519 Sine Wave Synthesizer :: Quicker, easier and cheaper to make your own chip!

How it works

tt_um_abeccari_swsynth is a CORDIC sine-wave synthesizer: it plays an equal-tempered musical note as an audio tone. You choose the pitch on the input pins, and the tone comes out three ways — a parallel digital sample bus and two 1-bit pulse-density-modulated (PDM) streams.

GDS layout assembled with IHP PDK.

Signal chain:

Note select — ui[3:0] picks a semitone (see the table below) and ui[7:4] an octave above A0 = 27.5 Hz. The output frequency is f = 27.5 · 2^(note/12) · 2^octave Hz, with the octave clamped to ≤ 8 so the tone stays below the Nyquist rate.
NCO — a frequency map turns the note into a phase increment that drives a 20-bit phase accumulator, stepped at the 48 kHz sample rate (clk / 256).
CORDIC — a rotation-mode CORDIC converts the phase into signed sine and cosine samples (~8-bit accurate).
Outputs —
- uo[6:0] — 7-bit parallel sine, offset-binary (0x40 = zero crossing), refreshed once per 48 kHz sample. uio[0] (SAMPLE_EN) pulses high for one clock at each new sample.
- uo[7] — PDM_I, a 1-bit sigma-delta stream of the sine at the full clock rate; low-pass filtered, it reconstructs the analog sine.
- uio[7] — PDM_Q, the same for the cosine (90° out of phase with PDM_I) — an I/Q pair.
- uio[6] — SQR, a 1-bit square wave at the tone frequency (the sign of the sine); a clean digital output that needs no filter.
- uio[5] — SAW, a 1-bit sigma-delta sawtooth at the tone frequency (the raw phase ramp); low-pass filter to reconstruct.
- uio[4] — NOISE, a 1-bit pseudo-random bitstream from a maximal-length 20-bit LFSR, advanced one step per 48 kHz sample. Unlike the PDM outputs this is a baseband signal, not a density-coded one — its 0–24 kHz band already is audio-band white noise, so no demodulation is needed. Being a zero-order-hold at 48 kHz it carries spectral images above f_s/2; a gentle low-pass at ≈ 20–24 kHz rejects those ultrasonic images (anti-imaging / reconstruction) for clean band-limited noise and to spare the amplifier ultrasonic switching energy. The sequence is deterministic and repeats every 2²⁰−1 samples (≈ 21.8 s).

Semitone codes on ui[3:0] (codes 12–15 have no distinct note and fold back to A):

`ui[3:0]`	0	1	2	3	4	5	6	7	8	9	10	11	12–15
Note	A	A#	B	C	C#	D	D#	E	F	F#	G	G#	A

The clock is 12.288 MHz = 256 × 48 kHz.

How to test

Clock — drive clk at the 12.288 MHz design point; the pitch and 48 kHz sample rate scale with it (f_s = clk / 256), and being fully synchronous it also runs correctly at lower rates. Clocks up to ~60 MHz are within the process corners (worst-case Fmax ≈ 66 MHz at the slow corner, from signoff STA); the demo board's on-board clock tops out at 50 MHz, which the design meets with margin, so drive clk externally to go higher.
Reset — hold rst_n low for a few clock cycles, then release it high.
Pick a note — set ui[7:4] = octave and ui[3:0] = semitone. For example ui_in = 0x40 → octave 4, note 0 (A) → 440 Hz (concert A); ui_in = 0x80 → note A, octave 8 → 7040 Hz.
Monitor —
- Put a logic analyzer on uo[6:0] (the 7-bit sine) and use uio[0] (SAMPLE_EN, 48 kHz) as the sample clock / scope trigger. The value traces a sine centered on 0x40.
- Scope uo[7] (PDM_I) and uio[7] (PDM_Q): fast 1-bit streams whose pulse density follows the sine and cosine.
Change the note on ui_in at any time; the tone follows on the next samples.

External hardware

On the TinyTapeout demo board the on-board RP2040 selects the design, supplies the clock, drives ui_in, and reads the outputs — so you can set the note from the on-board DIP switches or a MicroPython script and capture the 48 kHz parallel-sine samples in software, with no extra parts. All I/O is also broken out on the Pmod and SIL headers for the analog add-ons below.

Hear it: low-pass filter uo[7] (PDM_I) with a simple RC and drive a powered speaker or amplifier. The Audio Pmod does exactly this (RC reconstruction filter + amplifier + jack) and takes its input on uo[7], so it plugs straight into the output Pmod.
Quadrature (I/Q): filter uio[7] (PDM_Q) the same way for a second channel 90° out of phase — useful for demodulation experiments.
Parallel DAC: feed uo[6:0] (offset-binary, midscale 0x40) into a 7-bit R-2R resistor-ladder DAC on the output Pmod or a SIL header, latched by SAMPLE_EN (uio[0]), for an analog staircase sine; add a gentle RC afterward to smooth the sampling images.

PDM I and Q streams low-pass filtered back to sine and cosine (top), their spectra before/after filtering (middle), and the I/Q cross-correlation confirming 90° quadrature (bottom), note A, octave 8 = 7040 Hz

#	Input	Output	Bidirectional
0	NOTE0 (semitone LSB)	SINE0 (sine LSB, offset-binary)	SAMPLE_EN (48 kHz sample strobe)
1	NOTE1	SINE1	unused
2	NOTE2	SINE2	unused
3	NOTE3 (semitone MSB; 0=A..11=G#)	SINE3	unused
4	OCT0 (octave LSB)	SINE4	NOISE (1-bit white noise, maximal-length LFSR PRBS)
5	OCT1	SINE5	SAW (1-bit sigma-delta sawtooth at the tone frequency)
6	OCT2	SINE6 (sine MSB)	SQR (1-bit square wave at the tone frequency)
7	OCT3 (octave MSB; above A0=27.5Hz)	PDM_I (1-bit sigma-delta audio out)	PDM_Q (1-bit sigma-delta audio out - 90 degrees out of phase from PDM_I)

519 Sine Wave Synthesizer