The TX TwoPot

DIY Project by Alan Probandt

TX TwoPot
  • The TX TwoPot project files including source code!
Released Mar 19, 2009 101 KB

YAMAHA TQ5/TX81-Z 'TwoPot' Program Operation Overview

This is a bare-bones Real-Time controller for the Yamaha TQ5 4-operator FM tone-module audio synthesizer. It also works also with the popular Yamaha TX81-Z 4-op FM tone module. A standard PC keyboard plugs in and plays MIDI notes like a piano. The .hex file can be loaded directly into a common AVR Mega8 microcontroller. These are available on eBay and from component suppliers like or More information on AVR microcontrollers is available at

What it does - How to use it.

The main code checks repeatedly for a keypress from the PS2 keyboard. When a key is pressed, the code checks if it is a keypad digit, an arrow key, or a key from the main area. The main keys are arranged like two halves of a piano; the ~12345 row is the black keys (sharps and flats) of the QWERTY row of white keys. The 'Z' key is middle C. Pressing a key sends Note ON to the synth, releasing the key turns the note off. Up to eight keys be sounded at once, but due to the PS2 internal wiring, some key combinations don't work.

The 'up' arrow shifts the keyboard notes down one octave and the 'down' key shifts the notes played up one octave. The left and right keys change the voice number. Changing the voice causes all the parameter values for that voice to be loaded into the processor. The top line of the LCD shows the new voice name and number. The left and right keys roll over: pressing left key at voice #0 goes to #127, pressing right at #127 goes to #0. Pressing two keypad digits changes the voice number. From voice #23, pressing keypad '5' selects voice #25. Then pressing '8' loads voice #58.

Pressing a function key (1 to 8) selects a new algorythm for the voice. The display on the top right changes to show the new algorythm selection. The INSERT key writes the edited parameters to the synth's battery-backed memory (at the current voice location). The PAGE_UP key sends a text description of the 32 editable parameters to the serial port (38.4K baud 8/N/1; about 980 characters total). Use a standard PC terminal program to capture this download.

The Caps-Lock key toggles the Sustain feature of the synth. When Sustain is ON the voice is not turned off when its key is released. The entire voice envelope plays. After pressing eight notes, the first note turns off. This feature is great for developing sound environments using voices that have long AttackRate times and operators set to non-harmonic frequencies.

Every 1/10 of a second, the main code interrupts and does a scan of two variable resistors (potentiometer knobs or sliders). The new value for each 'pot' are compared to the value from the last set of conversions. If they are the same, then the interrupt exits and nothing happens. If the user moves the Parameter slider pot, then the code puts a new text string on the LCD, The parameter's current value gets displayed. This is the value of the parameter that was loaded into the TwoPot when the voice was loaded from the synth.

The Parameter slider is very sensitive - a second variable resistor of much lower value can be added in series to make it easier to select params that are close together. For example, a 5K ohm pot placed in series with a 50,000 ohm pot. The large value pot selects the operator range and the smaller pot selects the individual parameter.

              voicename  vx#   algorythm                    voicename  vx#   algorythm
           --------------------------                 F   --------------------------   
    LCD   :   GrandPiano  32    2    :                R  :   GrandPiano  32    2    :  
    16x2  :   AttackRt_1  31   10    :                E  :   Frq_4 2.00   8    0    :  
           --------------------------                 Q   --------------------------   
          param name  init_val  adj_val                  freq_opr  ratio Coarse Fine 

The LCD displays the voice name on the top line left, the voice number on top center, and the algorythm number on top right. The current selected parameter is on second line left, the original value for this param in the center, and the current adjusted value that was last sent to the synth on the right. In frequency-param mode, the Value slider has 256 steps. So it is more sensitive than the other modes that have a maximum of 128 steps for the Value slider.

The display is a little different when working with a frequency parameter. Now the second line shows the operator number on the second line left, the current frequency ratio value in the left center, and the current coarse and fine setting numbers on the right. The ratio string and settings display changes as the Value slider potentiometer is moved. When you change a parameter, the displayed values go back to the original display settings even though the synth has the adjusted values. I should probably change this to have the display always reflect the values last sent to the synth.

Changing the Value pot selects a new setting for the parameter. This pot value is adjusted according to the maximum value of the selected parameter. For example, a parameter that has only three settings (like Amp Mod Sensitivity) needs to have the knob turned (or slider pot moved) further than a parameter with 32 settings (like Attack Rate) to effect any change.

