An act of adopting one policy or way of life, or choosing one type of item, in place of another; a change, especially a radical one.
Lasc aims to be a small and flexible MIDI patch selection tool similar in function to the popular Tech21 MIDI Mouse. The code is written in C and targeted to an STMicroelectronics stm8s MCU and either a 7 segment LED or OLED display. It is intended to be cheap and simple to make though basic soldering and coding skills will be necessary. Software tools required are free and the hardware required to flash the MCU is typically very cheap and readily available.
With the obligatory pretentious bit out of the way, Lasc is the Irish word for 'switch'. There are a number of compact MIDI patch change devices (or switchers) produced by some well known and well respected manufacturers including Tech21nyc, Rocktron, T.C. Electronic and Disaster Control to name a few. I owned a Tech21 MIDI Mouse which is a simple, robust device that allows the user to send MIDI patch change messages. Effective though it is, for me it had 2 major disadvantages:
- they are a bit big and clunky and
- they are quite expensive.
They also only support a fixed range of PC messages and rather annoyingly enter config mode every power up.
Cue: Lasc - An attempt to squish the MIDI-mouse features into a smaller box and add a few improvements.
The original had 3 foot switches and two modes, one, immediate where hitting the UP button just switched one patch up and DOWN, one down. Simples. The second mode flashed the display digits and would allow selecting a patch number using the UP/DOWN buttons with patch numbers changing more quickly the longer the switch is held down. Hitting the 3rd MODE switch sends the selected patch and exits the mode. Bit trickier.
Since one of the constraints on size is the space for 3 foot switches, the obvious mod is to allow for just the two, UP and DOWN, switches and emulate the MODE switch by pressing both UP and DOWN at the same time. Define 'HAS_MODE_FS' in the Makefile for the 3 button version.
The MIDI mouse (and many other controllers) are restricted to selecting all 'standard' patch numbers 1 - 128 - MIDI PC 00 - 127 ) however many MIDI devices support banks of patches accessed using MIDI bank select CC messages. The bank select messages are comprised of the LSB and MSB of the desired bank which are sent immediately prior to the PC message. Some devices choose to ignore the LSB and instead just use the MSB and Lasc allows choosing this behaviour.
There is provision in the code for setting a number of maximum patch numbers and selecting between them. By default there are 5 ranges defined: 0 - 127, 0 - 199, 0 - 299, 0 - 799 and 0 - 998.
Sending MIDI CC bank select messages to a device that doesn't support them should just be ignored. If it does cause a problem, defining DONTSENDBANK will do what it says. Obviously this will only apply to the patch range 0 - 127.
When initially powered up, the Tech21 MIDI mouse enters its config mode. This times out after a couple of seconds but is still a delay, particularly if the device gets reset while in use. Instead, Lasc will only enter config mode if a switch is held down during power up. The config itself is stored in built in EEPROM. On first power up, the EEPROM is empty which gives Lasc default MIDI channel 1 and range 0 - 127 which are reasonable defaults.
Holding down a button during power-up will enter one of the config modes described below.
If the RESTORELASTPC define is set, the MIDI patch number is saved to EEPROM and will be restored on power up. The downside of this is that the EEPROM has a finite lifetime of 30K writes so could wear out.
If USE_EXTERNAL_LED is defined, Lasc toggles a GPIO on MIDI message send and during config mode and when in mode 2. This can be connected to an external LED. The display also blinks during the latter two operations so the external LED is entirely optional.
Lasc stores its configuration in EEPROM. On power-up it reads these values then enters operational mode. The default configuration is MIDI channel 1, PC 0 - 127, display 1 - 128.
To enter a configuration mode, keep one or both foot switches down during power-up until the display and, if fitted, the external LED flashes. Holding down the UP button will enter the MIDI config, holding down the DOWN button will enter the display config mode and holding down both will enter bank select config mode.
In MIDI config mode, the current MIDI channel is displayed followed by an upper or lower case character, the latter to indicate the range of available patches. The defaults are:
- lower case 'c' at the bottom of the display: 0 - 127
- lower case 'c' at the top of the display: 0 - 199
- upper case 'C': 0 - 299
- upper case 'u': 0 - 799
- upper case 'U': 0 - 998
Pressing the UP or DOWN foot switch changes the MIDI channel, pressing the MODE switch (or pressing both UP and DOWN together on a two switch device) selects the range.
In display config mode, the display flashes and shows a '0' or a '1' to signify the mode. '0' means that the number displayed by Lasc is the actual MIDI PC number sent, eg 0 - 127 whereas '1' means that Lasc will display a number one greater than the MIDI PC value, eg when sending PC 00, Lasc will display '1'. This is intended as a convenience to allow the Lasc display to match the controlled device numbering.
In bank select config mode, the display falshes and shows a '0' or a '1' to signify the bank select mode. If '0', Lasc will set the bank select MSB to 0 and the LSB to the bank number. If the mode is '1', Lasc will set the bank select MSB and the LSB to the bank number.
