@jmknapp

your table is of type uint16 and you use only halve its range (0..65535) so there is a few bits precision to be won?

check - http://arduino.cc/forum/index.php/topic,69723.0.html -

< 50 usec per call
- handles values outside 0..360
- tablesize: 182 bytes (uses interpolation)

Hi Rob,

I don't think going to 65535 would give even one more bit of precision--the error comes mostly from the table size of 1024, so that for example the step from sintab[0] to sintab[1] is 50, and 50/32768 = 0.00152. If the table went to 65535, the step between sintab[0] and sintab[1] would just go to 101 and give almost the same ratio.

You mention sin() taking 120 usec--what board is that with? I get 255 usec with a 16 MHz Diecimila, or at least I did some time back when I looked at it.

joe

Floating point divisions are more expensive, around 34 usec, but I don't think you have to use them. I.e., to convert from degrees to radians instead of 2*pi*d/360., just use 0.0174533*d.

So I gather since you want a speedup of a factor of three, you're looking for a total time of less than about 100 usec for the sin() call? It's an interesting exercise that might come in handy for my purposes too, so I gave it a whack. Here's some code that does a sin() function using table lookup in progmem:

Code:

#include <avr/pgmspace.h>
#define N 5000   // number of calls to make during test
#define NS 1024   // number of entries in sin table
#define MAXI 32768  // max integer value in table
#define I2F (1./MAXI) // conversion from integer to float

int i ;
long startTime, endTime ;
float dt ;
float d = 0.0 ;  // argument to sin function, degrees
float s ;  // result of sin function

