An 8-bit emulated push button programmable microcontroller with debugger

             by Jos van den Enden
Elektronik-Labor  Literatur  Projekte  Lernpakete  TPS

8-bit TPS system with an ATmega8. An LM35 temperature sensor is plugged into the breakout connectors to log temperature.


TPS programming spreadsheet, also containing the instruction table:

 link to the assembler source code:

link to the hex file:

1. Introduction

This work elaborates on the work of Burkhard Kainka on a microcontroller learning system marketed by Franzis and Conrad (, using a HT46F47 microcontroller.

Having a microcontroller’s firmware emulate a microcontroller opens the opportunity to set up a very reduced microcontroller instruction set, suitable for both learning purposes and to quickly build simple practical applications. By writing the user interface in such a way that the emulated instructions can be entered by push buttons (“Tasten-programmierbare Steuerung” - TPS) a very compact and cheap battery driven ‘creditcard sized’ system can be built.

In the present work an ATmega8 controller was used. The emulation software was programmed in assembler and is therefore very fast, viz. about 50 microseconds per TPS-instruction at a moderate ATmega clock speed of 4 MHz.

It was chosen to write the emulated instructions in AVR-assembler style, rather than the more Basic-like style the original system of Burkhard Kainka was written in. This makes the system very suitable as an introduction to AVR-assembler itself. Among other things, the user gets familiar with subjects like adressing modes (direct, indirect, immediate), double precision arithmetic, the benefit of a status register, comparing data and subsequent conditional branching, etc.

A most notable extention to the original system is the output format being increased from 4 to the full 8-bit byte register width and the addition of two extra buttons to edit and debug TPS applications. The system has one more register because a register pair was introduced for more precise calculations. With push-pop instructions and storage of variables and constants in program memory it gives more flexibility to store data. It provides shift instructions and more sophisticated ‘immediate’ addressing. Besides that, musical tones and data logging functionality, retrievable in easy readable BCD format, is provided.

The system also includes a simple debugger to set breakpoints and step through the TPS programs, offering the possibility to inspect register contents ‘on the go’. It also supports slowing down execution time and modification of TPS programs by inserting and deleting program lines.

As for the original system, the TPS instruction set fits in a 16x16 table in which instructions are grouped into arithmetic and logic, branching, data transfer, suspending and tone groups. The two axis of the table represent the ‘nibble hi’ and ‘nibble lo’ part of the instructions. A strict separation between command and data in the opcode bit pattern could not be uphold, as is the case for AVR-assembler opcode itself. Using the table, one can already write simple programs.

An Excell spreadsheet was developed to support complexer TPS programming. The 16x16 table is central to the spreadsheet. Furthermore an instruction list is taken up in the spreadsheet with specific information, remarks and TPS code samples for quick reference. Link to the spreadsheet:

1 ATmega8 microcontroller with TPS firmware, operating at a clock speed of 4 MHz internal RC oscillator, 256 bytes TPS program storage and 256 bytes log space both in EEPROM
Operating voltage: 5V out of a 9V battery pack, stabilized with an L7805 Voltage regulator IC
Power consumption: < 4 mA
8 digital output ports able to supply 10 mA each
4 digital input ports with external pulldown resistors and connected to 4 dip switches
4 digital input ports with internal pullup resistors and connected to push buttons
1 reset push button
1 10-bits ADC input 0 - 2.56V
1 10-bits ADC input 0 - 5V
2 PWM outputs shared with 2 programmable 12 chromatical tones audio outputs over 3 octaves

2. Circuit diagram

The push buttons b0..3 and the LED's 1..8 are used to enter, modify and debug TPS programs and to retrieve logged data. The LED's can be used under TPS program control as well; the push buttons too can be called inside TPS programs to obtain user input.

The digital inputs DIN.0..3 are specifically meant to be called inside TPS programs. They are connected to dip switches that can be used to start certain program fragments. When switched on, they pull the inputs to logical one (+5V). When off, external resistors pull the inputs to logical zero. In this situation the inputs can be externally driven. Switches and pulldown resistors are not drawn.

