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33 Simple Signal Generator

33 : Simple Signal Generator

Author: Matt Venn
Description: Square wave generator with a phase-shifting second channel for metastability experiments
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Clock: 50000000 Hz

How it works

Two square wave generators drive uo[1:0].

Channel 0 (uo[0]): independent square wave. High-time (on_count) and low-time (off_count) are set independently via SPI, giving full control over frequency and duty cycle.

Channel 1 (uo[1]): phase-shifted replica of ch0. It uses the same on_count and off_count as ch0, but its rising edge is offset by a programmable delay. This is designed for exploring metastability on an SR latch: by placing ch1's edge close to ch0's edge, the two inputs can be brought arbitrarily close together.

Phase control

The total delay applied to ch1's rising edge is:

total_delay = spi_offset + enc_int + sigma_delta_carry

spi_offset (registers 4–5): signed 16-bit static offset in clock cycles, set via SPI. Positive = ch1 lags ch0; negative = ch1 leads ch0. Range: ±32767 cycles.

Encoder (ui[4]=A, ui[5]=B): a quadrature encoder adjusts the phase in real time. Each click changes the phase by enc_step / 256 cycles (Q8 fixed-point). The integer part of the encoder accumulator is added directly to the delay; the fractional part is dithered via first-order sigma-delta modulation, so sub-cycle offsets are averaged accurately over multiple periods. Encoder range: ±127 cycles (use spi_offset for coarse positioning).

enc_step (register 10): step size per encoder click in 1/256-cycle units. Examples:

enc_step = 1 → ~0.004 cycles/click (~78 ps at 50 MHz), finest resolution
enc_step = 64 → 0.25 cycles/click
enc_step = 128 → 0.5 cycles/click
enc_step = 255 → ~1 cycle/click

Encoder button (ui[2], PmodEnc's SWT pin, active-high): while held, multiplies enc_step by 8x for fast scanning across the phase range. Releasing it reverts the very next click to the normal step size.

ch1's delay (spi_offset + enc_int + sigma_delta_carry) covers the full [0, period) range with no clamping — it wraps modulo the period, so ch1's pulse can land anywhere relative to ch0's, including spanning the period boundary. (For very small periods combined with large spi_offset/encoder values — larger in magnitude than the period — the wrap is computed mod one period only, so the resulting delay may not match the mathematical mod for |total_delay| >= period; this is a pre-existing edge case unrelated to normal use.)

Frequency formula

At 50 MHz, for a 50% duty-cycle square wave at frequency F:

on_count = off_count = 25_000_000 // F

Both counts are 16-bit → minimum frequency ≈ 382 Hz (on_count = off_count = 65535).

Setting on_count = 0 silences both channels.

The SPI peripheral is reused from calonso88/tt07_alu_74181.

How to test

The SPI master (e.g. RP2350 running MicroPython) programs the design using machine.SoftSPI. Each SPI frame is 2 bytes:

Byte 0: 0x80 | reg_addr (write), or reg_addr (read)
Byte 1: data

SPI mode: CPOL and CPHA are set via ui[0] and ui[1] respectively (both 0 for mode 0).

Register map

Reg	Field	Notes
0	ch0 `on_count[15:8]`	MSB
1	ch0 `on_count[7:0]`	LSB
2	ch0 `off_count[15:8]`	MSB
3	ch0 `off_count[7:0]`	LSB
4	`spi_offset[15:8]`	Signed 16-bit MSB; +ve=lag, −ve=lead
5	`spi_offset[7:0]`	Signed 16-bit LSB
6	`enc_step[7:0]`	Q8 step per encoder click (0–255)
7	—	reserved

MicroPython example — 10 kHz, 500-cycle lag, encoder fine-tune

from machine import Pin, SoftSPI

spi = SoftSPI(baudrate=100_000, polarity=0, phase=0, bits=8,
              firstbit=SoftSPI.MSB,
              sck=Pin(30), mosi=Pin(31), miso=Pin(28))
cs = Pin(29, Pin.OUT, value=1)

def write_reg(addr, val):
    cs(0); spi.write(bytes([0x80 | addr, val])); cs(1)

# ch0: 10 kHz, 50% duty (on=2500, off=2500 @ 50 MHz)
write_reg(0, 0x09); write_reg(1, 0xC4)  # on_count  = 2500
write_reg(2, 0x09); write_reg(3, 0xC4)  # off_count = 2500

# ch1: 500-cycle static lag (10 µs)
offset = 500  # cycles
write_reg(4, (offset >> 8) & 0xFF)
write_reg(5,  offset       & 0xFF)

# Encoder: 0.25 cycles/click for fine phase adjustment
write_reg(10, 64)

For RP2350 on the TT demo board, scripts/demo.py provides ready-made helpers (set_ch0, set_ch0_counts, set_phase_offset, set_enc_step, silence). Use scripts/run_freq.py to program from the command line:

python scripts/run_freq.py 1000 --offset 500 --enc-step 64

Encoder hardware

Connect a Digilent PModEnc to the bottom row of the input Pmod connector on the Tiny Tapeout demo board. This maps the encoder A/B outputs to ui[4] and ui[5], and the encoder's pushbutton (SWT) to ui[2] (fast-scan, see "Phase control" above).

Testing

Simulation (cocotb)

source /home/matt/oss-cad-suite/environment
cd test && make

Test	What it checks
`test_spi_registers`	Round-trip read/write of all config registers
`test_frequency_generation`	ch0 outputs correct frequency (edge count)
`test_ch1_inphase`	offset=0 → ch1 fires simultaneously with ch0
`test_spi_phase_offset`	Positive SPI offset → ch1 lags ch0 by correct cycle count
`test_channel_silence`	on_count=0 holds both outputs LOW
`test_encoder_integer_phase`	Encoder clicks accumulate to integer cycle offset
`test_encoder_sigma_delta`	Fractional enc_step dithers delay via sigma-delta
`test_encoder_button_fast_scan`	Holding the encoder button multiplies enc_step by 8x
`test_encoder_fast_scan_clamp_no_wrap`	Holding the button at ENC_MAX/ENC_MIN clamps correctly instead of wrapping (16->17-bit overflow fix)

Hardware-in-the-loop (HIL)

test/test_hil.py verifies frequency accuracy on real hardware using a Keysight oscilloscope and the RP2350. Probes on uo[0] (CH1) and uo[1] (CH2).

External hardware

RP2350 (or any SPI master) connected to uio[6:4] (MOSI, CLK, CS_N) and uio[3] (MISO)
Digilent PModEnc on the bottom row of the input Pmod connector (→ ui[4]=A, ui[5]=B, ui[2]=button/SWT for 8x fast-scan)
Oscilloscope on uo[1:0]
SR latch with S=uo[0] and R=uo[1] (or vice versa) for metastability experiments

IO

#	Input	Output	Bidirectional
0	SPI CPOL	Channel 0 square wave output
1	SPI CPHA	Channel 1 square wave output
2	Encoder button (hold for 8x fast-scan)
3			SPI MISO
4	Encoder A		SPI CS_N (active low)
5	Encoder B		SPI CLK
6			SPI MOSI
7

33 Simple Signal Generator