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viki2000
Joined: 08 May 2013
Posts: 233
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| How to increase the SPI speed? |
 Posted: Mon Jan 30, 2017 9:44 am |
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I use MCP4921 SPI DAC to generate a rectified sine wave 1Vpp 100Hz.
Starting from next Code Library:
http://www.ccsinfo.com/forum/viewtopic.php?t=53205&highlight=mcp4921
I customized it for PIC18F4550 to generate a rectified sine-wave.
I was inspired by next webpage, but I wanted to test it with CCS C compiler:
http://pic-tutorials.blogspot.de/
The PIC18F4550 works with an external crystal 20MHz and with PLL heaving then the clock 48MHz.
This is my schematic:
And this is code:
| Code: | #include <18F4550.h>
#use delay(clock=48000000,crystal=20000000,restart_wdt)
//#define SPI_MODE_0 (SPI_L_TO_H | SPI_XMIT_L_TO_H)
#define CLK PIN_B1
#define CS PIN_B0
#define SDI PIN_C7
void init_dac();
void DAC_write(unsigned char cmd, unsigned long data);
CONST unsigned long SINE_WAVE[250] = {0, 10, 21, 31, 41, 51, 62, 72, 82, 92, 103, 113, 123, 133, 143, 153, 164, 174, 184, 194, 204, 214, 224, 233, 243, 253, 263, 273, 282, 292, 301, 311, 321, 330, 339, 349, 358, 367, 376, 386, 395, 404, 412, 421, 430, 439, 447, 456, 465, 473, 481, 490, 498, 506, 514, 522, 530, 538, 545, 553, 561, 568, 575, 583, 590, 597, 604, 611, 618, 624, 631, 638, 644, 650, 656, 663, 669, 674, 680, 686, 692, 697, 702, 708, 713, 718, 723, 727, 732, 737, 741, 745, 750, 754, 758, 761, 765, 769, 772, 776, 779, 782, 785, 788, 791, 793, 796, 798, 800, 803, 804, 806, 808, 810, 811, 813, 814, 815, 816, 817, 817, 818, 818, 819, 819, 819, 819, 819, 818, 818, 817, 817, 816, 815, 814, 813, 811, 810, 808, 806, 804, 803, 800, 798, 796, 793, 791, 788, 785, 782, 779, 776, 772, 769, 765, 761, 758, 754, 750, 745, 741, 737, 732, 727, 723, 718, 713, 708, 702, 697, 692, 686, 680, 674, 669, 663, 656, 650, 644, 638, 631, 624, 618, 611, 604, 597, 590, 583, 575, 568, 561, 553, 545, 538, 530, 522, 514, 506, 498, 490, 481, 473, 465, 456, 447, 439, 430, 421, 412, 404, 395, 386, 376, 367, 358, 349, 339, 330, 321, 311, 301, 292, 282, 273, 263, 253, 243, 233, 224, 214, 204, 194, 184, 174, 164, 153, 143, 133, 123, 113, 103, 92, 82, 72, 62, 51, 41, 31, 21, 10};
void main(){
setup_adc(ADC_OFF);
//setup_spi(SPI_MASTER | SPI_MODE_0 | SPI_CLK_DIV_4);
unsigned long int i=0;
while(TRUE){
if (i++ == 249)
i = 0;
DAC_write(0x30, SINE_WAVE[i]);
}
}
void init_dac()
{
output_high(CS);
output_high(CLK);
output_high(SDI);
}
void DAC_write(unsigned char cmd, unsigned long data)
{
unsigned char s = 16;
unsigned long value = 0;
value = cmd;
value <<= 8;
value |= (data & 0xFFF);
output_low(CS);
while(s > 0)
{
if((value & 0x8000) != 0)
{
output_high(SDI);
}
else
{
output_low(SDI);
}
output_low(CLK);
output_high(CLK);
value <<= 1;
s -= 1;
}
output_high(CS);
//delay_us(10);
} |
As you may see above, I use lookup table with 250 values to generate 1Vpp 100Hz rectified sine-wave.
The values are generated with the software SMART SINE from here:
http://tahmidmc.blogspot.de/2012/10/smart-sine-software-to-generate-sine.html
That is already far better than only 100 values lookup table with I2C here:
http://www.ccsinfo.com/forum/viewtopic.php?p=209877#209877
The oscilloscope screenshots with MCP4921 SPI DAC are here:
When I measure the SPI clock signal with the oscilloscope, then I see 500KHz.
I would like to increase that clock to higher values and to enlarge the lookup table.
Of course another solution would be a trigonometric function, which I may consider later for comparison.
How do you setup/increase the SPI bus clock/frequency? |
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temtronic
Joined: 01 Jul 2010
Posts: 9230
Location: Greensville,Ontario
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| Posted: Mon Jan 30, 2017 9:53 am |
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simple answer is to read the CCS manual, #use spi().......
kinda funny how stuff like this is in there....
BTW pressing F11 while your project is open, actually opens up the manual.
I'm pretty sure without a 'baud=xxxx' entry in SPI(....), CCS will default to as fast as possible.
Others can confirm/deny as this PC isn't my eng PC.
Jay |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19524
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| Posted: Mon Jan 30, 2017 10:17 am |
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The code you are pointing to, uses software SPI.
Don't. Use the hardware instead.
Connect the CS to a different pin. SDI, is the spi data input for the hardware, You need to be using a pin that is not part of the hardware for the CS.
Assuming B2 below (change to suit what you can do).
#USE SPI, and spi_xfer.
Use this.
#USE SPI(SPI1, mode=0, bits=16, enable=PIN_B2, STREAM=MCP4921)
Then:
spi_xfer(MCP4921, val);
where 'val' is an int16, will clock the data out at 12Mb/sec....
The dac_write in the code you point to, becomes:
| Code: |
void DAC_write(unsigned char cmd, unsigned long data)
{
int16 value;
value = cmd;
value <<= 8;
value |= (data & 0xFFF);
spi_xfer(MCP4921, value);
}
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Honestly, save time, and make the #defines for the commands be the 16bit values. So (for instance):
#define BUFFER_ON 0x4000
etc..
Then you can get rid of the rotations, as:
| Code: |
void DAC_write(unsigned int16 command, unsigned int16 data)
{
spi_xfer(MCP4921,data | command);
}
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viki2000
Joined: 08 May 2013
Posts: 233
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| Posted: Mon Jan 30, 2017 11:51 am |
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Thank you for suggestions.
I will check them during next days. |
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