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External SDRAM support for libDaisy Nim wrapper

This module provides support for the external SDRAM chip on Daisy Seed. The SDRAM provides 64MB of additional RAM for audio buffers, delay lines, wavetables, and other large data structures.

Basic Usage:

import nimphea
import nimphea/sys/sdram

var hw = newDaisySeed()
var sdram = newSdramHandle()

# Large buffer in SDRAM
var delayBuffer {.codegenDecl: "$# $# __attribute__((section(\".sdram_bss\")))".}: array[480000, float32]

proc main() =
  hw.init()
  
  # Initialize SDRAM
  if sdram.init() != SDRAM_OK:
    return
  
  # Now you can use delayBuffer and other SDRAM variables
  for i in 0..<len(delayBuffer):
    delayBuffer[i] = 0.0
  
  while true:
    hw.delayMs(100)

SDRAM Memory Details:

• Base Address: 0xC0000000
• Size: 64MB (on Daisy Seed)
• Speed: 100MHz
• Chip: AS4C16M16SA or equivalent

Important Notes:

• SDRAM must be initialized before accessing SDRAM variables
• Use codegenDecl pragma with section attribute for SDRAM placement
• SDRAM is not suitable for real-time audio processing (use internal RAM)
• Perfect for buffers, wavetables, delay lines, sample storage

Delay Line Example:

const SAMPLE_RATE = 48000
const MAX_DELAY_SEC = 10

var delayLine {.sdramBss.}: array[SAMPLE_RATE * MAX_DELAY_SEC, float32]
var delayIndex = 0

proc audioCallback(input: ptr ptr cfloat, output: ptr ptr cfloat, size: csize_t) {.cdecl.} =
  for i in 0..<size:
    # Write to delay line
    delayLine[delayIndex] = input[0][i]
    
    # Read from delay line (simple)
    output[0][i] = delayLine[delayIndex]
    
    delayIndex = (delayIndex + 1) mod len(delayLine)

Wavetable Example:

const TABLE_SIZE = 2048
const NUM_TABLES = 64

var wavetables {.sdramBss.}: array[NUM_TABLES * TABLE_SIZE, float32]

proc initWavetables() =
  for tableNum in 0..<NUM_TABLES:
    for i in 0..<TABLE_SIZE:
      let phase = float32(i) / float32(TABLE_SIZE)
      let idx = tableNum * TABLE_SIZE + i
      wavetables[idx] = sin(phase * 2.0 * PI * float32(tableNum + 1))

Sample Buffer Example:

# Store 60 seconds of stereo audio at 48kHz
const BUFFER_SIZE = 48000 * 60 * 2

var sampleBuffer {.sdramBss.}: array[BUFFER_SIZE, float32]
var recordPos = 0
var playPos = 0

proc recordSample(left, right: float32) =
  if recordPos < BUFFER_SIZE - 1:
    sampleBuffer[recordPos] = left
    sampleBuffer[recordPos + 1] = right
    recordPos += 2

proc playSample(): tuple[left, right: float32] =
  if playPos < recordPos - 1:
    result.left = sampleBuffer[playPos]
    result.right = sampleBuffer[playPos + 1]
    playPos += 2
  else:
    result = (0.0'f32, 0.0'f32)