841 Munch :: Quicker, easier and cheaper to make your own chip!

How it works

This generates VGA output for a munching squares animation plus some other stuff, and some simple music. It uses the VGA and audio PMODs listed below.

Clock generation

A clock generator module divides the main clock clk and vsync into two derived clocks - a 393kHz PWM clock for audio output, and a 10Hz "audio tick" clock that drives the pattern sequencer. The clock generator also provides counters for PWM, volume modulation, and audio pattern sequencing.

LFSR

An 11-bit LFSR provides a noise source. It is an XNOR type, shifting down towards the LSB and inserting the new bit at MSB. XNOR taps are on bits 0 and 2. Bits 0-5 of the LFSR register are used to provide a noise channel and randomized video noise dithering.

Video

The video output is the standard 640x480 @ 60Hz, using a 25.175MHz pixel clock and negative polarity HSYNC/VSYNC. Timing is implemented with a simple two-counter design shamelessly stolen from the Tiny Tapeout VGA playground.

A fixed palette of eight colors is used, and eight brightness levels are created by mixing random bits with the 2-bit per channel brightness levels. Video is output on three layers - the background and layers 0 and 1. A non-black pixel overrides a pixel on any lower layer.

Audio

Audio comes from a basic PSG inspired by the SN76489. There are four channels of sound based on 12-bit timers - three pulse channels and one noise channel. The only real difference between a pulse channel and a noise channel is that the pulse channel flips state when the timer counts down, and the noise channel takes a random state from the LFSR. Each channel also has a two-bit volume level.

The 25.175MHz clock is divided by a 6-bit counter to create a 393KHz PWM output. 6-bits gives 64 possible levels. The PWM high period is a simple sum of the four channels' volumes at any given instant (multiplied by two with the low bit dithered from the LFSR). This does mean the PWM will glitch if volume levels change in the middle of a PWM cycle, but that's fine in practice since it's all low-pass filtered anyway.

The four channels are programmed through a sequencer that provides note and volume data to the PSG. The sequencer is clocked by dividing VSYNC by 6, so the sequencer moves through pattern rows at 10Hz, or 600 ticks per minute. Each pattern of 16 ticks represents one measure, four beats, which means the music proceeds at 150 BPM.

The sequencer cycles through pre-programmed patterns of notes. Note timer data is read from an indirected list of notes, then connected directly to the PSG reload values. This does mean the oscillators are not synchronized at note start. Volumes are modulated through a single repeating pattern per channel, indexed from the top two bits of the sequencer div-by-6 clock divider. This means the volume is a three-step pattern cycling at the start of each pattern tick.

Text Generator

On-screen text uses a segmented approach, where each segment is defined by a mathematical description of a line segment. Each character is then defined by which segments are off or on, like a multi-segment LED display. So text is generated at full resolution despite its large size; each character is 50x100 pixels.

The text generator is just a sequencer over an input bit stream, indexed by the horizonal and vertical position. In this implementation the input is at most six characters long. The text can be positioned arbitrarily, but for this demo it is fixed.

Stage Sequencer

A slower stage clock is derived from the pattern clock. It ticks once every pattern cycle, and drives an overarching "stage sequencer". Each stage counts down for a pre-programmed number of patterns, then switches to the next. The stage number is used in various logic to change the text and colors over time.

Extra outputs

In addition to the audio and video, the three highest bits of the internal pattern counter are output on uio_out[6:4]. The two highest bits count out the four beats in a pattern, and bit 1 has a negative edge at the beginning of each beat. This could be used for beat synchronization with external systems - I just used it for debugging.

How to test

Set the input clock for 25.175MHz. The Pico/RP2040 can output 25.177MHz on GPOUT0 with a 125MHz main clock and a divider of 4 [integer part] and 247 [fractional part]. This worked on my TV.

Reset, and enjoy. :)

#	Output	Bidirectional
0	R1
1	G1
2	B1
3	VSYNC
4	R0	beat clock bit 1 (output)
5	G0	beat clock bit 2 (output)
6	B0	beat clock bit 3 (output)
7	HSYNC	audio (output)

841 Munch

841 : Munch