ThrottleBlaster

ThrottleBlaster

A Pi Pico-based Turbo circuit with a MHz display that reduces the effective frequency of fast CPUs by pulling down the STPCLK# pin at a specified frequency and duty-cycle.

This allows you to play speed-sensitive games, like Digger on a 1200MHz Athlon!

Since rev.0.8e we also support the Pico 2!

Videos:

• Part 3 (rev 0.3): https://www.youtube.com/watch?v=g4OluJwGDEQ

• Part 2: https://www.youtube.com/watch?v=nGy8OmOe_34

• Part 1: https://www.youtube.com/watch?v=9uNml2j6sy0

• Mounted on a 5.25 panel: https://www.youtube.com/shorts/n1aFvRNtOcw (by Michael Swimm)

• Controllable by serial: https://www.youtube.com/watch?v=71rln-R2mis (by Michael Swimm)

Download firmware ThrottleBlaster_pico1.uf2 (or ThrottleBlaster_pico2.uf2 for the Pico 2): https://github.com/scrapcomputing/ThrottleBlaster/releases

Download gerbers (ThrottleBlaster_gerbers_rev.X.X.zip): https://github.com/scrapcomputing/ThrottleBlaster/releases

A front panel design by Michael Swimm: https://www.thingiverse.com/thing:6619261

How it works

The Throttle Blaster is a fancy PWM controller that pulls the CPU's STPCLK# to ground.

It is tailored to the needs of vintage PC enthusiasts, so it drives a 4-digit 7-segment display and is pre-loaded with presets that span several hardware generations.

Features

• Four modes of operation to fit your needs, selected by jumpers:
  1. single push-button control. This is great for re-purposing the Turbo button (NOTE: the on/off turbo switch would need to be replaced with a push-button).
  2. Two-button control (left/right).
  3. rotary-encoder with a push-button. This is great for placing it on a drive bay panel.
  4. analogue potentiometer with/without a push-button. This could be used for configurations without a display.
• 7-segment TM1637-based display that shows the effective frequency. This can be considered optional, but is highly recommended.
• Cycle through preset frequencies or select a frequency at a 1MHz granularity.
• Each preset can be tuned/programmed both in terms of the PWM level but also the PWM frequency.
• Configuration changes are permanently stored in the Pico's flash memory.
• The PWM pulses are generated by the Pico's PIOs and are clock-accurate, with no jitter.

Modes of operation

Mode 1: Rotary Encoder (ROT)

• The devices starts at the "Presets" state (mode).
• Turn the knob to select a frequency
• Short push to switch to fine-grain frequency selection
• Long push to enter configuration of the current preset:
  • Fine-tune actual frequency, short push to switch to PWM period tuning. Long-push escapes.
  • Fine-tune PWM period, short push to switch to preset deletion. Long-push escapes.
  • Select if preset should be deleted, short push to confirm.

Initial configuration

• The default CPU frequency is 200MHz. Long push to configure the CPU speed. Select the frequency and short push to get back to the presets.

Mode 2: Single-button (1Btn)

• The device starts at the "Presets" state.
• Three types of push:
  1. Short push (release immediately)
  2. Medium push (for ~0.5 seconds)
  3. Long push (about 2 seconds)
• Cycle through presets with a short push. A medium push brings us back to the default CPU frequency.
• Long push to enter programming mode.

Mode 3: Two-button (2Btn)

• The device starts at the "Presets" state.
• Two types of push: long and short
• Three types of actions:
  1. Short Left or Right push (release immediately)
  2. Both Left and Right short push
  3. Both Left and Right long push.
• Cycle through presets with a L or R short push.
• Go to 1-1 manual mode with both L and R short push.
• Long push (L or R) to enter programming mode, or to configure the CPU Frequency (CPUF) when showing the maximum frequency.
• Reset to defaults by long press L and R twice.

Mode 4: Potentiometer (POT)

• Can be used in conjunction with the single-button operation. Turning the potentiometer overrides the preset selected by the button.

