitance. If the connection to the VREG capacitor

becomes open, the digital supply voltage will oscillate and cause either an undervoltage, or overvoltage failure within one internal

sample time. This failure will result in one of the following:

1. The DEVRES flag in the DEVSTAT register will be set. MMA685x will respond to SPI acceleration requests as

defined in Table 27.

2. MMA685x will be held in RESET and be non-responsive to SPI requests.

3.3.2

CREGA Failure Detection

The analog supply voltage regulator is designed to be unstable with low capacitance. If the connection to the VREGA capacitor

becomes open, the analog supply voltage will oscillate and cause either an undervoltage, or overvoltage failure within one internal

sample time. The DEVRES flag in the DEVSTAT register will be set. MMA685xMMA685x will respond to SPI acceleration re-

quests as defined in Table 27.

Note: This feature is only supported with a VCC supply voltage in the range of 4.75V to 5.25V.

3.3.3

VSS and VSSA Ground Loss Monitor

MMA685x detects the loss of ground connection to either VSS or VSSA. A loss of ground connection to VSS will result in a VREG

overvoltage failure. A loss of ground connection to VSSA will result in a VREG undervoltage failure. Both failures result in a device

reset.

3.3.4

SPI Initiated Reset

In addition to voltage monitoring, a device reset can be initiated by a specific series of three write operations involving the

RES_1 and RES_0 bits in the DEVCTL register. Reference Section 3.1.5.1. for details regarding the SPI initiated reset.

3.4

Internal Oscillator

MMA685x includes a factory trimmed oscillator as specified in Section 2.6.

3.4.1

Oscillator Monitor

The COUNT register in the customer accessible array is a read-only register which provides the current value of a free-running

8-bit counter derived from the primary oscillator. A 10-bit pre-scaler divides the primary oscillator by 1024. Thus, the value in the

COUNT register increases by one count every 128

μs, and the register rolls over every 32.768 ms. The SPI master can periodi-

cally read the COUNT register, and verify the difference between subsequent register reads against the system time base.

1. The SPI access rates and deviations must be taken into account for this oscillator verification.

MMA685X bảng dữ liệu(PDF) 22 Page - Freescale Semiconductor, Inc