UPC EETAC Bachelor's Degree in Telecommunications Systems and in Network Engineering EEL

Chapter 2 problems

- D2.23 -

Earbuds control buttons (FPGA-VHDL)

Products

D2.22

D2.24

1. Specifications

Design the button decoder (Button_controller) that is typical in many commercial Bluetooth earbuds. The idea is to assign a binary code to each function once detected. Fig. 1 shows a typical pair of commercial earbuds.

The same project designed programming a μC is in D3.23.

Typical functions

Fig. 1. Example wireless commercial earbuds. Only one tactile pushbutton or capacitive touch sensor is available for commanding operations.

Once paired, depending on the apps selected in the mobile phone, earbuds can be used for answering phone calls, voice assistance or playing music.  

Let us simplify the option #1 in Fig. 2 where only typical music player functions are listed.

Option #1 example functions and actions:  

Functions
a)

Fig. 2 a) Example specifications when used as music player.

Option #2 example functions and actions for multiple applications: call, music, gaming.   

Similar functions for touch buttons
b)

Fig. 2b) Example functions when used for phone, music and gaming apps. 

An internal switch S may be used to select left or right earbuds. For example S = '0' is the right earbud, S = '1' is the left earbud.

The idea is organise a FSM to detect one, two or five continuous short clicks, and also a long press. Fig. 3 shows a block diagram of an earbud from Toshiba company.

Button controller

Fig. 3. Block diagram of an earbud. Reference paper: wireless earphone solution for long time operation with a small battery on a wireless earphone. TVS stands for transient voltage suppressor diodes to prevent the destruction of the electronics due to electrostatic discharges (ESD). LDO is a low dropout voltage regulator.

Experiment in a protoboard and characterise the signals from a typical push-button. What is the typical time duration of a short and a long pulse?

Probably a tactile button will generate digital noise that we can eliminate using a debouncing filter. Better to run the FSM using clean short and long pulses.

 

2. Planning

A commercial product has to be organised planning several design phases.

Design phase #1: basic features. Dedicated processor

Solve the circuit for the design step #1. Only when it is fully tested working correctly and reported, solve the design step #2.

Design step #1. Let us start detecting (1) a single short click, (2) two short clicks and (3) a single long click.

Design step #2. (Optional) Add the five short clicks detection to disconnect the Bluetooth pairing and other features. 

Let us solve design step #1. How the captured signal will look like? How many samples do you need to identify the three patterns. What is the typical short click duration? Draw some waveforms in a timing diagram as in Fig. 4.

Let us imagine that a short click event duration is between 130 ms and 240 ms. Thus, a good sampling frequency is 8 Hz (TCLK = 125 ms). In this way detecting a short click will generate one or two consecutive '1'. The same with the time separation between two clicks, let it be one or two consecutive '0'. We can also imagine that more that three consecutive '1' is a long click. In this way we can figure out the buffer size of the capture window.

Click patterns

Fig. 4. Example timing diagram when clicking PB several times. We can consider several situations, one short chick is one of two '1' detected. The separation  between short clicks is one or two CLK periods. Thus, a signal capture window of seven samples is required before analysing the waveform and deciding which function assign. 

Once the event is detected, we need to assign the output function, depending on the earbud side (R / L). When one short click is detected in the right side (S = '0'), we need to toggle an internal play flag signal (P_flag) to set the correct function code: Play or Pause. We can draw the Button_controller symbol and the function encoding as in Fig. 5. Function codes and function available pulse will be interfaced as inputs to the main earbud audio integrated circuit. 

Button controller symbols and functions

Fig. 5. Button_controller symbol and output function codes.

With these initial ideas, we can try to fix an internal architecture as shown in Fig. 6 based, as usual, in the idea of a dedicated processor. The first sampled '1' will allow to fill in the click window. And, after seven CLK cycles, the output function will be set by the FSM. If the function detected is play or pause, the internal P_flag will be updated.

