Freescale Semiconductor
Document Number: AN4296
Rev 1, 02/2013
Application Note
Data Manipulation and the Basic
Settings of Xtrinsic MMA8491Q
Accelerometer
by: Fengyi Li, Applications Engineer
This document shows you how to configure the
MMA8491Q to extract and manipulate acceleration data.
The MMA8491Q is a straightforward implementation
for taking acceleration samples, and enables you to take
samples at any time or to stream data at any chosen
sampling rate, particularly at an extremely low speed.
You can drive this device with one GPIO pin and one
timer from the host microcontroller. The manipulation of
the data into different formats is important for algorithm
development and for display.
This application note accompanies the MMA8491
Driver Code, and explains:
• Configuring the MMA8491Q as a tilt sensor
• Polling XYZ accelerometer data
• Streaming data
• Setting the sampling rate
• Setting when the acceleration data is read
• Changing the accelerometer data format (hex to
counts or to decimal numbers)
• Installing/using the MMA8491 driver
© 2012-2013 Freescale Semiconductor, Inc. All rights reserved.
Contents
1
2
3
4
5
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 MMA8491Q data sampling operations . . . . . . . . . . 2
Configuring the MMA8491Q . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Tilt sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Additional accelerometer XYZ data polling . . . . . . . 4
Streaming Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Stream data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Setting the sample rate . . . . . . . . . . . . . . . . . . . . . . 9
3.3 Setting when the acceleration data is read . . . . . . 10
Converting 14-bit data . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1 Converting the 14-bit 2's complement hex number to a
signed integer (counts) 12
4.2 Converting the 14-bit 2's complement hex number to a
signed decimal fraction (in g's) 14
Using the MMA8491 HyperTerminal Driver. . . . . . . . . . 20
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2 Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1
Introduction
The MMA8491Q device is an industrial, low voltage, low power 3-axis multifunctional accelerometer and
tilt sensor with digital output, housed in a 3 mm 3 mm 1 mm QFN package (0.65 mm pitch). The
device can accommodate two accelerometer configurations:
• As an easy-to-implement 45° tilt sensor (out of the box),
using one line output per axis.
• As a digital (I2C) output accelerometer with I2C serial communication ports,
14-bit ±8 g raw data can be read from the device with a 1 mg/LSB sensitivity.
Features
• Sensor g-range: ±8 g
• Fixed oversampling rate: 2
• Digital resolution: 14-bit
• Resolution: 1 mg/LSB
NC
NC
12
11
Byp
1
10
Xout
VDD
2
9
Yout
SDA
3
8
Zout
EN
4
7
Gnd
5
6
SCL Gnd
Figure 1. Pins of MMA8491Q
There is a driver available that runs on the MMA8491 Sensor Toolbox kit, which provides an example in
CodeWarrior for everything discussed in this application note. The driver may be used with RealTerm or
HyperTerminal, to capture and log data in different formats.
1.1
MMA8491Q data sampling operations
1. The accelerometer is turned on at the rising edge of the EN pin, and acquires one sample for each
of the three axes.
2. Samples are acquired, converted, and compensated for zero-g offset and gain errors, and then
compared to an internal threshold value of 0.688 g and stored.
3. If the absolute value of any of the X, Y, Z axis samples exceeds this threshold, then the
corresponding outputs on these axes drive logic Highs, and vice versa. (If the absolute value of any
of the X, Y, Z axis samples are equal to or lower than this threshold, then the corresponding outputs
on these axes drive logic lows.) These signals can be used as tilt event triggers to MCUs, and
indicate that new data is ready.
4. Valid accelerometer data is stored into Data Registers. To determine whether the new sample data
is ready, read Status Register 0x00.
5. To take a sample, pull the EN pin High.
To acquire the next sample, deassert the EN pin and then reassert the EN pin. This toggle of the EN
pin can be easily done by using a GPIO pin (or a timer output) from the host microcontroller.
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•
•
NOTE
Send a rising edge to the EN pin to take one sample.
The EN pin should not be asserted before VDD reaches 1.65 V.
Generate a rising edge to EN pin,
to generate 1 new sample (for example, use PortC pin2)
PTCD_PTCD2 = 1
Now wait for the minimum turn-on time tON for valid data
outputs.
For the tON value, see the PMMA8491Q data sheet.
For the tilt results on the X, Y, and Z axes, read pins Xout,
Yout, and Zout.
• When 45 tilt is detected, then output = 1.
• When 45 tilt is not detected, then output = 0.
