Update hardcoded I2S::SYSCLK exact mappings (#3497)

Adds a utility to run on the Pico and Pico2 to identify the lowest
frequency error possible for a given output sample rate.

Use that data on the Pico and Pico2 to implement a smarter I2S::setSysClk()
method which varies the SYSCLK depending on the sample rate and if the
I2S is bidirectional.

Originally had the same hardcoded values for RP2040 and RP2350, and the
RP2040 values were calculated from the release max F_CPU of 133, not the
new official 200MHz support.

Fixes #3496

Author: earlephilhower

libraries/I2S/examples/I2SBrute/I2SBrute.ino

@@ -0,0 +1,46 @@
+// Brute force SYSCLK checker for I2S.  Counts down from F_CPU and
+// calculates the absolute I2S speed error for a given frequency,
+// reporting the minimum encountered and at which frequency
+// (Note 64K is 32K bidirectional, see #3496)
+// Run it on any new CPUs or updated default F_CPUs.
+
+// End users should not need to ever run this
+// Released to the public domain 2026 by Earle F. Philhower, III
+
+void setup() {
+  delay(5000);
+  int F[11] = { 8000, 16000, 32000, 64000, 48000, 96000, 192000, 11025, 22050, 44100, 88200 };
+  for (auto x = 0; x < 11; x++) {
+    int f = F[x];
+    Serial.printf("Checking for %d\n", f);
+    int khz = (F_CPU * 1.10) / 1000;
+    float err = 999999.9;
+    int bestMatch = khz;
+    while (khz > 100000) {
+      uint a, b, c;
+      bool r = check_sys_clock_khz(khz, &a, &b, &c);
+      if (!r) {
+        khz--;
+        continue;
+      }
+      float edgerate = f * 64; // 32 bits, high and low events
+      float v = (khz * 1000) / edgerate;
+      float e = v - floor(v);
+      e /= v;
+      e *= 100.0f;
+      if (e < err) {
+        err = e;
+        bestMatch = khz;
+        Serial.printf("%d %f\n", bestMatch, (double)err);
+        if (err == 0) {
+          break;
+        }
+      }
+      khz--;
+    }
+    Serial.printf("%d: %d, err=%1.6f%%\n", f, bestMatch, (double)err);
+  }
+}
+
+void loop() {
+}

libraries/I2S/src/I2S.cpp

@@ -162,13 +162,75 @@ bool I2S::setFrequency(int newFreq) {
 }
 
 bool I2S::setSysClk(int samplerate) { // optimise sys_clk for desired samplerate
-    if (samplerate % 11025 == 0) {
-        return set_sys_clock_khz(I2SSYSCLK_44_1, false);
+    if (_isInput && _isOutput) {
+        samplerate *= 2; // We need 4 clocks per bit, not 2, in dual mode
+    }
+    int rate;
+    // Numbers taken from I2SBrute either at 0.0% error or a "reasonable" if not minimal
+#ifdef PICO_RP2350
+    switch (samplerate) {
+    case 8000:
+    case 16000:
+    case 32000:
+    case 48000:
+    case 96000:
+        rate = 153600; // Very mild overclock
+        break;
+    case 64000:
+        rate = 102400;
+        break;
+    case 192000:
+        rate = 147600;
+        break;
+    case 11025:
+        rate = 156000; // Mild overclock
+        break;
+    case 22050:
+        rate = 120000;
+        break;
+    case 44100:
+        rate = 144000;
+        break;
+    case 88200:
+        rate = 135600;
+        break;
+    default:
+        rate = F_CPU; // unknown, don't change
+        break;
     }
-    if (samplerate % 8000 == 0) {
-        return set_sys_clock_khz(I2SSYSCLK_8, false);
+#else
+    switch (samplerate) {
+    case 8000:
+        rate = 192000;
+        break;
+    case 16000:
+    case 32000:
+    case 48000:
+    case 96000:
+        rate = 153600;
+        break;
+    case 64000:
+        rate = 213000; // Mild overclock, OTW we drop 50% in speed
+        break;
+    case 192000:
+        rate = 196800;
+        break;
+    case 11025:
+        rate = 204000; // Very mild overclock
+        break;
+    case 22050:
+        rate = 216000; // Mild overclock
+        break;
+    case 44100:
+    case 88200:
+        rate = 192000;
+        break;
+    default:
+        rate = F_CPU;
+        break;
     }
-    return false;
+#endif
+    return set_sys_clock_khz(rate, false);
 }
 
 bool I2S::setMCLKmult(int mult) {

libraries/I2S/src/I2S.h

@@ -188,8 +188,4 @@ class I2S : public Stream, public AudioOutputBase {
     PIOProgram *_i2sMCLK;
     PIO _pio, _pioMCLK;
     int _sm, _smMCLK;
-
-    static const int I2SSYSCLK_44_1 = 135600; // 44.1, 88.2 kHz sample rates
-    static const int I2SSYSCLK_8 = 153600;  // 8k, 16, 32, 48, 96, 192 kHz
-
 };