ToneSweep improvements (thanks Pio)

https://forum.pjrc.com/threads/45246-Issues-with-tonesweep-in-audio-library

gui/index.html

 	<p class=func><span class=keyword>isPlaying</span>();</p>
 	<p class=desc>Returns true (non-zero) while the output is active.
 	</p>
+	<p class=func><span class=keyword>read</span>();</p>
+	<p class=desc>Returns the current frequency, or zero if the output is not active.
+	</p>
 	<h3>Examples</h3>
 	<p class=exam>File &gt; Examples &gt; Audio &gt; HardwareTesting &gt; ToneSweep
 	</p>

synth_tonesweep.cpp

   if(t_hi < 1)return false;
   if(t_hi >= (int) AUDIO_SAMPLE_RATE_EXACT / 2)return false;
   if(t_lo >= (int) AUDIO_SAMPLE_RATE_EXACT / 2)return false;
-  if(t_time < 0)return false;
+  if(t_time <= 0)return false;
   tone_lo = t_lo;
   tone_hi = t_hi;
   tone_phase = 0;
     uint64_t tone_tmp = (0x400000000000LL * (int)(tmp&0x7fffffff)) / (int) AUDIO_SAMPLE_RATE_EXACT;
     // Generate the sweep
     for(i = 0;i < AUDIO_BLOCK_SAMPLES;i++) {
-      *bp++ = (short)(( (short)(arm_sin_q31((uint32_t)((tone_phase >> 15)&0x7fffffff))>>16) *tone_amp) >> 16);
+      *bp++ = (short)(( (short)(arm_sin_q31((uint32_t)((tone_phase >> 15)&0x7fffffff))>>16) *tone_amp) >> 15);
 
       tone_phase +=  tone_tmp;
       if(tone_phase & 0x800000000000LL)tone_phase &= 0x7fffffffffffLL;
         }
         tone_freq += tone_incr;
       } else {
-        if(tmp < tone_hi) {
+        if(tmp < tone_hi || tone_freq < tone_incr) {
           sweep_busy = 0;
 
           break;

synth_tonesweep.h

   boolean play(float t_amp,int t_lo,int t_hi,float t_time);
   virtual void update(void);
   unsigned char isPlaying(void);
+  float read(void) {
+    __disable_irq();
+    uint64_t freq = tone_freq;
+    unsigned char busy = sweep_busy;
+    __enable_irq();
+    if (!busy) return 0.0f;
+    return (float)(freq >> 32);
+  }
 
 private:
   short tone_amp;