// sin table
// values for first quadrant, other quadrants calculated by symmetry
PROGMEM prog_uint16_t sintab[NS] = {
0,
50,100,150,201,251,
301,351,402,452,502,
552,603,653,703,753,
804,854,904,954,1005,
1055,1105,1155,1206,1256,
1306,1356,1407,1457,1507,
1557,1607,1658,1708,1758,
1808,1858,1909,1959,2009,
2059,2109,2159,2210,2260,
2310,2360,2410,2460,2510,
2560,2611,2661,2711,2761,
2811,2861,2911,2961,3011,
3061,3111,3161,3211,3261,
3311,3361,3411,3461,3511,
3561,3611,3661,3711,3761,
3811,3861,3911,3961,4011,
4061,4110,4160,4210,4260,
4310,4360,4409,4459,4509,
4559,4609,4658,4708,4758,
4808,4857,4907,4957,5006,
5056,5106,5155,5205,5255,
5304,5354,5403,5453,5503,
5552,5602,5651,5701,5750,
5800,5849,5898,5948,5997,
6047,6096,6146,6195,6244,
6294,6343,6392,6442,6491,
6540,6589,6639,6688,6737,
6786,6835,6884,6934,6983,
7032,7081,7130,7179,7228,
7277,7326,7375,7424,7473,
7522,7571,7620,7669,7717,
7766,7815,7864,7913,7961,
8010,8059,8108,8156,8205,
8254,8302,8351,8400,8448,
8497,8545,8594,8642,8691,
8739,8788,8836,8884,8933,
8981,9029,9078,9126,9174,
9223,9271,9319,9367,9415,
9463,9512,9560,9608,9656,
9704,9752,9800,9848,9896,
9944,9991,10039,10087,10135,
10183,10230,10278,10326,10374,
10421,10469,10517,10564,10612,
10659,10707,10754,10802,10849,
10897,10944,10991,11039,11086,
11133,11181,11228,11275,11322,
11369,11416,11464,11511,11558,
11605,11652,11699,11746,11793,
11839,11886,11933,11980,12027,
12073,12120,12167,12213,12260,
12307,12353,12400,12446,12493,
12539,12586,12632,12678,12725,
12771,12817,12864,12910,12956,
13002,13048,13094,13140,13186,
13232,13278,13324,13370,13416,
13462,13508,13554,13599,13645,
13691,13736,13782,13828,13873,
13919,13964,14010,14055,14100,
14146,14191,14236,14282,14327,
14372,14417,14462,14507,14552,
14598,14642,14687,14732,14777,
14822,14867,14912,14956,15001,
15046,15090,15135,15180,15224,
15269,15313,15357,15402,15446,
15491,15535,15579,15623,15667,
15712,15756,15800,15844,15888,
15932,15976,16019,16063,16107,
16151,16195,16238,16282,16325,
16369,16413,16456,16499,16543,
16586,16630,16673,16716,16759,
16802,16846,16889,16932,16975,
17018,17061,17104,17146,17189,
17232,17275,17317,17360,17403,
17445,17488,17530,17573,17615,
17658,17700,17742,17784,17827,
17869,17911,17953,17995,18037,
18079,18121,18163,18204,18246,
18288,18330,18371,18413,18454,
18496,18537,18579,18620,18662,
18703,18744,18785,18826,18868,
18909,18950,18991,19032,19073,
19113,19154,19195,19236,19276,
19317,19358,19398,19439,19479,
19519,19560,19600,19640,19681,
19721,19761,19801,19841,19881,
19921,19961,20001,20040,20080,
20120,20159,20199,20239,20278,
20318,20357,20396,20436,20475,
20514,20553,20592,20631,20671,
20709,20748,20787,20826,20865,
20904,20942,20981,21020,21058,
21097,21135,21173,21212,21250,
21288,21326,21365,21403,21441,
21479,21517,21555,21592,21630,
21668,21706,21743,21781,21818,
21856,21893,21931,21968,22005,
22042,22080,22117,22154,22191,
22228,22265,22301,22338,22375,
22412,22448,22485,22521,22558,
22594,22631,22667,22703,22740,
22776,22812,22848,22884,22920,
22956,22992,23027,23063,23099,
23134,23170,23205,23241,23276,
23312,23347,23382,23417,23453,
23488,23523,23558,23593,23627,
23662,23697,23732,23766,23801,
23835,23870,23904,23939,23973,
24007,24041,24075,24109,24144,
24177,24211,24245,24279,24313,
24346,24380,24414,24447,24480,
24514,24547,24580,24614,24647,
24680,24713,24746,24779,24812,
24845,24877,24910,24943,24975,
25008,25040,25073,25105,25137,
25169,25201,25234,25266,25298,
25330,25361,25393,25425,25457,
25488,25520,25551,25583,25614,
25645,25677,25708,25739,25770,
25801,25832,25863,25894,25925,
25955,25986,26016,26047,26077,
26108,26138,26169,26199,26229,
26259,26289,26319,26349,26379,
26409,26438,26468,26498,26527,
26557,26586,26615,26645,26674,
26703,26732,26761,26790,26819,
26848,26877,26905,26934,26963,
26991,27020,27048,27076,27105,
27133,27161,27189,27217,27245,
27273,27301,27329,27356,27384,
27411,27439,27466,27494,27521,
27548,27576,27603,27630,27657,
27684,27711,27737,27764,27791,
27817,27844,27870,27897,27923,
27949,27976,28002,28028,28054,
28080,28106,28131,28157,28183,
28208,28234,28259,28285,28310,
28335,28361,28386,28411,28436,
28461,28486,28511,28535,28560,
28585,28609,28634,28658,28682,
28707,28731,28755,28779,28803,
28827,28851,28875,28898,28922,
28946,28969,28993,29016,29039,
29062,29086,29109,29132,29155,
29178,29201,29223,29246,29269,
29291,29314,29336,29359,29381,
29403,29425,29447,29469,29491,
29513,29535,29557,29578,29600,
29621,29643,29664,29686,29707,
29728,29749,29770,29791,29812,
29833,29854,29874,29895,29915,
29936,29956,29977,29997,30017,
30037,30057,30077,30097,30117,
30137,30156,30176,30196,30215,
30235,30254,30273,30292,30312,
30331,30350,30368,30387,30406,
30425,30443,30462,30480,30499,
30517,30535,30554,30572,30590,
30608,30626,30644,30661,30679,
30697,30714,30732,30749,30766,
30784,30801,30818,30835,30852,
30869,30886,30902,30919,30936,
30952,30969,30985,31001,31018,
31034,31050,31066,31082,31098,
31114,31129,31145,31161,31176,
31192,31207,31222,31237,31253,
31268,31283,31298,31312,31327,
31342,31357,31371,31386,31400,
31414,31429,31443,31457,31471,
31485,31499,31513,31526,31540,
31554,31567,31581,31594,31607,
31620,31634,31647,31660,31673,
31685,31698,31711,31723,31736,
31749,31761,31773,31785,31798,
31810,31822,31834,31846,31857,
31869,31881,31892,31904,31915,
31927,31938,31949,31960,31971,
31982,31993,32004,32015,32025,
32036,32047,32057,32067,32078,
32088,32098,32108,32118,32128,
32138,32148,32157,32167,32176,
32186,32195,32205,32214,32223,
32232,32241,32250,32259,32268,
32276,32285,32294,32302,32311,
32319,32327,32335,32343,32351,
32359,32367,32375,32383,32390,
32398,32405,32413,32420,32427,
32435,32442,32449,32456,32463,
32469,32476,32483,32489,32496,
32502,32509,32515,32521,32527,
32533,32539,32545,32551,32557,
32562,32568,32573,32579,32584,
32589,32595,32600,32605,32610,
32615,32619,32624,32629,32633,
32638,32642,32647,32651,32655,
32659,32663,32667,32671,32675,
32679,32682,32686,32689,32693,
32696,32700,32703,32706,32709,
32712,32715,32718,32720,32723,
32726,32728,32730,32733,32735,
32737,32739,32741,32743,32745,
32747,32749,32750,32752,32754,
32755,32756,32758,32759,32760,
32761,32762,32763,32764,32764,
32765,32766,32766,32767,32767,
32767,32767,32767
} ;