New sample
After reading the accelerometer results on I2C bus, deassert
the EN pin to prepare for the next rising edge (and to save
power).
PTCD_PTCD2 = 0
Figure 2. Sampling accelerometer data using MMA8491Q
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3
2
Configuring the MMA8491Q
2.1
Tilt sensor
MMA8491Q is an out-of-the-box 45° tilt sensor. The corresponding tilt sensing results are output on the
XOUT, YOUT, and ZOUT pins. These pins are set to 1 when the tilt greater than 45° is sensed.
Assert the EN pin to acquire a new tilt result. The tilt sensing flow is the same as in Figure 2. This toggle
of the EN pin can be easily done by using a software controller GPIO pin (or a timer output) from the host
microcontroller. Both cases are described in Section 3, “Streaming Data,” on page 8.
2.2
Additional accelerometer XYZ data polling
The MMA8491Q accelerometer data output can be read via the I2C communication bus, using an I2C
address of 0x55. The MMA8491Q data can be polled whenever the device is in (should be) ACTIVE mode
(EN pin remains high after a sample is taken. For details about operational modes, see the data sheet.). It
does not have a dedicated data ready pin (like other general purpose accelerometers do).
However, users might find this case interesting. They can use XOUT, YOUT, ZOUT pins as interrupts, to
trigger a data reading right after the tilting events. To quickly read out the accelerometer data via I2C
protocol, follow the procedure in Figure 3.
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SlaveAddressIIC = 0xAA;
(I2C address is 0xAA)
Send a rising edge to EN pin, to generate 1 new sample (for
example, use PortC pin2)
PTCD_PTCD2 = 1;
Now wait for the minimum turn-on time tON for valid data
outputs.
For the tON value, see the PMMA8491Q data sheet.
Read Status register.
RegisterFlag.Byte =
IIC_RegRead(SlaveAddressIIC, 0x00);
Are the data for all axes ready?
if(RegisterFlag.ZYXDR_BIT ==1);
No
Yes
New sample
Read MSB_X, LSB_X, MSB_Y, LSB_Y, MSB_Z, LSB_Z registers,
either individually or in bursts.
IIC_RegReadN(SlaveAddressIIC, OUT_X_MSB_REG, 6, &value[0]);
Or
MSB_X = IIC_RegRead(SlaveAddressIIC, 0x01);
LSB_X = IIC_RegRead(SlaveAddressIIC, 0x02);
MSB_Y = IIC_RegRead(SlaveAddressIIC, 0x03);
LSB_Y = IIC_RegRead(SlaveAddressIIC, 0x04);
MSB_Z = IIC_RegRead(SlaveAddressIIC, 0x05);
LSB_Z = IIC_RegRead(SlaveAddressIIC, 0x06);
After reading the tilt results on I2C bus, deassert the EN pin to
prepare for the next rising edge (and to save power).
PTCD_PTCD2 = 0;
Figure 3. Reading data quickly over I2C
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To determine if the data is ready, read the status bit XYZDR (XYZ data ready) bit:
• When new data is ready, the data ready bits are set to 1.
• After the corresponding data registers are read, the data ready bits are updated to 0.
Bit
Address offset
Register
0x00
STATUS
7
6
5
4
3
2
1
0
0
0
0
0
ZYXDR
ZDR
YDR
XDR
Table 1. STATUS register
Field
Description
ZYXDR
X, Y, Z-axis new Data Ready
ZYXDR signals that a new sample for all channels is available.
• ZYXDR is cleared when the high-bytes of the acceleration data (OUT_X_MSB, OUT_Y_MSB, OUT_Z_MSB)
of all channels are read.
0 No new set of data ready (default)
1 A new set of data is ready
ZDR
Z-axis new Data Available
ZDR is set whenever a new acceleration sample related to the Z-axis is generated.
• ZDR is cleared any time that the OUT_Z_MSB register is read.
0 No new Z-axis data is ready (default)
1 A new Z-axis data is ready
YDR
Y-axis new Data Available
YDR is set whenever a new acceleration sample related to the Y-axis is generated.
• YDR is cleared any time that the OUT_Y_MSB register is read.
0 No new Y-axis data ready (default)
1 A new Y-axis data is ready
XDR
X-axis new Data Available
XDR is set whenever a new acceleration sample related to the X-axis is generated.
• XDR is cleared any time that the OUT_X_MSB register is read.
