Friday, 1 June 2018

It speaks!

Porting TTS to the ESP8266 processor has been on my to-do list for a while now, prompted by a comment on an issue I'd created to remind me. To do this, I realised that I'd have to reacquaint myself with PWM on the ATmega328p.

At its most basic, a PWM waveform is a square-wave with a varying duty-cycle. Both the frequency of the square-wave and the duty-cycle are determined on an AVR by the its built-in timers. PWM on Arduino pin 9 (hardware pin 15, OC1A), for example, is driven by Timer-1, a 16-bit timer.

Several PWM modes are available; TTS chooses the "phase and frequency corrected" option.

The diagram shows how this works to change the duty-cycle of the output pin:
  • the timer counts from BOTTOM to TOP and back down again
  • when it crosses the OCR1 threshold going up, it sets the OC1A output high,
  • when it crosses this threshold on the way down again, it sets the OC1A output low. 
Therefore by dynamically changing OCR1, we can change the "volume" on the output pin.

The final piece of the puzzle is how the frequency of the square wave is determined. Since the timer ticks on every clock cycle, it will take 2*TOP cycles to cycle from BOTTOM to TOP and back. The AVR has a clock frequency of 16MHz and PWM_TOP in the TTS library is defined as 1200/2. This gives a frequency of about 13kHz.

By default PWM on Arduino/ESP8266 has a range of 1023 and frequency of 1kHz. However it provides APIs to change these values. By setting them appropriately, the ESP8266 speaks!

Wemos Mini D1 with TTS amplifier and super-cheap speaker
The picture shows a Wemos Mini D1 connected to a modified version of the simple LM386 amplifier described at the TTS GitHub. (The modifications arose from a Hackaday article on the LM386 and are summarised in the GitHub's README.)

This amplifier sounds surprisingly good, considering its total cost was about €1. Hooking it up to a signal generator and oscilloscope produced the following pretty pictures.

At low volume, the output is a pretty faithful inversion of the input signal (a 1kHz sine wave):

Undistorted Signal
At high volume, the output is clipped by the supply rails (but still sounds pretty good to my untrained ear):
Clipped Signal
These pictures were produced using an Xprotolab Plain, an amazing combination of Oscilloscope, Signal Generator and Protocol Analyzer implemented on an AVR XMEGA. Buy one!
Xprotolab Plain

Tuesday, 29 May 2018


One of the very few downsides of ordering PCBs from Seeed Studio's Fusion service is the shipping cost. While an individual PCB can cost less than $0.50 in multiples of 10, shipping to Western Europe can add another $20.

One way to amortize these costs is to add more PCBs to an order, either from my own "design pipeline" or by searching online for cool projects. One very cool project discovered in this way, was ArduTouch from Cornfield Electronics.

One the hardware side, this is basically a battery-powered Arduino Uno with an LM386 amplifier driving a 4-Ohm mini-speaker, or your HiFi via line-in. The keypad is very nicely integrated into the PCB, see my construction below.
ArduTouch with Batteries and Cheap Speaker

Software-wise its GitHub provides five (at the last count!) different synthesizer sketches and a library for constructing your own. As usual, sketches are compiled in the Arduino IDE and uploaded by FTDI cable. A nice touch is that they uploaded a BOM for Mouser, making ordering the parts a snip!

I have several PCBs left over from this order. If you want one, contact me through the Tip Jar on the right. However I would encourage you to support the guys who made this great project possible by buying one of their kits.

Saturday, 13 January 2018

Web-Configured ESP8266

A unique feature of Internet-enabled "things" is that they often have an Internet connection and little else. First-configuration of these devices therefore presents a challenge, which most solve by creating their own wireless access point, to allow their owner to supply credentials for the local wireless network.

Web-Configured ESP is an attempt to abstract away the boilerplate required for this task, and other related ones. It's a "starter" sketch providing the following basic features:

  • Creation of a temporary Access Point ("MyESP") allowing the configuration at the conventional url,
  • A basic web-server running at this address, serving config.json and storing updates to it
  • An HTTP update server, allowing upload of new sketches "over the air"
  • An mDNS responder allowing access at http://myesp.local
Together, these features allow bootstrapping of sketches without requiring the devices to be tethered to a development machine, and user-provisioning of the finished article in the field.