Not all the parameters used by the Yamaha 4-op series of modules are listed. Only the 32 that have the most direct change on the sound of the patch are here. Here is a list in order:

1   Operator_1 Frequency_Ratio           17  Operator_1 Decay1Rt   
2   Operator_2 Frequency_Ratio           18  Operator_2 Decay1Rt   
3   Operator_3 Frequency_Ratio           19  Operator_3 Decay1Rt   
4   Operator_4 Frequency_Ratio           20  Operator_4 Decay1Rt   
5   Operator_1 AttackRt                  21  Operator_1 Decay2Rt   
6   Operator_2 AttackRt                  22  Operator_2 Decay2Rt   
7   Operator_3 AttackRt                  23  Operator_3 Decay2Rt   
8   Operator_4 AttackRt                  24  Operator_4 Decay2Rt   
9   Operator_1 OutLevel                  25  Operator_1 Decay1Lv   
10  Operator_2 OutLevel                  26  Operator_2 Decay1Lv   
11  Operator_3 OutLevel                  27  Operator_3 Decay1Lv   
12  Operator_4 OutLevel                  28  Operator_4 Decay1Lv   
13  Operator_1 Osc_Wave                  29  Feedback              
14  Operator_2 Osc_Wave                  30  EffectType            
15  Operator_3 Osc_Wave                  31  Effect_Spd            
16  Operator_4 Osc_Wave                  32  Effect_Bal            

If you are using a TQ5 tone module, the effects parameters are at the very end of the list (the top of the slider pot).

Only the single voice settings are changed, the multi voice settings are not implemented. Nor are TX81-Z effects or microscale tuning tables. All these features can be set by pressing front panel buttons. Switching Memory-Protect to OFF is done for each model.

