Gate Array Learn

	apt install yosys fpga-icestorm nextpnr-ice40 verilator flashrom
	make
	doas make load

Raspberry Pi UART
	Enable serial communication in raspi-config.

UART
	stty -F /dev/ttyUSB1 3000000 raw cs8
	hexdump -ve '1/1 "%u\n"' /dev/ttyUSB1 | pv > pcm.txt

	pv /dev/ttyUSB1 > pcm.raw
	hexdump -ve '1/1 "%u\n"' pcm.raw > pcm.txt
	play -r 48k -e unsigned-integer -b 8 -c 1 pcm.raw

References
	Adafruit PDM Microphone Breakout
		https://learn.adafruit.com/adafruit-pdm-microphone-breakout/
	Cascaded Integrator-Comb (CIC) Filters
		https://www.dsprelated.com/showarticle/1337.php
	CIC filter in Verilog
		https://github.com/ericgineer/CIC/blob/master/CIC.v
	Pulse Density Modulation
		https://curiouser.cheshireeng.com/2015/01/21/pdm-in-attiny85-source-code/
		https://en.wikipedia.org/wiki/Pulse-code_modulation
		https://tomverbeure.github.io/2020/10/04/PDM-Microphones-and-Sigma-Delta-Conversion.html
		https://tomverbeure.github.io/2020/12/20/Design-of-a-Multi-Stage-PDM-to-PCM-Decimation-Pipeline.html
	RMS in FPGA
		https://www.controlpaths.com/2022/07/18/true-rms-compute-in-fpga/
	Verilator
		https://www.veripool.org/verilator/
		https://verilator.org/guide/latest/
	Verilog by Example (Blaine C. Raedler 2021)
		http://readler.com/books2.html