In either config mode, after around 3 seconds with no button press, the channel and range selected or display mode are saved to EEPROM so they will be the default at next power on.
The display then shows the initial patch number (or the last PC selected before power off if RESTORELASTPC was defined) - NB this is not transmitted. From here on, operation should be as described above.
The target MCU for this project is the ST Micro stm8s for which there are many very small and cheap dev boards. The device used for development was an STM8S103F3P6
https://www.st.com/en/microcontrollers-microprocessors/stm8s103f3.html#documentation
The MCU board has a regulator on-board, good for a supply of up to 15VDC so driving this from a conventional 9VDC effect pedal supply should be fine.
The code currently supports two different displays/drivers. The first uses a MAX7219 and 3x7 segment LED display. The second uses a 128x64 dot matrix OLED display with SSD1306 driver. The latter provides a small, bright, sharp, low power display with great visibility and needs just 4 wires however the code to drive it is much bigger. The chosen MCU has only 8K of flash which is a challenge.
Compilation was done with the free and excellent SDCC - http://sdcc.sourceforge.net/index.php. Initially the code was compiled using the version 3.8.0 compiler but the OLED version exceeded the code flash space on the mcu (8KB). Fortunately the SDCC-4.* compiler generates more compact code and allows it to be shoehorned into the mcu .
Flashing the MCU is straightforward using the SWIM protocol
via a cheap STLINK2 clone bought on ebay
and the free https://github.com/vdudouyt/stm8flash utility.
The project also utilises the sdcc-stm8 port of the official ST Micro Standard Peripheral Library - https://github.com/roshbaby/stm8s-sdcc
The build is controlled by a single Makefile, you will need to edit this to suit your paths and the options you want - that should be pretty straightforward. In addition to building the code there are additional targets to simplify flashing, testing and zeroing the EEPROM if ncessary.
Lasc code was developed on a Linux machine but most/all tools should run on Windows or Mac.
The display driver, a MAX7219, is connected to 3 GPIO pins:
SCK: PC5 -> MAX7219 pin 13
MOSI: PC6 -> MAX7219 pin 1
CS/SS: PA3 -> MAX7219 pin 12
It is also connected to 3v3 (pin 19) and ground (pin 4). LED display wiring varies but the cheapo Chinese ones seem pretty similar if not identical. Assuming the display is oriented pins facing down and with the decimal points on the right, viewed from above, numbered like an IC with pin 1 at top left, common cathode display.
| MAX7219 | LED pin | LED segment |
|---|---|---|
| 2 | 2 | digit 3 |
| 6 | 6 | digit 1 |
| 11 | 3 | digit 2 |
| 14 | 5 | segment a |
| 15 | 4 | segment f |
| 16 | 1 | segment b |
| 17 | 11 | segment g |
| 20 | 10 | segment c |
| 21 | 7 | segment e |
| 23 | 8 | segment d |
There is a 27K resistor from 3v3 to pin 18 and 10uF electrolytic and 0.1uF ceramic across the power supply. The MAX7219 is supposedly a 5v device but seems to work fine on 3v3.
While these displays are available with an SPI interface, the I2C version seems more popular and cheaper. The display is connected to 2 GPIO pins:
SCL: PB4
SDA: PB5
It is also connected to 3v3 (pin 19) and ground (pin 4). The I2C address is defaulted to 0x78, change SSD1306_I2C_ADDRESS if necessary.
MIDI TX is a single GPIO via a 47R resistor to pin 5 of DIN connector. Pin 4 of the DIN is connected to 3v3 via another 47R resistor. Pin 2 is grounded (https://www.pjrc.com/teensy/td_libs_MIDI.html).
MIDI TX: PD5
The code supports 2 or 3 SPST momentary foot switches, each wired between its respective GPIO and ground.
F/S UP: PC3
F/S DOWN: PC4
F/S MODE: PC7
To minimise space used, the 2 switch version emulates the MODE switch when both buttons are pressed simultaneously. It is fairly easy to fit the 2 switch version in a 1590B using through hole components, a 0.56" display and strip board. The SSD1306 version easily fits into a 'Hammond' 1590A sized enclosure.
For me, the hardest part is cutting a rectangular hole in the die-cast aluminium 'Hammond' enclosure. I used a Dremel-like tool with small cutting disks. There are probably way better methods. For the OLED version, since there are only a few connections it is pretty straightforward to just run wires from the dev board GPIO vias to the OLED, power, foot switches and MIDI port. I glued the OLED into the box with cyanoacrylate (superglue) which worked pretty well.
For the max7219/7 segment LED version, I used a small piece of
strip board and through hole components. There is a simple
diagram included.
I used 'sockets' for the 7 segment display pins but
in retrospect it would probably have been simpler to solder them
direct, your mileage may vary though I'd suggest the OLED version
looks better anyway.
I did this for fun and to learn a bit more about micro-controllers. I don't make any guarantees it will work though mine works really well. Officially I don't offer support but if you do have a go at making one and get stuck, get in touch. Similarly if you have any good ideas for improvements let me know.
Simon - simon@panicpants.com