If a Yamaha TX81Z or TQ5 is not connected, the display will show "Keyboard Only". The PS2 keyboard will send polyphonic Note ON/OFF messages to any MIDI synth connected. Output on MIDI chan 1.

 ~~~~~~~~~~~~~~   Hardware design notes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                     |  Basic parts list                             DigiKey #
    Atmel AVR Mega8  microcontroller   8K Flash      |                                               
                       _______                       |  2   DIN5 (180 degree) MIDI jacks             DIN-5500-5S
 +5V--10K----<  /reset |1  28|1  ADC5 SCL            |  1   MiniDIN6 jack for PS2 keyboard           CP-2460-ND
     MIDI in   RxD PD0 |2  27|2  ADC4 SDA            |  1   MIDI opto-isolator (4n24..PC817)         
     MIDI out  TxD PD1 |3  26|3  ADC3  Param pot     |  1   Power jack (2.1mm pin=ground)            PJ-102A
    PS2 kbd clock INT0 |4  25|4  ADC2  Value pot     |  1   LM78L05 +5 volt regulator IC             LM78L05ACZFS-ND
    PS2 kbd data   PD3 |5  24|5  ADC1                |  1   Atmel AVR Mega88 microcontroller         ATMEGA88P-20PU-ND
                   PD4 |6  23|6  ADC0                |  1   16x2 LCD display (standard HD44780 style)
          +5V <-   VCC |7  22|7  GND                 |  2   Slide potentiometers  (10K-100K ohm is OK)
                   GND |8  21|8  AREF  -> +5V        |  1   small 'tach' switch for reset                     
                   PB6 |9  20|9  AVCC  -> +5V        |  2   220 - 360 ohm resistors (1/4 watt) for MIDI 
                   PB7 |10 19|10 PB5 LCD data7       |  1   470 to 1K ohm resistor for MIDI-In opto-isolator
                   PD5 |11 18|11 PB4 LCD data6       |  1   Power adapter +9 (to +15 volts) DC ~ 50 milliAmps 
                   PD6 |12 17|12 PB3 LCD data5       |      - runs processor, LCD display, and PS2 keyboard
                   PD7 |13 16|13 PB2 LCD data4       |  
         LCD_RS    PB0 |14 15|14 PB1 LCD_E           |  see GIF file for schematic drawing

Other AVR devices work well also. Adjust the interrupt vector table, the .org addresses in the SRAM init section, the peripheral (USART, timer, etc...) names, and the doADC code for your selected device. Other AVRs should have USART, ADC, and at least eight I/O port pins. The AVRs that don't have USARTs can run with software UART code. Check the Projects section of the the website for examples (and my other MIDI projects).

Almost any opto-isolator will work. I've built MIDI interfaces with the 4N25, PS817, H1L11, 6N138 and other devices. The only requirement is that the opto-isolator be able to handle the 31250 MIDI baud rate. The resistor that pulls-up the output voltage to +5 should be selected to make a sharp falling edge between +5 volts and 0.5V. Different opto-isolators will need different values, but they are usually between 390 and 1500 ohms. See MIDI hardware and opto-isolator tutorials on the web for more info.

You can assemble this code with the free assembler available from the Atmel website: AVRASM2.exe From the PC command line, use:

C:\AVR\avrasm2 TP_M8_v4.asm -fI -l TP_M8_v4.lst -o TP_M8_v4.hex

Load the code into the AVR device using an Atmel STK500 dev system or AVR's inexpensive ISP (in-system programmer). There are many DIY AVR programmers available on the web. My favorite is the free, high-quality programmer "SP12.exe" that is available at: Here are some examples of using SP12 with the PC command-line:

    C:\AVR\SP12 -i     { identify the AVR IC and verify communication}
    C:\AVR\SP12 -wpf TP_M8_v4.hex    { load the assembled code into the AVR device}
    C:\AVR\SP12 -rpfh WhatsThis.txt   { Read the AVR IC. Put the code into a text file resembling a binary dump}

The AVR devices come from the factory with internal fuses used to configure the device. The fuses need to be set differently from the factory default for this program to work correctly. AVR device programmers (such as the STK500 or SP12) allow you to change these fuses as needed. The primary change is adjusting the AVR system clock. AVRs are set to run at a 1_MHz system clock and this needs to be changed to 8_MHz for the MIDI timing to be correct.

Each device has several banks of fuses. The fuses that need changing are in the low bank. Take care when changing these fuses as it is easy to put the AVR device into a mode that requires a high-voltage programmer like the STK500 or the AVR-Dragon to re-configure it. For example, the fuse settings can be changed to require an external system clock signal instead of the AVR's internal clock. If the device is set into external clock mode, then it needs a 500KHz square-wave signal applied to the XTAL pin to return it to internal-clock mode. The SP12 programmer has some safeguards against setting the AVR into a mode where it can't be easily reprogrammed.

The microcontroller used (AVR Mega8) needs to have its fuses set differently from the factory default state:

   high fuse byte:  unchanged from factory default
    low fuse byte  clk/8=off, no clk out, full-swing, slow-rise power, internal_RC: 8MHz system clock

The SP12 command to set the fuses correctly for the Mega8 is:

  C:\AVR\sp12 -wF11100100  

The AVR microcontroller is running on its internal system clock which is generated by a tiny resistor/capacitor combination built into the IC. The manufacturing process can cause these R-C values to vary enough to throw off the precise MIDI timing. Each AVR chip coming of the line at the factory gets calibrated. This calibration value gets put into the AVR's internal clock-calibration register (OSCCAL) at when the device is powered-on. The clock speed register can be adjusted for more precise clock speeds. In my experience, the Mega48/88 devices don't need adjustment. MIDI timing using the factory calibration value has always worked OK.

However, the Mega8 devices have different calibration values for the 1_MHz and the 8_MHz internal clock values. The calibration value for the 1_MHz speed gets loaded into OSCCAL at power-up. This value may not work for the 8_MHz internal clock speed that this program uses. There is a routine at the beginning of the program that reads the byte at the highest internal AVR EEPROM location and puts this value into the OSCCAL clock speed register. You need to put the correct value for 8_MHz operation into the high EEPROM location to avoid MIDI timing errors with the Mega8. Here are the SP12 instructions to read the 8_MHz calibration value and put it into the AVR EEPROM:

   C:\AVR\sp12 -rc11        returns the factory-calibrated value for internal 8MHz operation
   C:\AVR\sp12 -wea 0x1ff:0xnn    writes 'nn' [two digit hex value] that was returned by the previous instruction
   C:\AVR\sp12 -rea 0x1ff   returns the value found at the highest Mega8 internal EEPROM location

The Memory_Programming section of the Mega88 and Mega8 datasheets talks about the fuses and calibration. Questions can be posted to the AVR Freaks website general forum at These questions come up often and there are a lot of threads and tutorials about the details and complications of AVR configuration/programming.

Files in the ZIP

TX81_Two_Pot_schematic.gif -  schematic drawing of electronics
TX_ReadMe.txt              -  this file
TP_Mega8.asm               -  AVR Mega8 source code in assembler
TP_Mega8.hex               -  Intel Hex file of code, ready for loading with standard AVR device programmers

If you don't want to bother with programming the AVR device or are having difficulty getting a single chip from an electronics supplier without a large minimum order, I'll sell you a fully-tested Mega8 or Mega88 programmed with the latest version for $5.00 US (plus USPS shipping $2 in the USA, a little more worldwide). Send me a message at:

~~ Enjoy

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All of the material on this page was created by Alan Probandt.
Direct questions about the TX TwoPot to Alan, since I know nothing about this type of stuff:
Direct questions about this website to me (Matt) or if you would like me to host your own DIY project:
Last modified March 31, 2009