I am using the SLKS12 with option "C128", MC9S12 C128 MCU, 80 LQFP. The accelerometer module that I am trying to communicate with is the HitachiH48C3 from Parallax. It only has one data pin so I have no choice but to use bidirectional mode. The datasheet is not very specific about the communication protocol but I found the data sheet for the ATD chip on the accelerometer module. I have read a lot of threads concerning this mode but to no avail. Hopefully Peg or BigMac can help with these short comings. Here is the code I have been working with:
void main(void)
{
float gforce[] = {0.0,0.0,0.0}; //Calculated G-forces
//0 is X axis
//1 is Y axis
//2 is Z axis
SPIinit();
//Forever loop
for(;
{
if( gforce[0]!=0.0 || gforce[1]!=0.0 || gforce[2]!=0.0) //Test to see if motion
PTT_PTT1 = 1;
else
PTT_PTT1 = 0;
for (i=0 ; i<3 ; i++)
gforce[i] = axis_acceleration(i); //Interrogate accelerometer and calc G's
}/* loop forever */
/* please make sure that you never leave main */
}//end of main
void SPIinit(void)
{
SPICR1 = 0x70; //Load pattern for SPI control register 1
//Interrupt disabled
//System enabled
//Trans interrupt enabled
//Master mode
//Clock polarity (idle low)
//Clock phase (rising)
//Slave select output disabled
//MSB first
SPICR2 = 0x03; //Load pattern for SPI control register 2
//Mode fault disabled
//Bidirectional mode(recieve)
//Stop in wait mode
//Serial pin control(bidirectional)
SPIBR = 0x01; //Load pattern for SPI baud rate
//Bus clock divide by 4
//1MHz
//500ns minimum clokc for accelerometer
//2MHz clock maximum
}//SPI initialize subroutine
float axis_acceleration(int i)
{
//Developed from example code "HitachiH48C3AxisAccelerometer.pdf"
//Also referenced MCP3204/3208 datasheet from Microchip for SPI
//Routine for reading accelerometer
//To interogate accelerometer send 000110"axis"
//000 are just fill bits for the 8bit SPIDR register
//1 for start bit
//1 for single ended operation of the ATD chip
//0 is actually a don't care b/c there are only 4 ATD channels
//"axis" is where the analog inputs are connected to the ATD
//X axis is channel 00
//Y axis is channel 01
//Z axis is channel 10
//Ref is channel 11
//Next series of words will contain 12bit channel conversion
//First High-Z then "null" bit and then as MSB then LSB
//high-z,null,11,10,9,8,7,6
//5,4,3,2,1,0,1,2
//3,4,5,6,7,8,9,10
//11,0,0,0,0,0,0,0
//then repeating nulls until interrogated again
const int clkStall = 3; //Used to stall clock in communication with accelerometer
const float G_convert =0.055000055; //Constant used to convert ADC data to G-forces
//4095 for max voltage on 12bit ADC
//3.3V operational voltage
//0.3663V = 1g output
//G = (axis-vref)/4095*(3.3/0.3663)
//G = (axis - vref)*0.0022000022
//G/4 for shifting the data to align "0" bit
//G*100 to make output unit 0.01g
int j = 0; //Variable used for looping
float gforce = 0.0; //Calculated G-forces
word refCount = 0; //Refernce data from accelerometer
word axisCount = 0; //Axis data from accelerometer
//0 is X axis
//1 is Y axis
//2 is Z axis
//Get reference conversion
SPICR2_BIDIROE = 1; //Set pin for ouput
PTT_PTT0 = 0; //Turn accelerometer on
PORTA = SPI_trans(3 + 24); //Request reference
//Reads erroneous data
SPICR2_BIDIROE = 0; //Set pin for input
PORTA = SPI_trans(0xFF); //Save MSB data
PORTB = SPI_trans(0xFF); //Save LSB data
PTT_PTT0 = 1; //Turn accelerometer off
refCount = PORTAB; //Save reference count
for( j=0 ; j<clkStall ; j++ ){} //Delay loop for next accelerometer data
//500ns minimum
//3 * 250 = 750ns
//Get Axis conversion
SPICR2_BIDIROE = 1; //Set pin for output
PTT_PTT0 = 0; //Turn accelerometer on
PORTA = SPI_trans(i + 24); //Request axis
//Reads erroneous data
SPICR2_BIDIROE = 0; //Set pin for input
PORTA = SPI_trans(0xFF); //Save MSB data
PORTB = SPI_trans(0xFF); //Save LSB data
PTT_PTT0 = 1; //Turn accelerometer off
axisCount = PORTAB; //Save axis count
//calculate g-forces
gforce = (axisCount - refCount) * G_convert;
return gforce;
}//axis acceleration subroutine
byte SPI_trans(byte val)
//SPI tranmit subroutine developed from
//https://community.freescale.com/message/35017#35017
{
volatile byte garbage = 0; //Variable used to collect "garbage"
while (!SPISR_SPTEF); //Loops unitl Transmit Empty Flag is set
garbage = SPISR; //First step to clear flags
SPIDR = val; //Loads data and clears Flags
while (!SPISR_SPIF); //Loops until data is transfered(8 clocks)
garbage = SPISR; //First step to clear flags
return SPIDR; //Returns data in SPI data register to caller and clears flags
}//SPI transmit subroutine
Thanks!
Mac,
First I would like to thank you for all of your expertise! I now have it working. I did some more digging and found the datasheet for the accelerometer that parallax used. It turns out that it detects gravity, Z axis acceleration. I assumed that the module only detected changes in acceleration. I changed the variables back to float and it is working like a champ. Now on to the timer module to make my delays more suitable, not "poor man".
If you would like to see the full code let me know how to add the grey block like you did with your example code so that I am not character limited. Thanks again!
Lundin,
Thanks for your comments on the programming syntax. I am just now trying to use C++ instead of assymbly b/c it is a lot more readable. I will implement the more appropriate timer once I get the SPI module operational. One module at a time