Button controller

Fig. 6. Button_controller internal architecture. (redraw it as a dedicated processor :FSM +datapath)

Chip1: finite state machine (FSM)

With such hardware resources we can imagine how the circuit will be commanded by the FSM.

Try to draw a state diagram for the circuit's FSM as shown in Fig. 7. 

Example state diagram

Fig. 7. FSM example state diagram.

Draw some waveforms in a timing diagram for the FSM.

Example timing diagram

Fig. 8. Timing diagram showing several patterns captured. Some other captured patterns are represented in Fig. 4. The FSM will decide the function code assigned comparing the current pattern detected with models stored  internally (CC2 truth table).

Apply the FSM architecture to this problem.

Draw the state register memory based on D_FF and deduce the number of D_FF required when encoding the machine using the following options:

Option #1: radix-2 (sequential)

Option #2: Gray

Option #3: Johnson

Option #4: one-hot

Draw the CC2 truth table to obtain the circuit's outputs and its equivalent flowchart behavioural interpretation (plan B).  CC2 truth table will include the 7-bit comparator to set functions codes. For instance, when the capture pattern is CP = "1111100" and S = '1', FC = "100" (volume -); when CP = "1000000" or "1100000" and S = '0' and P_flag = '1',  FC = "010" (pause); etc.

Draw the CC1 truth table to obtain the circuit's state transitions and its equivalent flowchart behavioural interpretation (plan B). 

Project location:

 C:\CSD\P6\Button_controller\(files)

Write the FSM VHDL file.

Start a Quartus Prime synthesis project for one of the following programmable target chips:

Option #1: Cyclone IV EP4CE115F29C7

Option #2: MAX II EPM2210F324C3

Option #3: MAX 10 10M50DAF484C7  (*)

(*) Remember that this chip does not generate sdo delay files, thus use another one when gate-level simulations are required.

Inspect and annotate the RTL and technology views. Check the number of D_FF synthesised in this application.

Generate a VHDL testbench fixture schematic. Translate the timing diagram sketch from the specifications into de corresponding stimulus processes.  

Run functional simulations to verify your design. Visualise as well in the wave timing diagram the internal states.

Run gate-level simulations to measure the propagation time CLK to output (tCO). Measure the minimum TCLK period or the maximum frequency of operation of the FSM.

 

Dedicated processor Button_controller: FSM + datapath

Design or adapt the other components in the top entity Button_controller and synthesise and test it. For example, Fig. 9 shows how the left Shift_buffer_7bit.

Shift_buffer_7bit circuit

Fig. 9. Inventing the capture Shift_buffer_7bit chaining D_FF.

Project location:

 C:\CSD\P8\Button_controller\(files)

Write the FSM VHDL files.

Inspect and annotate the RTL and technology views. Check the number of D_FF synthesised in this application.

 

Prepare a VHDL testbench fixture to adapt the stimulus signals in the Fig. 4 sketch. Rely on the constant CLK_Period for setting the time scale.

Timing diagram

Fig. 10. Testbench fixture. What inputs, outputs and internal signals are to be monitored?

Run functional simulations to verify your design. Visualise as well in the wave timing diagram the internal states.

Measure the propagation time CLK to output (tCO)using gate-level simulations.

What may be a good estimation of the maximum frequency of operation? Explain your answer.

And more questions... for example, what will happen if the user clicks continuously the push-button?  What can be done to prevent such situation?

 

Design phase #2: Additional features to enhance the product such crystal oscillator for CLK signals and timers for saving energy when the product is not used.  

- Design step #1: Design a CLK generator to obtain the 8 Hz square wave from a 50 MHz quartz crystal oscillator like the one available in the DE10-Lite board. Deduce the number of D_FF that the project will contain.

- Design step #2: Replace the Shift_buffer_7bit by a shift register. 

- Design step #3: Add a timer so that after 2 minutes in pause mode, the system goes a new sleep state to save battery energy.