Show HN: An efficient SRL32 (cascading shift registers) clock prescaler in VHDL

gist.github.com

2 points

2 years ago

Having just started learning FPGA Hardware Description Languages by attempting to write a simple LED blinker, I found that the overwhelming majority of the Internet's solution to slowing down a fast clock (for making the pulsing of an LED visible to the human eye) was either using vendor-specific, proprietary clock managers and PLLs or implementing some twenty-something-bit-wide counter as to count hundreds of thousands of clock cycles and generate a 1 Hz output.

Although there is a world of difference between counters in hardware-accelerated designs and those in software-emulated ones, I nonetheless viewed the number of daisy-chained components resulting from a mere counter as far-from-ideal and absurd; I began searching for a more efficient method.

I came upon a rather obscure blog post from 2015 (http://www.markharvey.info/art/srldiv_04.10.2015/srldiv_04.1...) outlining the exact same issue while also referencing the Xilinx systems designer Mr. Ken Chapman's proposal: using FPGAs' shift register primitives (e.g., Xilinx's SRL32E) to alleviate that.

However, the method described therein would rely on the user to calculate the target frequency's factors between [2, 32) and painstakingly connect each and every instance of SRL32Es to one another, all in a manual manner, not to mention that the resulting pulse would have a low, one-cycle-long duty.

Thus, I wrote srl_prescaler.vhd, a fully automated template generator in VHDL for an efficient register-based cascaded clock divider based solely on SRL32 primitives alongside AND gates---the advantage of this module is that it is very generic and easy-to-use:

```

prescaler : entity work.srl_prescaler

generic map (100e6, 1)

port map (clk_in_100mhz, ce_out_1hz);

```

In the above example, an input clock of 100 MHz (i.e., `100e6` & `clk_in_100mhz`) gets divided into a clock enable signal of 1 Hz (i.e., `1` & `ce_out_1hz`). Among the other improvements, a third optional parameter (i.e., the duty cycle) may also get supplied as a real number (0.00, 1.00) to the generic map.

Overall, this small project makes an otherwise-niche method more accessible by actually making use of the many language features that VHDL has to offer (e.g., pre-computing factor results using functions, automating hardware creation via for...generate clauses, latching using registers and guarded signals, etc.), serving as a good yet practical learning point.