14 Oct 2015: Libre Laptop PCB 2 STM32F072 developments

Accessing the touchpanel over I2C proved successful after a lot of experimenting and debugging (including at the hardware-level). The first thing: libopencm3 does not have master branch support for the stm32f0. First experimentation, it was therefore impossible to know if it was the I2C code (cut across from the near-identical stm32f3 series) or the touchpanel. Therefore, a quick circuit was hacked together using an AT24C64 I2C EEPROM and two through-hole 2.2k resistors. no actual circuit board!

This circuit proved simple enough to be able to read from the EEPROM using the stm32f3 series source code, which was proof enough that the libopencm3 I2C interface worked. Returning then to the Yunlea touchpanel, success was achieved first time when previously it had been a 100% failure rate. This, it has to be said with hindsight, was probably down to the fact that the cable was not pushed fully into the 12-pin 0.5mm pitch FPC connector...

There are so many things that can possibly be wrong, this just demonstrates once again that it is imperative, especially with hardware and software debugging, to be only dealing with a single unknown. Next on the list is the SPI interface to the HXC8357 on the Yunlea LCD/Touchpanel...