Serial Mode (UART): Control by the serial port, works in conjunction with all other modes.

You can connect to the Throttle Blaster via the serial port and set the Frequency and PWM Period. This is convenient for launching a game with a .bat file that first configures the Throttle Blaster and then launches the game.

Reset to factory defaults during power on (since rev 0.9)

Press the button (rotary encoder, or any individual button in other modes) and plug in the ThrottleBlaster to power. The led on the Pico will start flashing fast for around 2 seconds and the MHz display will show "RES".

This helps revert the deleted presets or any potential flash-related issue.

UART Circuit

• Connect the Throttle Blaster's Tx pin to the PC's serial port Rx pin (that is pin 2 of the serial connector), the Throttle Blaster's Rx pin to serial Tx (pin 3) and ground to ground (pin 5)

UART Software

• Serial port settings: 9600 8N1, no flow control
• The string to send is in the form F<MHz>P<Period>\r, where:
  • <MHz> is the desired effective frequency in MHz (float),
  • <Period> is the PWM Period level (1-256),
  • \r (also seen as ^M) is the Carriage-Return character (ASCII 13 0x0d).
  • For example F4.77P8 sets the frequency to 4.77MHz and the PWM period to 8 which is a around 50us.
• In DOS you can use a terminal emulator, like Kermit, to connect and send the command.
  • You could use this one-liner in a batch file: kermit set port COM1, set speed 9600, output F4.77P8\13. This will set the frequency to 4.77 MHz, the period to 8 and will send a Carriage-Return character (\13).

Note Changing the frequency is not instantaneous. So if you are setting the frequency in a batch file, please consider adding a delay before launching the game.

Note Please note that the serial-port functionality is totally optional. You don't need to populate the MAX3232 IC and its capacitors if you are not planning to use it.

Preset Buttons (since rev 0.7)

Since revision 0.7 you can also directly select a preset by pushing one of the 8 available buttons. This functionality is compatible with all other modes of operation.

Please note that you don't need a Rev 0.7 PCB to use this feature. You can simply connect buttons to the Pico's GPIOs, with one button pin to the GPIO and the other to ground, see table below:

Reset Detection (Optional and experimental) (since rev 0.8)

Connect the computer case reset button to RES1 and another cable from RES2 to the computer motherboard. Now when you press the reset button, the Throttle Blaster will detect it and will temporarily set the frequency to maximum for a faster boot. During this time the MHz display will show "boot" and show a count down until the frequency is restored.

Note Polarity matters: If you connect the reset cables to the RES headers the wrong way your PC won't boot!

Reverse Direction jumper JP3 (since rev 0.4)

Since revision 0.4 the default rotation direction of the knob has been changed and a new jumper JP3 has been added. Closing the jumper will reverse the direction for both the rotary encoder and the potentiometer.

Presets

Firmware

Dependencies

• C++17 cross compiler for pico (see README)
• pico-sdk 2.2 or later
• (Optional) TM1637 display library 1.2.1

Build Instructions

• Download release (v1.2.1) https://github.com/wahlencraft/TM1637-pico and extract it.
• cd firmware/ && mkdir build && cd build
• cmake -DCMAKE_BUILD_TYPE=Release -DPICO_SDK_PATH=/path/to/pico-sdk/ -DPICO_BOARD=<pico|pico2> -DPICO_TM1637_PATH=/path/to/TM1637-pico/ ../src/ && make
• This will place the firmware into: ThrottleBlaster_pico1.uf2 (or ThrottleBlaster_pico2.uf2) in the build directory.

Installing the firmware

• Unplug the Pico
• Press and hold the small "BOOTSEL" button on the Pico
• While holding the BOOTSEL button, connect the Pico to your PC with a micro-USB cable
• The Pico should show up as a mass-storage device
• Copy the ThrottleBlaster_pico1.uf2 (or ThrottleBlaster_pico2.uf2 for the Pico 2) firmware to the drive associated with the Pico
• Safely eject the mass-storage device

The Pico should boot and you should see the Pico's LED light up.