0 No new X-axis data ready (default)
1 A new X-axis data is ready
After reading the axes data, you should clear the EN pin for two good reasons:
• To prepare for the EN pin rising edge
• To preserve the power
Depending on your application, you can either clear the EN pin, or de-assert the power and EN pins. For
more information about saving power, see the MMA8491Q data sheet.
Example 1. MMA8491Q data polling code in main.c
for(;;)
{
/* Use the microcontroller GPIO PortC2 pin to drive MMA8491Q EN pin */
PTCD_PTCD2 = 1; // pull EN pin HIGH
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/* Poll data via I2C if data is ready */
RegisterFlag.Byte = IIC_RegRead(SlaveAddressIIC, STATUS_00_REG);
if (RegisterFlag.ZYXDR_BIT == 1)
{
/* Read the XYZ sample data*/
IIC_RegReadN(SlaveAddressIIC, OUT_X_MSB_REG, 6, &value[0]);
/*
//
//
//
//
//
//
Or read individual registers */
MSB_X = IIC_RegRead(SlaveAddressIIC,
LSB_X = IIC_RegRead(SlaveAddressIIC,
MSB_Y = IIC_RegRead(SlaveAddressIIC,
LSB_Y = IIC_RegRead(SlaveAddressIIC,
MSB_Z = IIC_RegRead(SlaveAddressIIC,
LSB_Z = IIC_RegRead(SlaveAddressIIC,
0x01);
0x02);
0x03);
0x04);
0x05);
0x06);
}
/* Use the microcontroller GPIO PortC2 pin to drive MMA8491Q EN pin */
PTCD_PTCD2 = 0; // pull EN pin LOW
}
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3
Streaming Data
In this section, a suggested flow diagram is presented. It uses a real-time clock (RTC) to time the
MMA8491 device’s data streaming events: assert the EN pin to start to sample, wait for data register to be
ready to read, and set the sampling rate. The flow is the expansion of Figure 3 and shown in Figure 4.
Start Sampling
RTC Interrupt Routine
Enable RTC
RTC Service Routine
Is Data Ready
flag set?
Yes
Start RTC
service routine
No
TMR_Counter++
Read Status register.
RegisterFlag.Byte =
Are the data
for all axes ready?
if(RegisterFlag.ZYXDR_BIT
==1);
Is this the start of
Sampling cycle?
TMR0_Counter
case 1
Yes
Pull EN pin High
PTCD2 = 1; (pin 7)
Yes
Set up Data Ready flag
to enable data reading
DataReadyFlag = 1;
Yes
Clear TMR_Counter to
start next sampling cycle
TMR_Counter = 0;
No
No
Yes
Read MSB_X, LSB_X, MSB_Y, LSB_Y, MSB_Z,
LSB_Z registers, either individually or in bursts.
IIC_RegReadN(SlaveAddressIIC,
OUT_X_MSB_REG, 6, &value[0]);
Or
MSB_X = IIC_RegRead(SlaveAddressIIC,
LSB_X = IIC_RegRead(SlaveAddressIIC,
MSB_Y = IIC_RegRead(SlaveAddressIIC,
LSB_Y = IIC_RegRead(SlaveAddressIIC,
MSB_Z = IIC_RegRead(SlaveAddressIIC,
LSB_Z = IIC_RegRead(SlaveAddressIIC,
Has it passed tON?
TMR0_Counter
case 2
No
0x01);
0x02);
0x03);
0x04);
0x05);
0x06);
Is this the end of
Sampling cycle?
TMR0_Counter
case 3
PTCD2 = 0; (pin 7)
No
DataReadyFlag = 0;
No
Has Sampling
finished?
End RTC service
routine
Yes
Disable RTC
End Sampling
Figure 4. Sampling MMA8491Q using the RTC timer (for sampling rate and wait time)
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3.1
Stream data
You can stream data from the MMA8491Q device by controlling the RTC module:
• Streaming starts when the RTC module is enabled and starts to tick.
• Streaming finishes when the RTC module is disabled.
MCU pulls the device EN pin HIGH to take samples at the set sampling rate.
