FRAM-based E-Paper Screen Controller

I really like the e-paper screens, especially those from Pervasive Displays, ideal for low-power IoT applications like the Low Power Home Network Weather Monitoring.

However, they raise two challenges:
  • Almost all the pins of a standard LaunchPad are used,
  • A large memory is required as frame-buffer.​

The idea for the FRAM-based E-Paper Screen Controller project is to use a MSP430FR5994 as a controller. 

The hardware was sampled by Texas Instruments and Pervasive Displays.

Hardware

The MSP430FR5994 acts as a controller of the screen, and uses the large FRAM as frame-buffer. The FRAM retains the memory even when the controller is powered off.

The same MSP430FR5994 provides a fast connection as slave to the client, here a MSP432P401R. 

The Flash of the Pervasive Displays BoosterPack stores all the fonts.

Software

Both the slave and the client rely on variations of the same LCD_screen Library Suite.

The controller exposes high level commands for graphics and texts.

The library on the client weights less than 15 kB typically. The library on the controller weights less than 35 kB and keeps plenty of FRAM available for the frame-buffer of the screen.

​Both libraries were developed with the Energia framework on embedXcode.
​

For the proof of concept, connection goes through UART instead of the targeted SPI. The Energia SDK doesn't feature slave SPI yet.
​

Proof of Concept

The proof of concept focuses on testing the use of FRAM as frame-buffer.

On the power front, there are four different profiles:
  • Power-up, up to 29.1 mA,
  • Normal operation, below 3,6 mA,
  • Screen refresh, less than 21,2 mA,
  • Deep sleep LPM4, as low as 0,12 mA.

The SuperCap on the MSP4320FR5995 LaunchPad has been disconnected.
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The initial spike to power-up the screen needs to be investigated.

Next steps include migrating from UART to SPI communication and addressing the power issue.

​Prototype

The prototype inaugurates the SPI connection, thanks to the availability of the SPI Slave library for Energia. 

As always, the Saleae logic analyser was instrumental in debugging, identifying and fixing communication issues.
The MSP430FR5994 acts as an SPI slave to the client, and SPI master to the screen and fonts in Flash.

Additional connections allow the client to check the controller's readiness, wake it up and reset it.
I faced unexpected issues when trying larger screens, the iTC 7.4" BWR and the iTC 12.2" BWR. The screens didn't refresh.

​After contacting the support teams:
  • Recommended SPI bus speed is 1 MHz, contrary to the 5 and 10 MHz mentioned at §1.5 SPI Timing Format on the application notes.
  • The 3.3 V regulator of some LaunchPad can't deliver enough power. An external power source is required.
  • The EPD Extension Board Gen2 (EXT2)-02 needs a minor correction on the loop control of the DC/DC booster circuit, by replacing R6 = 0.47 Ω by 0 Ω. 
With all three fixes implemented, the screens were refreshing successfully.

New Improved Fonts

The new fonts bring more flexibility in size, more compact files and ultimately a better readability. The 1 MB Flash manages to pack all the 48 fonts!

The Pervasive Displays iTC 12.2" BWR screen really excels with its 103 dpi resolution!

On the right: Monospaced, Sans Serif and Serif, from 12 to 64, standard and bold. 

​Links
Posted: 16 Jan 2019
Updated: 25 Jan 2019, 04 Mar 2019