Circuit

Schematic

The Throttle Blaster circuit is fairly simple:

• The Rotary Encoder, the Push Button and the Potentiometer are connected to the Pico's GPIOs.
• The Display is also connected directly to GPIOs
• The STPCLK# pin is driven by a N-channel MOSFET, a 2N7000, and its gate pin connects to the Pico's GPIO via a 1K resistor and the transistor's drain connects to STPCLK# via a 100 Ohm resistor.
• The circuit is powered directly from the PSU's 5V power supply via diode (preferrably a Schottky).
• The serial port circuit relies on a MAX3232 for converting the RS232 levels to Pi-Pico levels.

PCB

Bill Of Materials (BOM)

Download gerbers: https://github.com/scrapcomputing/ThrottleBlaster/releases

Using the circuit for the first time

• Select the operation mode using jumpers JP1/JP2.
• Connect J1 to your CPU's STPCLK# pin.
• Power it on and you are good to go.
• You can reverse the knob direction by closing JP3 (since rev.0.4)

Basic Troubleshooting

• Check that the Throttle Blaster's ground is connected to the motherboard's ground
• Check the voltage at the STPCLK# pin:
  • Max frequency: The voltage should be matching the CPU's I/O high ~3.3V for most CPUs
  • Any other frequency: You should be seing pulses using an oscilloscope, or values between 0 and ~3.3V using a multimeter.
• Confirm that it's working as expected by running benchmarks from Phil's DOS Benchmark Pack

How to find the STPCLK# pin

Just look for it in your CPU's datasheet. This table lists the STPCLK# pin number for your reference:

How about older CPUs that don't have a STPCLK pin?

486 or older CPUs don't usually have a STPCLK# pin. Instead they have the HOLD pin which has somewhat similar functionality and is active high. Some boards use PWM on this HOLD pin when the Turbo button is pressed.

The HOLD pin is not pulled down internally by the CPU. So it is up to the motherboard to pull it down. The problem is that this is usually actively pulled down by the CPU chipset, meaning that if we try to pull it up with the Throttle Blaster we may damage the chipset. So I don't think there is a way to get the Throttle Blaster to work universally with older systems.

Change Log

• Rev 0.9: Allows deletion of presets and adds flash reset when button pressed during boot.
• Rev 0.8e: Fixes single/two-button configuration bug (introduced in 0.8b). Adds Pico2 support.
• Rev 0.8d: Fixes reset detection
• Rev 0.8c: Fixes bugs introduced in Rev 0.8b.
• Rev 0.8b: Improves responsiveness of rotary encoder. WARNING: This revision is buggy, please upgrade to Rev 0.8c (thanks @nahimov for reporting the bug).
• Rev 0.8: Adds RESET detction circuit (optional) which will temporarily set the speed to max for 20 seconds.
• Rev 0.7: Adds support for 8 preset buttons.
• Rev 0.6: Firmware bug fixes: (i) fix saving MHz/Period adjustments to flash and (ii) one-button mode frequency glitch.
• Rev 0.5: Replaces potentiometer capacitor with SMD and several firmware fixes.
• Rev 0.4: Reverses knob direction and adds jumper JP3 for selecting direction.
• Rev 0.3: Adds UART support in both firmware and PCB.
• Rev 0.2: Adds two-button mode "2Btn".
• Rev 0.1: Initial release.

Acknowledgements

• Many thanks to all of you who have reported bugs or asked for clarifications or features requests. By doing so you are helping improve the project! Special thanks to Michael Swimm for extensive testing, reporting bugs, coming up with awesome ideas for new features, and for sharing fancy mounting hardware for the Throttle Blaster!

License

The project is GPLv2 except for Pwm.pio which comes from the Pi Pico SDK examples and is under SPDX-License-Identifier: BSD-3-Clause.