Example 2. RTC module initialization variable definition in system.h
#define RTC_ENABLED
#define RTC_DISABLED
(RTCSC_RTIE = 1)
(RTCSC_RTIE = 0)
Example 3. RTC disable control in main.c
// When DATA stream is completed or not started, RTC is disabled
if (XYZ_STREAM == FALSE)
{
RTC_DISABLED;
}
Example 4. RTC enable control in mma8491_terminal.c
//When user command 'S' is read in microcontroller, RTC is enabled
case 'S':
XYZ_STREAM = TRUE;
sample_num_dec = sample_num;
PROMPT_WAIT = TRUE;
SCISendString("Start to take samples. \r\n");
Tmr_counter = 0;
RTC_ENABLED;
break;
3.2
Setting the sample rate
To stream MMA8491Q data at a fixed rate, you can adopt a timer (by employing a timer on the host
microcontroller). In this scheme, a control variable (TMR_Counter) is used alone with the RTC to record
the lapse time, TMR_Counter should only be updated by the RTC service routine. The TMR Counter
increases by one on every RTC interrupt, and this number is then compared to a preset value. The sampling
period is a multiple of the RTC interrupt period and this preset value. The TMR_Counter should be cleared
at the beginning and end of each sampling cycle.
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The sampling period is determined by:
Sampling period = RTC interrupt period preset_sampling_rate_counter_value
3.3
Setting when the acceleration data is read
Understand that the acceleration data is valid after the turn-on time (tON) and that any I2C transition values
are not valid beforehand (before the turn-on time has elapsed). You can use this property to time the Data
Register reading, which can help you eliminate unnecessary I2C transactions on the bus and save total
system power consumption.
To ensure a valid I2C reading value, the data reading time delay should be set larger than tON. If the timer
to track the data reading time is the same timer as the timer used to set the sampling rate, then the control
variable TMR_Counter can be used.
The data read period is determined by:
Data ready period = RTC interrupt period preset_data_ready_counter_value
When the TMR_Counter is equal to the preset counter preset_data_ready_counter_value, the control
variable DataReadyFlag is set, which signals to the main program that it (the main program) should
conduct an I2C reading transaction. In this scheme (shown in Figure 4), only one I2C transaction is
allowed. After the I2C transaction is conducted, the DataReadyFlag variable is cleared.
Example 5. RTC module initialization variable definition in system.h
/****************************************************************************
** Real Time Clock Interrupt (RTC)
**
** 0x000C KBISC
KBI Interrupt Status and Control Register
** 0x000D KBIPE
KBI Interrupt Pin Select Register
** 0x000E KBIES
KBI Interrupt Edge Select Register
** 0x000F IRQSC
*/
// internal clock 16 kHz, 8 prescale to 250 us per tick
#define init_RTCSC
0b01000001;
#define init_RTCMOD
0b00000001;
// mod by 2 to scale the tick to 500 us
#define init_RTC_period
0x64; // set sample rate at 1 sec / sample
#define CLEAR_RTC_INTERRUPT
(RTCSC_RTIF = 1)
Example 6. Sample rate and XYZ data ready delay setting in main.c
/*********************************************************\
* Real timer service routine
\*********************************************************/
interrupt void isr_RTC(void)
{
/*
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*
RTC timer is set to trigger every 500 us
*
Set EN = 1 at 0 us
*
Set read I2C data flag POLL_ACTIVE at 500 us, clear EN afterwards
*
Mark Error if missing data
*/
Tmr_counter ++;
// Used to test turn on time
switch (Tmr_counter)
{
case 1:
// start sampling
PTCD_PTCD2 = 1;
break;
case 3:
// Data Ready Delay
if (POLL_ACTIVE == TRUE)
POLL_ERROR = TRUE;
else
POLL_ACTIVE = TRUE;
break;
default: break;
}
if (Tmr_counter > (RTC_period - 1)) // Sample rate variable: RTC_period
Tmr_counter = 0;
CLEAR_RTC_INTERRUPT;
// Clear RTC Interrupt
}
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4
Converting 14-bit data
The accelerometer signal is directly converted and stored as 14-bit data to registers when it is ready, after
the device turn-on time tON. The data stays in the registers as long as VDD and EN pins stay HIGH. You
can use I2C communication to read the data registers (0x01 – 0x06).
Table 2. Data registers map(1)(2)
Type
Register Address
Auto-Increment Address(3)
Default
STATUS
R
0x00
0x01
0x00
OUT_X_MSB
R
0x01
0x02
Output
[7:0] are the 8 MSBs of the 14-bit sample
OUT_X_LSB
R
0x02
0x03
Output
[7:2] are the 6 LSB of 14-bit sample
OUT_Y_MSB
R
0x03
0x04
Output
[7:0] are 8 MSBs of the 14-bit sample
OUT_Y_LSB
R
0x04
0x05
Output
[7:2] are the 6 LSB of 14-bit sample
OUT_Z_MSB
R
0x05
0x06
Output
[7:0] are the 8 MSBs of the 14-bit sample
OUT_Z_LSB
R
0x06
0x00
Output
[7:2] are the 6 LSB of 14-bit sample
Name
Comment
Read time status
1. Register contents are preserved when EN pin is set high after sampling.
2. Register contents are reset when EN pin is set low.
3. Auto-increment is the I2C feature that automatically updates the I2C read address after each read.
Auto-increment addresses that are not a simple increment are highlighted in bold.
