614 Ring osc on VGA :: Quicker, easier and cheaper to make your own chip!

How it works

Manually-instantiated gf180mcuD inverter cells form a chain out of chain segments of varying lengths, allowing the user to select given points in the overall chain to loop back to produce a ring oscillator. This makes a configurable ring oscillator that is expected to be able to oscillate from about 15MHz up to 350MHz.

This (or an external clock) then can be selected to drive a "worker" module: a counter which counts up to 3000.

Alongside this is a VGA sync generator which takes its pixel colour from whatever is in the upper 6 bits of the worker's counter at the time. The worker is reset during HBLANK of each VGA line.

It's expected that at the faster ring oscillator speeds, the counter will reach its target of 3000 sooner than the width of the VGA line but with some jitter... or the counter/compare logic will break down because it's too fast.

How to test

Set clksel2[1:0] to 0.

Set clksel[3:0] to (say) 10, or anything greater than 1.

Set mode[1:0] to 0 (though these are unused at the time of writing; TBA).

Set vga_mode to 0.

Attach a Tiny VGA PMOD to uo_out.

Supply a 25MHz clock to the system clk, and assert reset for at least 2 clocks.

Expect to see vertical coloured bars on screen, but expect some jitter. Their width should increase as you increase clksel.

Measure the ring oscillator (or rather, the selected clock source) on uio_out[7:4]: uio_out[4] is the raw oscillator output, and the higher bits are the oscillator divided by powers of 2.

More testing notes:

When vga_mode==1, clk should be 26.6175MHz (106.47 MHz ÷ 4) to drive a 1440x900 60Hz VGA display.
When clksel2 is:
- 0: Just rely on clksel.
- 1: Use fixed 5-deep inv_1 ring oscillator.
- 2: Use fixed 5-deep inv_4 ring oscillator.
- 3: Use inverted clk.
- NOTE: options 1 and 2 require clksel > 1 (any value will do) to enable the rings.
When clksel2==0 and clksel is:
- 0: Use clk.
- 1: Use altclk.
- For values 2 and above, use an inv_2-based ring oscillator tapped at...
- 2: => 3
- 3: => 5
- 4: => 7
- 5: => 9
- 6: => 13
- 7: => 17
- 8: => 21
- 9: => 25
- 10: => 33
- 11: => 41
- 12: => 49
- 13: => 65
- 14: => 97
- 15: => 161

External hardware

Tiny VGA PMOD and a VGA monitor.

#	Input	Output	Bidirectional
0	clksel[0]	r7	IN: clksel2[0]
1	clksel[1]	g7	IN: clksel2[1]
2	clksel[2]	b7
3	clksel[3]	vsync
4	altclk	r6	OUT: osc
5	mode[0]	g6	OUT: div2
6	mode[1]	b6	OUT: div4
7	vga_mode	hsync	OUT: div8

614 Ring osc on VGA