A simple time stamper, frequency counter.

Let Raspberry Pi Pico count the number of clock cycles between two (or more) rising edges of an input signal. Resulting time mark of each rising edge, or clock count between adjacent rising edges or measured frequency is written to serial or USB output.

The code allows for different external/internal clock sources to be used.

^* Using PLL with external clock source requires hardware modifications and Pico SDK modifications. On the HW side it is required to remove onboard XOSC and to connect the external clock to XIN pin of the RP2040 chip. External clock source has to have squared 3.3V signal up to 25 MHz (by experience). To upload a new code 12 MHz is required. See additional notes below.

RP2040 allows external clock in range 1-15 MHz. GPIO pins are able to handle up to 50 MHz. See RP2040 datasheet for details. Internal PLL can be sourced only from the onboard XOSC.

The code can be modified (see below) to output:

• timemarks for each channel a timemark (in seconds from first sensed pulse) and channel name. These can be directly used in e.g. TimeLab. Four simultaneus channels are possible.
• frequency calculated from counted sys clock periods within one or mode input signal periods. If 1 period used (AVG_PERIODS 1) 2 input channels can be used. When averaging on more periods required sensing only one input channel is possible (because of low PIO memory?)
• cycle count number of sys clock periods within one or mode input signal periods. If 1 period used (AVG_PERIODS 1) 2 input channels can be used. When averaging on more periods required sensing only one input channel is possible (because of low PIO memory?)

Examle output for timemarks

3.999672500	 ChC
3.999989410	 ChB
4.999672500	 ChC
4.999985890	 ChB
5.999672500	 ChC
5.999982350	 ChB
6.999672500	 ChC
6.999978820	 ChB

Indicator GPIOs will output 10 Hz signal while channel input HIGH. You will see pulsing led on 1 PPS 50-50 duty cycle input or just one blink for short pulse (<100ms) PPS input.

• Connect input pulse signal one (or more) input channel GPIOs.
• Connect external clock input to GPIO20 (pin 26).
• Connect LED indicators for each channel
• Connect to GPIO0 (pin 1) for serial output (115200 bauds), optional.
• PPS clock output available at GPIO21 (pin 27).

Note all Raspberry Pi Pico GPIOs are 3.3V only. Do not connect 5V signals unless you known what you are doing.

To compile your own build you need to have installed C/C++ RPi Pico toolchain. Consult Getting started guide on how to prepare your environment.

Then consider following source code modifications:

In the CMakeLists.txt file enable USB nd disable UART for USB output or vice versa by modification of these 2 lines

pico_enable_stdio_usb(picoPET 1)
pico_enable_stdio_uart(picoPET 0)

Set 1 for enabling the output interface and 0 for disabling.

NOTE: Using of USB requires system clock of 48 MHz or above. It will not work e.g. external 10 MHz without not using the internal PLL.

In the picoPET.c file uncomment one of the appropriate clock sources or feel free to implement you own.

//#define CLK_SRC_XOSC
#define CLK_SRC_XOSC_PLL
//#define CLK_SRC_EXT_CLOCK
//#define CLK_SRC_EXT_CLOCK_PLL

When using external clock (CLK_SRC_EXT_CLOCK) with other than 10MHz frequency, change also the uint clk_src_freq variable.

uint clk_src_freq = 10*MHZ;             // change this only if CLK_SRC_EXT_CLOCK defined

In the picoPET.c file uncomment one of the appropriate output types or feel free to implement you own.

#define OUTPUT_FREQUENCY
//#define OUTPUT_TIMEMARK
//#define OUTPUT_CYCLE_COUNT

More the one period of the input signal can be sensed and thus increasing the gate time and resolution. The number of input signal periods is configured by AVG_PERIODS constant in the picoPET.c file.

#define AVG_PERIODS 1                   // number of periods to average; has to at least 1, for steady results use odd numbers 1, 3, 5...

You can change the input signal pin for up to 4 channels and indicator LED pins by changing INPUT_SIGNALx_GPIO and INPUT_SIGNALx_LEDGPIO constants. Change the SM_COUNT to number of input channels required, this will free 2 PIOs for each unused channel.

#define SM_COUNT 2
#define INPUT_SIGNALA_GPIO 5
#define INPUT_SIGNALA_LEDGPIO 4
#define INPUT_SIGNALB_GPIO 6
#define INPUT_SIGNALB_LEDGPIO 7
#define INPUT_SIGNALC_GPIO 9
#define INPUT_SIGNALC_LEDGPIO 8
#define INPUT_SIGNALD_GPIO 13
#define INPUT_SIGNALD_LEDGPIO 12

NOTE: External clock must be connected to GPIO20 (default) or GPIO22.

Using external clock with PLL requires connecting the external clock signal to RP2040's XIN pin and also modify the Pico-SDK and set new frequency.

Connecting external clock to XIN is best achieved by removing the onboard XOSC and bypassing the XOSC ouput pad with nearest (e.g. GPIO10) pin. To upload new code to the Pico 12 MHz signal has to be used. Convenient, reasonable stable 12 MHz 3.3V square signal can be obtained e.g. from the ublox GNSS modules. These use 48 MHz internal clock and 12 MHz is natural number divider so jitter is minimalized.

New XOSC frequency of the external clock has to be configured in the Pico SDK. Make a new copy of the SDK and modify these files

Change the XOSC_MHZ constant from 12 MHz to frequency of your external clock (e.g. 10 MHz)

#define XOSC_MHZ 10

Change the XOSC_MHZ constant from 12 MHz to frequency of your external clock (e.g. 10 MHz)

#define XOSC_MHZ _u(10)

The clock.c hardwires the 12 MHz frequency of onboard XOSC and we want to make it variable refering XOSC_MHZ constant. Locate the clocks_init() procedure update the configuration of clk_ref by substiting 12 * MHZ constant with XOSC_MHZ. Change the line

    clock_configure(clk_ref,
                    CLOCKS_CLK_REF_CTRL_SRC_VALUE_XOSC_CLKSRC,
                    0, // No aux mux
                    12 * MHZ,
                    12 * MHZ);

to

    clock_configure(clk_ref,
                    CLOCKS_CLK_REF_CTRL_SRC_VALUE_XOSC_CLKSRC,
                    0, // No aux mux
                    XOSC_MHZ,
                    XOSC_MHZ);

• Uses Ublox GPS output as 12 MHz reference clock instead of onboard TCXO.
• GNSS locked stated sensed from NMEA GGA sentence. Red "locked" indication led blinks when unlock, steadily on when locked.
• External 10 MHz clock source can be switched on using hardware switch. USB output still active.
• External signal sensed using GPIO 11 (better would be GPIO22 :( ). Green indication led blinks when signal connected, steadily on when used as reference.
• System PLL configured for 240 MHz with 12 MHz reference and 200MHz with 10 MHz reference, giving 10ns resolution @10 MHz reference signal.
• Configurable divided clock output frequency using internal switches.
• Schematic available