The auto-increment addressing is only enabled when device registers are read using I2C burst read mode; therefore the
internal storage of the auto-increment address is cleared whenever a stop bit is detected.
Each axis has 2 data registers, MSB and LSB. The corresponding 14-bit result is in left-justified format
(2’s complement numbers). See Table 3.
Table 3. Register bit map
Address Offset
Name
0x00
STATUS
R
0x01
OUT_X_MSB
R
0x02
OUT_X_LSB
R
0x03
OUT_Y_MSB
R
0x04
OUT_Y_LSB
R
0x05
OUT_Z_MSB
R
0x06
OUT_Z_LSB
R
4.1
7
6
5
4
3
2
1
0
0
0
0
0
ZYXDR
ZDR
YDR
XDR
0
0
0
0
0
0
XD[13:6]
XD[5:0]
YD[13:6]
YD[5:0]
ZD[13:6]
ZD[5:0]
Converting the 14-bit 2’s complement hex number to a signed
integer (counts)
The data stored in the data registers are 14-bit 2’s complement numbers. Converting this data to a signed
value (in counts) means that the 2’s complement hex number is converted to an integer number (with a +
or – sign). For example:
0xABCC = –5389
0x544 = +337
The sign of the result is easy to determine, by checking if the high byte of the value is greater than 0x7F.
If so, then the value is a negative number. It needs to be transformed by performing a 2’s complement
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conversion. This involves executing a 1’s complement (i.e., switch all 1s to 0s and all 0s to 1s) and then
adding 1 to the result.
Located in the driver program, the code below performs this conversion. It also adds the additional output
formatting step of replacing each leading zero digit with a space character, which is done by passing 0xF0
to SCI_NibbOut(). Looking more closely, you can see that this routine will add 0x30 to 0xF0, resulting in
a value of 0x120, which gets truncated to 0x20 (the ASCII space character).
Example 7. SCI_s14dec_Out function in sci.c
void SCI_s14dec_Out (tword data)
{
byte a, b, c, d;
word r;
/*
** Determine sign and output
*/
if (data.Byte.hi > 0x7F)
{
SCI_CharOut ('-');
data.Word = ~data.Word + 1;
}
else
{
SCI_CharOut ('+');
}
/*
** Calculate
*/
a = (byte)((data.Word >>2) / 1000);
r = (data.Word >>2) % 1000;
b = (byte)(r / 100);
r %= 100;
c = (byte)(r / 10);
d = (byte)(r % 10);
/*
** Format
*/
if (a == 0)
{
a = 0xF0;
if (b == 0)
{
b = 0xF0;
if (c == 0)
{
c = 0xF0;
}
}
}
/*
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** Output result
*/
SCI_NibbOut (a);
SCI_NibbOut (b);
SCI_NibbOut (c);
SCI_NibbOut (d);
}
4.2
Converting the 14-bit 2’s complement hex number to a signed
decimal fraction (in g’s)
Based on different microcontroller architecture, the conversation of a 14-bit 2’s complement hex number
to a signed decimal fraction is different. The 32-bit architecture MCU has enough bit length to calculate
the mg-values following just one simple formula. Conversion of a 16-bit number using a lower bit-length
microcontroller architecture requires multiple steps: calculate the integer and the fractions separately, and
then add them together.
4.2.1
Conversion using a 32-bit MCU
The formula below converts the 14-bit 2’s complement to a fractional g-value, using a 32-bit architecture
MCU. The 32-bit MCU has a 32-bit arithmetic unit that can contain the calculation result without
truncating it. In the cases where a 32-bit output variable can be arranged via software, this formula can also
be considered.
Result = (1000 data.DWord + 512) >> 10
The above formula is based on a MMA8491Q device property. The device has an 8-g accelerometer scale
and a 14-bit digital resolution, where 1 g = 1000 mg = 1024 counts.