All digital input ports are software debounced. The ADC1 input has double sensitivity compared to the ADC2 input (2.56 and 5V respectively). Output pins 15 and 16 have double, TPS software controlled functions. Either they can be configured as PWM outputs or as musical tone outputs. The reset button b4 resets the controller. Each ATmega8 pin has a breakout connector, not drawn.

3. Physical layout

The ATmega8 registers are 1 byte wide. A byte has 8 bits. Therefore, the 8 output LED's can display register contents directly. The LED's are split into two groups of 4, as one byte is devided in two nibbles of 4 bits. Most instructions are 8 bit sized, so they too can be displayed as a whole. Instructions that comprise 16 bits, the second part being data, must be devided over two lines of code and displayed in two steps. The digital outputs are also available from the breakout strips to drive other circuits.

The system is controlled with the four push buttons b0..b3. To give a first idea: b1 is the Enter-button, whereas b2 and b3 are used to input nibbles lo and hi respectively. The most important function of b0 is, in combination with reset, to call the debugger. With b1, in combination with reset, programming mode is entered. Button b0 also enables ‘page’ stepping through TPS programs. The four buttons can be called in TPS programs to receive user input. When not used the buttons can be externally driven at the breakout strips.

Digital input to TPS user programs is given with the 4 dip switches. When switched off, digital input can also be given at the breakout strips.

There are breakout strips for +5V and ground, and there is an on/off switch. The voltage regulator stabilizes the 9V battery voltage of to 5V.

See the circuit diagram for more in- and output information.  Other details below.

The breakout strips are on a 1:1 basis to the ATmega8 pins. Below a specification of the pins:

pins 1..5 and 11..13, PORTD.0 .. PORTD.7…… outputs (DOUT.0 .. DOUT.7)
pin 14, PORTB.0…………………………………….......push button input (b0)
pins 15..16, PORTB.1 .. PORTB.2………………….PWM and tone outputs (PWM1..2,TON1..2)
pins 17..19, PORTB.3 .. PORTB.5………………….push button inputs (b1 .. b3)
pins 23..26, PORTC.0 .. PORTC.3……………… inputs (DIN.0 .. DIN.3)
pins 27..28, PORTC.4 .. PORTC.5………………....ADC inputs (ADC1 .. ADC2)


10. Example programs

Below some examples of real programs, transcripions of “Beispiel” programs of Burhard Kainka, publiced by Conrad and mirrored under the link below:

In these examples Addr. is the address of the instruction, Hex. is the instruction itself as a hex number, and Mnemonic is the instruction in a symbolic language more readable than hex code.

“Wechselblinker” program

The program moves a 10101010 pattern back and forth, slightly different to the original program, where just one “1” jumps back and forth on the LED display.

The two mnemonics at addresses 00 and 01 must be red as one instruction with immediate addressing, the data part of it being at address 01. The hashtag indicates that there a coupling between the two bytes. For details on addressing, see above. The data AA (1010 1010) is loaded into r0. Then, at addresses 02 and 03 the data is displayed by the LED’s for 1 second. At addr. 04 and 05 the process is repeated, now with the data 55 (0101 0101). At the end a jump is carried out to the beginning, so that there is a repeated change in the LED light pattern (Wechselblinking).

“Binairy counter with PWM output” program

The program counts in binairy and modulates the light of an extra LED based on a binairy number.

At start it is assumes that r0 = 0, which on reset of the controller is true. In the first instruction at 00 the register pair r1:r0 is increased by one. For details about register pairs, see above. Then, r0 is output to the LED’s. This program does not output r1. In the third instruction, r0 is written to the PWM output 1. A LED connected to the PWM1 output will vary its light intensity depending on the value of r0. At address 03 the program is suspended for a short periode of 100 ms, resumed at address 04 and then proceeds to addr. 00 due to the rjmp- 4 instruction. If, because of the repeated inc’s at address 00, all 8 bits of r0 have been set (1111 1111), r0 becomes 0 (0000 0000) again. The program thus displays the following sequence of binairy numbers:

<0000 0000> < 0000 0001> <0000 0010> < 0000 0011> < 0000 0100> < 0000 0101> etc. to zero

Elektronik-Labor  Literatur  Projekte  Lernpakete  TPS