Question About Sine Table

lardom · 2020-11-23 17:58

The sine table in main ROM goes from $E000 to $F001. There are 2,049 16-bit samples in that memory block. I guess that means there are 4,098, or 4097, 32-bit registers. Does that mean sine samples repeat every two longs?

localroger · 2020-11-23 18:15

Addresses in Hub RAM are byte addresses, so the range $E000 to $F000 is 4 kilobytes ($1000 or 4096). The samples are words which are read from even byte offsets ($E000, $E002, $E004 etc.) and there are 2049 of them with a single word sample overlap at $F000-1, which simplifies some wraparound algorithms.

lardom · 2020-11-23 18:47

@localroger Thank you. I've been trying for several days to figure that out. I've seen "$F001-1" before and it had me scratching my head.
I wrote some test Spin code that displayed 4 digit integer sine values from 0° to 90°. The results were close enough but a lot still wasn't clear to me.
I'll order a DAC and see if I can produce a sine wave.

Duane Degn · 2020-11-23 20:33

You mentioned "MathTestSine" in a note. I just want to make sure you're also aware of this forum thread.

I thought you found "MathTestSine" in that thread but I suppose it's also possible to have found it on my GitHub.

BTW, I'm glad you found the program helpful.

Tracy Allen · 2020-11-23 21:46

You can use the Duty mode of the counter as a DAQ in order to produce your sine wave. It is pulse density encoded and can be easily smoothed with an RC circuit.

The attached tutorial program in Spin produces a 1Hz sine and suggests outputting it to an LED see the pulsations.

Below is the table lookup method. You can see that a full circle is 13 bits, and the top two point to quadrants 0--3. The lower 11 bits get shifted one place left in order to point to a word address in the sine table. The address at $F001 helps to tie the ends together as it transitions into quadrants 1 and 3.

PUB Fullsine(x) | q                                  ' x is from counter, 13 msbs 0--8192 represents 0--360 degrees
  q := x >> 11                                          ' quadrant, 0 to 3
  x := (x & $7ff) << 1                                  ' 0 to 90- degrees, adjust to 2048 word addresses.
  case q                                                ' by quadrant
    0 : result := word[$E000 + x]                       ' 
    1 : result := word[$F001 - x]
    2 : result := -word[$E000 + x]
    3 : result := -word[$F001 - x]
  return

lardom · 2020-11-23 23:38

@"Duane Degn" I found your post on the forum. The value @E801 is 46340 and I could not figure how to convert that to sine 45° 0.7071. You pointed out that the key was to divide the raw table values by '1' as a 16-bit value or 65535. I tested it for 30°, 45° and 60°. It worked and I exhaled. I didn't want to bump up a post from 2015 just to say thank you so I left it on your message board instead.
@"Tracy Allen" All four quadrants using duty mode with an RC circuit sounds like an excellent test. That's what I'll do. Thank you.
@localroger so hub ram is byte addressed and cog ram is long addressed. Got it.👍

Tracy Allen · 2020-11-24 00:02

I forgot to attach the tutorial program to my previous post; now it's there. Oh, edited again, to default to a standard 5MHz crystal and pin numbers for LEDs on the old Parallax demo board.
https://forums.parallax.com/discussion/83915/spin-code-examples-for-the-beginner-public-version/p7

lardom · 2020-11-24 14:40

@"Tracy Allen" This is a wonderful sight. 🙂 Thank you.

lardom · 2020-12-14 23:19

I'm posting a zip for those who search the subject in the future. I hope they can benefit from what I've learned.

Question About Sine Table

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