For example:
0x7FFC count = +1023 mg
0xFFFC count = –1 mg
0xF000 count = –1024 mg
Data.DWord is the signed accelerometer value extended to 32 bits. The formula below can be used to
extend the data to any processor word length. To start the extension, the left-aligned 14-bit data must first
be right-aligned (by a 2-bit right shifting operation). Next, the data’s signed bit will be compared, to
identify if this sign value is negative. If the data’s sign value is negative, then the negative sign is extended
by minus 0x4000.
Example 8. Extend a signed number to a microcontroller ALU length
Data.DWord = Data.Word >> 2;
If (Data.DWord >= 0x2000)
Data.DWord -= 0x4000;
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A 10-bit right shift operation is used to accomplish the division by 1024. If the division operation is
chosen, the fractions that are greater than or equal to 0.5 are truncated by the shifting operation. To
compensate for this drawback that the shifting operation brings, and to get a more accurate rounded
number, 512 is added before shifting. This addition allows the fractions greater or equal to 0.5 to be
rounded up to an integer after the shift.
Example 9. Fractional conversion for a 32-bit architecture MCU
void SCI_s14frac_Out (tword data)
{
dword result;
byte a, b, c, d;
word r;
data.DWord = data.Word >> 2;
if (data.DWord >= 0x2000)
data.DWord = data.DWord - 0x4000;
result = (1000 * data.DWord + 512) >> 10;
/*
** Determine sign and output
*/
if (result > 0x80000000)
{
SCI_CharOut ('-');
result = ~result + 1;
}
else
{
SCI_CharOut ('+');
}
/*
** Convert mantissa value to 4 decimal places
*/
r = result % 1000;
a = (byte)(result / 1000);
b = (byte)(r / 100);
r %= 100;
c = (byte)(r / 10);
d = (byte)(r%10);
/*
** Output mantissa
*/
SCI_NibbOut (a);
SCI_NibbOut (b);
SCI_NibbOut (c);
SCI_NibbOut (d);
SCI_CharOut ('mg')
}
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4.2.2
Conversion using a non-32-bit MCU (8-bit, 16-bit)
This section describes how to do the fractional conversion when using MCUs that use an 8-bit or 16-bit
(less than 32-bit) architecture.
Converting to a signed value into g's requires performing the same operations as shown previously
(Section 4.2.1, “Conversion using a 32-bit MCU”), with an added step of resolving the integer and
fractional portions of the value. The scale of the accelerometer's Active Mode (i.e., either 2 g, 4 g, or 8 g)
determines the location of the inferred radix point separating these segments, and thereby the overall
sensitivity of the result.
For the MMA8491Q:
• The most significant bit (bit 13) represents the sign of the result (either positive or negative).
• MMA8491Q has only one scale (8 g, where 1 g = 1024 counts). Therefore, bits 12, 11, and 10 will
contribute to an integer value of 0, 1, 2, 3, 4, 5, 6, and 7; bits 9 through 0 will be fractional values.
Table 4. 8-g scale value with corresponding integer and fraction bits
Full Scale Value
Conts/g
Sign Bit
Integer Bits
Fraction Bits
(212
8g
1024
12
= 4096)
11 (211 = 2048)
10 (210 = 1024)
13
0 through 9
The fractional portion of the result can be extracted, by logically shifting the sample to the left by 4 binary
locations. Table 6 shows this result; Table 7 provides the corresponding decimal values.
Table 5. 8-g full scale 14-bit data conversion to decimal fraction number
Word Format
MMA8491Q (14-bit)
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
13
12
11
10
9
8
7
6
5
4
3
2
1
0
x
x
Integer/Fraction
±
I
I
I
F
F
F
F
F
F
F
F
F
F
x
x
MSB/LSB
M
M
M
M
M
M
M
M
L
L
L
L
L
L
0
0
Table 6. 8-g full scale 14-bit in word format, after left shift to eliminate integer and sign bits
Word Format
MMA8491Q (14-bit)
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
x
x
x
x
x
x
Integer/Fraction
F
F
F
F
F
F
F
F
F
F
x
x
x
x
x
x
Fraction bits
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
x
x
x
x
x
x
MSB/LSB
M
M
M
M
L
L
L
L
L
L
0
0
0
0
0
0
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Table 7. 8-g mode fraction values
8g Mode
2-1
29 = 512
Calculation
256 counts/g
Rounded to 4th
Decimal Place
Integer
512/1024 = 0.5
0.5000
5000
0.2500
2500
-2
28
= 256
256/1024 = 0.25
2-3
7
2 = 128
128/1024 = 0.125
0.1250
1250
2-4
26 = 64
64/1024 = 0.0625
0.0625
625
5
32/1024 = 0.03125
0.0313
313
4
16/1024 = 0.01563
0.0156
156
2
-5
2
-6
2
2 = 32
2 = 16
-7
3
2
2 =8
8/1024 = 0.007812
0.0078
78
2-8
22 = 4
4/1024 = 0.003906
0.0039
39
-9
2 =2
2/1024 = 0.001953
0.0020
20
-10
20
1/1024 = 0.000976
0.0010
10
2
2
1
=1
For each of the 10 fraction bits, if the value of the bit is set—then the corresponding decimal value will be
added to the total. For example, if bit 9, 6, and 4 are set, then the total will be (5000 + 625 + 156 = 5781),
which corresponds to 0.5781 g:
• The highest fractional value occurs when all fraction bits are set (5000 + 2500 + 1250 + 625 + 313
+ 156 + 78 + 39 + 20 + 10 = 9991), which corresponds to 0.9991g.