float romsin(float x) {
int ix ;   // index into sin table
int is ;   // value read from sin table
int q ;    // quadrant number 0,1,2,3
        int j ;

x *= 1./360. ;

x -= (int)x ;

if (x < 0)
x += 1.0 ;   // x is now between 0 and 1, representing 0 to 360 degrees

        q = x * 4 ;   // get quadrant number

        ix = (int)(x*4*NS) % NS ;  // get index into table

switch(q) {
case 0:   // 0-90
is = pgm_read_word_near(sintab + ix);
break ;
case 1:   // 90-180
ix = NS - ix - 1 ;  // reflect
is = pgm_read_word_near(sintab + ix); ;
break ;
case 2:   // 180-270
is = -pgm_read_word_near(sintab + ix); ;  // negate
break ;
case 3:   // 270-360
ix = NS - ix - 1;   // reflect
is = -pgm_read_word_near(sintab + ix); ;   // negate
break ;
}

        return((float)is*I2F) ;
}

void setup() {
  Serial.begin(9600) ;
  Serial.println("Sin in ROM test....") ;
}

void loop() {
  startTime=millis() ;
  for (i = 0 ; i < N ; i++)
    s = romsin(d) ;
  endTime=millis() ;
  dt = 1000.*(float)(endTime - startTime)/N ;  // time per call in usec
  
  Serial.print("sin(") ;
  Serial.print(d) ;
  Serial.print(")=") ;
  Serial.print(s) ;
  Serial.print(", ") ;
  Serial.print((int)dt) ;
  Serial.println(" usec/call") ;
  
  d += 1.0 ;
}

The results are around 80 usec per call. The error is at most 0.0016, near 0 and 180 degrees. That could be improved with a bigger table if necessary, and/or using interpolation although in the latter case there would be a time hit.

I'm not sure why people warn against the slowness of progmem access. From what I can see, a read takes less than a microsecond, so that's no worry in this context.

cos(), atan2() etc. are left as an exercise for the reader!

Joe

FYI, I measured the speed of a sin() call on a 16 MHz Diecimila and got 255 usec. A floating point multiply clocked in at 9 usec.

Joe

I've been playing with that sensor. It works well out to about 50 feet with something like a 120 degree beam pattern in azimuth, 60 degrees in elevation. Not so great for weather systems!

Within that beam pattern it'll tell you the speed of objects, but not the range. Doppler radars with range capabilities are "pulsed," where a pulse is sent out and the phase of the received pulse is measured to get the range per the speed of light. This sensor doesn't have this capability. You can operate it in what the data sheet calls "pulsed" mode, but that just allows you to save on power consumption and doesn't involve any capability to measure the phase of the returned pulses. It's pretty much a black box. You apply power and ground and you get an IF signal out, containing the difference (doppler) frequencies, conveniently located in the audio range. E.g., a 100 mph object produces a ~3100 Hz signal. The most basic mode is "continuous wave" mode where the transmit antenna (consisting of two small patches on the pcb) continuously emits the x-band signal, and the receive antenna (similarly two patches on the pcb) continuously receives the reflected signal. As I say, the pulsed mode allows you to reduce the duty cycle to conserve power, although the unit isn't too power hungry--30 ma @ 5V.

Joe

The output resistance of your current source is RV1 which is 1K max. So it seems that U2 is really just an inverting voltage amplifier with gain 430/RV1.

That's a valid way of looking at the circuit. Combining "Vout = Vcell * 430/RV1" with "Vcell = Icell * RV1", we get "Vout = Icell * 430". Whis is exactly what @secretagent wants.

True, which is why I deleted my post just before you replied. 

Check out Digi's paper:

XBee & XBee‐PRO OEM RF Module Antenna Considerations
http://ftp1.digi.com/support/images/XST-AN019a_XBeeAntennas.pdf

Summarizing, they compare both the 1mW XBee and 60 mW XBee-Pro, each with a chip or whip antenna.

