ADCMXL3021 datasheet(21/50 Pages) AD

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Data Sheet

ADcmXL3021

Rev. 0 | Page 21 of 50

Busy Signal

The factory default configuration provides the user with a busy

signal (BUSY), which pulses low when the output data registers

are updating (see signal orientation of busy signal in Figure 46).

In this configuration, connect BUSY to an interrupt service pin

on the embedded processor, which triggers data collection,

when this signal pulses high.

BUSY

AVAILABLE

SPI

COMMAND

NOT AVAILABLE

Figure 46. Busy Signal (BUSY) Orientation after SPI command

During the start-up and reset recovery processes, the BUSY

signal can exhibit some transient behavior before data production

begins. Figure 46 provides an example of the BUSY behavior

during command processing. A low signal indicates SPI access

is not available with the exception of the escape code that can

terminate a capture. Figure 47 shows the BUSY signal during

power up.

VDD

BUSY

START-UP TIME

~200ms

TIME THAT VDD > 3V

MICROCONTROLLER IS BUSY

WHEN BUSY IS LOW

Figure 47. BUSY Response During Startup

RTS

The RTS function provides a method for reading data (time

domain acceleration data for each axis, temperature, status, and

CRC code) that does not require a stall time between each

16-bit segment and only requires one command on the DIN line

to initiate. System processors can execute this mode by reading

each segment of data in the response, while holding the CS line

in a low state, until after reading the last 16-bit segment of data.

If the CS line goes high before the completion of all data

acquisition, the data from that read request is lost.

The RTS burst contains 100 16 bits words (a header, including a

incrementing counter, 32 x-axis samples, 32 y-axis samples, 32

z-axis samples, temperature, status, and CRC). The external SCLK

rate must be between 12.5 MHz to 14 MHz to ensure the complete

burst is read out before current data in the register buffer is

overwritten. The maximum SCLK for RTS burst outputs is

14 MHz ± 1%. The minimum SCLK required to support the

transfer is 12.5 MHz. The RTS burst response uses the

sequencing diagrams shown in Figure 4 and Figure 5 and the

data format shown in Table 19.

When first entering RTS mode capture, the first eight samples

are all 0s and the CRC for the first frame is invalid. It is

recommended that the first frame be ignored and that the

data for the second frame and all subsequent frames be used.

Table 19. RTS Data Format

Byte Location in

Output Dataset

2-Byte Value Represents

Fixed header: 0xccAD; where cc is an

incrementing counter value from 0x00 to

0xFF, which returns to 0x00 after 0xFF

X-axis data [0] (oldest data from x-axis)

X-axis data [1]

X-axis data [2]

…

X-axis data [31]

Y-axis data [0] (oldest data from y-axis)

Y-axis data [1]

…

128

Y-axis data [31]

130

Z-axis data [0] (oldest data from z-axis)

132

Z-axis data [1]

134

Z-axis data [2]

…

192

Z-axis data [31]

194

Temperature

196

Status

198

CRC-16

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