• The resolution in full scale 8 g Active Mode is 0.977 mg. Adding the fractional value to 4
significant digits results in 1-mg resolution.
Example 10 shows a code example that performs the conversion of a 14-bit signed 2’s complement value
into a signed decimal fraction displayed in g’s. This routine (with modification) can also be used to convert
12-bit, 10-bit or 8-bit data, with 2 g or 4 g full scales. The extra unused bits will remain zeros.
Example 10. Fractional conversion SCI_s14frac_Out in sci.c
void SCI_s14frac_Out (tword data)
{
BIT_FIELD value;
word result;
byte a, b, c, d;
word r;
/*
** Determine sign and output
*/
if (data.Byte.hi > 0x7F)
{
SCI_CharOut ('-');
data.Word &= 0xFFFC;
data.Word = ~data.Word + 1;
}
else
{
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SCI_CharOut ('+');
}
/*
** Determine integer value and output
*/
SCI_NibbOut((data.Byte.hi & 0x70) >>4);
data.Word = data.Word <<4;
SCI_CharOut ('.');
/*
** Determine mantissa value
*/
result = 0;
value.Byte = data.Byte.hi;
if (value.Bit._7 == 1)
result += FRAC_2d1;
if (value.Bit._6 == 1)
result += FRAC_2d2;
if (value.Bit._5 == 1)
result += FRAC_2d3;
if (value.Bit._4 == 1)
result += FRAC_2d4;
//
data.Word = data.Word <<4;
value.Byte = data.Byte.hi;
//
if (value.Bit._7 == 1)
result += FRAC_2d5;
if (value.Bit._6 == 1)
result += FRAC_2d6;
if (value.Bit._5 == 1)
result += FRAC_2d7;
if (value.Bit._4 == 1)
result += FRAC_2d8;
if (value.Bit._3 ==1)
result += FRAC_2d9;
if (value.Bit._2 ==1)
result += FRAC_2d10;
if(value.Bit._1 ==1)
result += FRAC_2d11;
if(value.Bit._0 ==1)
result += FRAC_2d12;
/*
** Convert mantissa value to 4 decimal places
*/
r = result % 1000;
a = (byte)(result / 1000);
b = (byte)(r / 100);
r %= 100;
c = (byte)(r / 10);
d = (byte)(r%10);
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/*
** Output mantissa
*/
SCI_NibbOut (a);
SCI_NibbOut (b);
SCI_NibbOut (c);
SCI_NibbOut (d);
SCI_CharOut ('g');
}
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5
Using the MMA8491 HyperTerminal Driver
This section shows how to use the MMA8491 Driver (which is preinstalled) with the Windows
HyperTerminal program. Table 8 lists the tools you will need.
Table 8. Tools
For
Use
Hardware
MMA8491Q Sensor Toolbox Kit (LFSTBEB8491 or RDMMA8491).
See http://www.freescale.com/webapp/sps/site/overview.jsp?code=SNSTOOLBOX&fsrch=1&sr=1
Software
Windows HyperTerminal or any other communication tool
Programming
To program the driver to Sensor Toolbox board or to make any modifications to the program, use Code
Warrior v 6.3.
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=CW-MICROCONTROLLERS
5.1
Overview
The MMA8491 driver program enables you to quickly access the MMA8491Q data and tilt sensing result
via an I2C bus. TheMMA8491 driver is also a reference program that you can use. This program is based
on the MMA8491Q Sensor Toolbox (STB) kit. The driver is downloadable to the MC9S08QE8
microcontroller on the baseboard.