The radiation pattern for the whip antenna is similar to that of a dipole. That is to say, it
is shaped like a donut.  Thus, the performance of a module using a whip antenna, is
relatively insensitive to its orientation in the plane that is perpendicular to the whip
antenna.  On the other hand, the radiation pattern of the chip antenna is not as uniform
as that of the whip antenna.  Therefore, certain orientations will achieve better
performance than others.  As our evaluation was performed, the orientation was
selected to achieve the best performance. ...

After reviewing Table 1, we can make several important observations.
• The whip antenna has a range advantage over the chip antenna, but only outdoors.
• The XBee-PRO can achieve more range than the XBee.
• The XBee-PRO and XBee both achieve more range outdoors than they do indoors.

The whip antenna on the XBee module affords additional range in outdoor applications.
However, it also occupies more space.  If more range is required, and space is a
constraint, then the XBee-PRO with a chip antenna may be more appropriate.  On the
other hand, if more range is a requirement and cost, not space, is the constraint, then
the XBee with a whip antenna may be the best choice. 
It should also be clear that the XBee-PRO can achieve superior range when compared to
the XBee.  Thus, if the application requires more range than the XBee can provide, then
the XBee-PRO with a whip or a chip antenna could be used.  Again, the chip antenna is
best for tight spaces, while the whip antenna achieves more range. 

For enclosures, I've been thinking about using local laser-cutting facilities. Laser cutters can easily cut acrylic for pots, switches, displays, etc. There's a local hackerspace of sorts here for small-scale work, as well as several other options in town for larger runs. Of course, that means putting it together yourself, so maybe that's not a good solution for you.

There's a guy at the hackerspace (http://columbusideafoundry.com) who laser-cuts recycled wooden cigar boxes for electronic enclosures.

Joe

Just as an alternative, look at these items from Hobby Engineering:

http://www.hobbyengineering.com/H2251.html

transmitter data sheet:
http://www.hobbyengineering.com/specs/WENSHING-TWSBS3.pdf

receiver data sheet:
http://www.hobbyengineering.com/specs/WENSHING-RWS3716C.pdf

And check the schematic on the last page of those pdfs. That circuit shows 4 pushbuttons on the transmitter side that will appear as logic levels on the receiver side automagically. In addition to those two items, a decoder and encoder chip are needed, HT-12D and HT-12E, available from futurelec:

http://www.futurlec.com/Others/HT12Dpr.shtml
http://www.futurlec.com/cgi-bin/search/qty_price.cgi?part_no=HT12E

Joe

Sounds like a very cool device to experiment with. Based on the brochure at http://www.ecotesis.com/Biblioteca/Horizon/Horizon-FuelCell_H-1000xp_3_6kg_for_Eco-Marathon_2011-01.pdf it looks like the serial communication is aimed at driving a PC-type app that they provide, so you might have to reverse-engineer the messages going back and forth--or maybe the company would even send you the details if you ask.

Joe

Seems like Eagle costs a fair amount--the free version is pretty restricted, no? The hobbyist version (no commercial use) is $125.

ExpressPCB uses their own proprietary software--free--and there's an arduino shield footprint available:

http://www.joejaworski.com/arduino_shield.zip

Looks like 2500uF caps cost at least $13 each - and I'm going to need to build at least 5 of these rectifiers.

Well, like I said you could probably use 500 uF and get 5V of ripple (something like 11-16 V going into the 7805). You might even get by with 300 uF (8V ripple). Ripple rejection of the 7805 is over 60 dB and as high as 80 dB so there might be some millivolts of ripple on the 5VDC side if you can tolerate that.

Joe

Here's the formula for the capacitor required given a ripple voltage Vpp for a full-wave rectifier:

C = i/(2*f*Vpp)

So with i=300ma, f=60Hz and 1v ripple:

C = 0.3/(2*60*1) = 2500 uF.

To regulate that to 5VDC, I'd guess a 7805 voltage regulator would dissipate about 3 watts at your max current. You could probably get by with quite a bit more ripple--at 5V ripple, C=500 uF which would take some strain off the 7805 too.

Joe

I want pick the capacitive pulse with a ne555 like here http://www.sportdevices.com/rpm_readings/index.htm
and feed it to the LM2917..

now in my single cylinder bike there is one spark in every rotation..

so if the engine is running at 2000 rpm then what will be the frequency (Fin) ..??
2000 Hz or (2000/60 )Hz...??

Oh, sorry--right you are, it's 33 1/3 Hz. Funny, that!

Joe

Sounds like you want a contact microphone.

Here's one made from a Radio Shack piezo transducer:

http://home.earthlink.net/~erinys/contactmic.html

Joe

Maybe also a diode across the relay coil to limit any voltage spike when you turn off the transistor.

Joe