Question About Sine Table

lardom

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

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

@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

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

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

@"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

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

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

lardom

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