You can use any terminal emulator program to control the device operation from a computer. In this
application note, we use Windows HyperTerminal. A list of commands is available for you to choose from.
This driver configures the MMA8491Q device to take one sample per second. To trigger the sampling, the
driver uses the MCU’s GPIO pin C2. The rising edge of the GPIO pin is timed with an RTC (real-time
clock). The RTC runs using a 500-us step. The accelerometer data is read 1 ms after the EN pin is asserted,
and is then displayed on the HyperTerminal screen. The EN pin is turned off after the read, regardless of
the data display.
5.2
5.2.1
Procedures
Set up equipment and apply power
1. Connect the USB cable, to power the STB board.
2. Make sure that SW1 is on.
The power LED CR1should be lit.
5.2.2
Download HyperTerminal driver (if necessary)
1. Download the MMA8491Q HyperTerminal program from the MMA8491Q product site.
The driver can be found on the “software & tools” tab, with the name of AN4296SW.
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MMA8491Q
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NOTE
The MMA8491Q Sensor Toolbox kit comes pre-programmed with
HyperTerminal diver.
2.
3.
4.
5.
6.
Connect the programmer to the J1 connector on the USB communication board of the STB kit.
Open the MMA8491Q driver program (using Code Warrior 6.3).
Program the driver to the MC9S08QE8 microcontroller.
Disconnect the programmer.
Turn off SW1, then turn on SW1, to allow the part to restart.
NOTE
You can program the HyperTerminal driver into other Sensor Toolbox kits.
If you do, then the Sensor Toolbox kits will NOT be recognized by the STB
software afterwards.
5.2.3
Run communications program, start sampling
1. Run HyperTerminal.
2. Configure HyperTerminal port with these settings:
Table 9. HyperTerminal settings
Parameter
Bits Per Second
Data Bits
Parity
Stop Bits
Flow Control
Value
115200
8
None
1
None
3. Enter any key to bring up the initial prompt (Figure 5). This menu header only appears when the
STB board is first powered on.
Figure 5. HyperTerminal at power up
Data Manipulation and the Basic Settings of Xtrinsic MMA8491Q Accelerometer, Rev 1
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4. Entering “?” will bring up the menu shown in Figure 6.
Table 10 lists the HyperTerminal menu commands.
Figure 6. HyperTerminal menu
Table 10. HyperTerminal menu commands
Command
Function
Notes
Tn
Tilt Sensing Selector
• N=0 Tilt sensing function turned off
• N=1 Tilt sensing function turned on
Dn
Data output Selector
• N=0 Data output turned off
• N=1 Data output turned on
Nn
Stream data
Sample number
•
•
•
•
Fn
Read XYZ as signed counts
• N=0, read XYZ as signed counts
Example of response:
X= -250 Y= -126 Z= +968
• N=1, read XYZ as signed g’s
Example of response:
X= -0.1992g Y= -0.0577g Z= +0.9473g
S
Start sampling
N=0
N=1
N=2
N=3
1 sample
100 samples
500 samples
unlimited samples
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5. Enter the commands to configure the device sampling conditions or to start sampling.
Note that the commands are not case-sensitive.
6. Press any key to exit out of the data streaming.
5.2.4
Acquire and save data
1. In HyperTerminal, select TransferCapture Text.
2. Save the file to a known location, and select Start.
The data log will start.
Figure 7. Capture text dialog in HyperTerminal
3.
4.
5.
6.
Select one of the commands.
When a satisfactory amount of data has been collected, select TransferCapture TextStop.
Open Excel.
Open the log file.
Note that if the file does not appear, then select the Files Of Type drop-down menu, and select
All Files (*.*).
Data Manipulation and the Basic Settings of Xtrinsic MMA8491Q Accelerometer, Rev 1
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Figure 8. Select “Files of Type” to be “All Files”
7. The Text Import Wizard should now appear; select the Delimited option and then select Next.
8. Depends on the output data, choose delimiters accordingly. For example, when convert g’s values,
Select the Space option, and in Other type “=”. Next select Finish. The data set will have 6
columns: X, X Data, Y, Y Data, Z, Z Data. See Figure 9.
Data Manipulation and the Basic Settings of Xtrinsic MMA8491Q Accelerometer, Rev 1
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Figure 9. Imported sample data in Excel file
Data Manipulation and the Basic Settings of Xtrinsic MMA8491Q Accelerometer, Rev 1
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Document Number: AN4296
Rev 1
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