Clock
The information in this document applies to Tiny Tapeout 04 and beyond.
Tiny Tapeout includes a clock input signal (clk), provided externally through the mprj_io[6] pin of the caravel chip (pin number 37 in the QFN-64 chip package).
Limitations
Internally, both the clk and rst_n pins are handled like any other input pins. We expect a latency (insertion delay) of up to 10 nanoseconds between the chip’s I/O pad and your project’s clock.
The caravel chip uses the sky130_ef_io_gpiov2_pad macro for the I/O pads. The documentation specifies a maximum input frequency of 66 MHz. Therefore, we believe that the maximum clock frequency for your designs will be around 66 MHz.
Clock Generation
The Tiny Tapeout Demo board can generate the clock signal for your designs. The clock is generated by the on-board RP2040 microcontroller (firmware source code).
The RP2040 can generate a wide range of frequencies, including 50 MHz, 48 MHz (useful for USB), 40 MHz, 25.179 MHz (useful for VGA), 10 MHz, 1 MHz. If you need a very slow clock like 1Hz, this can be generated with a simple circuitpython program.
You can find the complete lists of verified frequencies in the firmware’s README file. If you need a frequency that does not appear on the list, feel free to open an issue and we’ll look into adding it to the list.
