Serial Mouse Blaster Firmware

This is the firmware for the Serial Mouse Blaster project's Raspberry Pi Pico.

The project converts a modern USB optical mouse into a serial mouse.

Download

Binaries are available in the releases. The file to download is the one with the .uf2 extension.

Currently there are two images:

SerialMouseBlasterFirmware_revX.Y_0_90v.uf2 : 0.90v core voltage (best)
SerialMouseBlasterFirmware_revX.Y_1_00v.uf2 : 1.00v core voltage (in case 0.90v does not work for you)

Serial mouse protocol

We are using just two of the serial port signals:

Rx Data (Pin 2) : This sends the mouse data to the mouse driver
RTS (Pin 7) : The mouse driver uses this for mouse detection

Currently we are emulating 2-button Microsoft serial mouse. The protocol is very simple. For each mouse state update the mouse sends 3 bytes. These 3 bytes encode the status of the Left/Right mouse buttons and the X/Y movement deltas. The following awesome ascii art is a verbatim copy from CuteMouse docs/protocol.txt:

Serial Microsoft mode: 1200 bps, 7 data bits, 1 stop bit, no parity

              1st byte          2nd byte          3rd byte
         +---------------+ +---------------+ +---------------+
         |0|1|L|R|Y|Y|X|X| |0|0|X|X|X|X|X|X| |0|0|Y|Y|Y|Y|Y|Y|
         +---------------+ +---------------+ +---------------+
              | | \ / \ /       \----+----/       \----+----/
              | |  |   |             |                 |
              | |  +---|-------------|---------+       |
              | |      +-----+       |         |       |
              | |           / \ /----+----\   / \ /----+----\
              | |          +---------------+ +---------------+
 Left Button -+ |          | | | | | | | | | | | | | | | | | |
Right Button ---+          +---------------+ +---------------+
(1 if pressed)                 X movement        Y movement


Movement values are 8-bit signed twos complement integers.
Positive movement value indicates motion to the right/downward.

The firmware receives data from the mouse with the help of TinyUSB. Once the data is received, it encodes it using the microsoft protocol and sends the 3-byte packet. This is done in a loop using a polling frequency of 40Hz.

Mouse Detection / Reset

The mouse driver briefly lowers RTS and expects to receive character 'M' (0x4D) after about 14ms on the Rx pin. This is for the two button Microsoft-compatible mouse. For more detailed information please refer to the mouse man page mouse(4).

The SerialMouseBlaster PCB connects the level-shifted RTS to the Pico's GPIO 6, and Rx to GPIO 4.

Build dependencies

Pico SDK 1.5.0. Please adjust the PICO_SDK_PATH in src/CMakeLists.txt
C++ 17 or later compiler
CMake 3.13 or later
Recent version of make or other build system supported by CMake

Build instructions

Create a build directory and cd into it: mkdir build && cd build
cmake ../src/
make. The firmware should be in build/SerialMouseBlasterFirmware.uf2

CMake configuration options

-DPICO_FREQ to force a frequency in KHz. The default is 0 which uses a special routine that runs everything off the USB PLL at 48MHz.
-DPICO_VOLT to set the core voltage. Accepted values can be found in <pico-sdk>/src/rp2_common/hardware_vreg/include/hardware/vreg.h. For example: VREG_VOLTAGE_0_85 for 0.85V, VREG_VOLTAGE_1_00 for 1.00v.
-DPICO_LED 0 or 1. If set to 0 disables the LEDs completely. The default consumes very little power, so overriding it is not very useful.
-DUSB_POLL_PERIOD controls the poll period of the USB mouse. The default is 25ms which is 40Hz.
-DENABLE_SERIAL_DBG 1 or 0. Setting it to 1 enables serial debug output using the SerialMouseBlaster's debug output (connected to the Pico's pins 1 to 3)

How to load the firmware

Unplug the mouse from the SerialMouseBlaster PCB
Press and hold the small button on the Pico
While holding the button, connect the Pico to your PC with a micro-USB cable
The Pico should show up as a mass-storage device
Copy the .uf2 firmware to the drive associated with the Pico
Safely eject the mass-storage device

The Pico should boot and you should see the LED blinking very briefly (and faintly) every second or so.

Serial port

The direction as relative to the PC. So Output means that the PC is sending data to the RS232 device.

Pin	Name	Direction	Description
1	CD	IN	Carrier Detect
2	RXD	IN	Receive Data
3	TXD	OUT	Transmit Data
4	DTR	OUT	Data Terminal Ready
5	GND		Ground
6	DSR	IN	Data Set Ready
7	RTS	OUT	Request To Send
8	CTS	IN	Clear To Send
9	IR	IN	Ring Indicator

Female connector looking at the holes:
 -----------------------
|     -------------     |
|     \ 5 4 3 2 1 /     |
| ( )  \ 9 8 7 6 /  ( ) |
|        -------        |
 -----------------------

Raspberry Pi Pico's current consumption at 5V

We set the core voltage with vreg_set_voltage(), and the individual clocks with clk_configure() (see the implementation of set_sys_clock_48mhz()). The default core voltage is 1.10V.

clk_usb must hast to be at 48MHz, but clk_sys can work at much lower frequencies. However, when clk_sys is much slower then clk_usb the connected USB devices no longer work. The slowest I could get clk_sys to operate at while USB was still working was 41.8MHz. This is unfortunate, because running clk_sys out of pll_usb with a divider of 4, at 12MHz, consumes only 3mA.

Stopping clk_rtc and clk_adc save about 0.1mA.

clk_sys(MHz)	pll_sys(MHz)	pll_sys	Core (V)	Current (mA)
125,pll_sys	125, VCO=1500	48, VCO=1440	1.10	19.0
125,pll_sys	125, VCO=1500	48, VCO=1440	1.00	17.7
48,pll_sys	125, VCO=1500	48, VCO=1440	1.10	10.6
48,pll_sys	125, VCO=1500	48, VCO=1440	1.00	9.7
48,pll_sys	125, VCO=1500	48, VCO=1440	0.90	8.9
41.8,pll_usb	41.6, VCO=750	48, VCO=768	0.90	6.3
48,pll_usb	Disabled	48, VCO=1440	1.10	8.9
48,pll_usb	Disabled	48, VCO=1440	1.00	8.1
48,pll_usb	Disabled	48, VCO=1440	0.90	7.4
48,pll_usb	Disabled	48, VCO=768	0.90	6.05

Pico's LED consumes about 2mA when always on.

scrapcomputing / SerialMouseBlasterFirmware