1
Open 7400 Competition / [Entry] SPI LCD
This idea might not be new, but I haven't seen able to find a design for a LCD backpack with less than 3 interface wires. Most designs either use a microcontroller or additional pins
Features of my LCD Backpack design:
- Interfaces via 3 wire SPI (Clock, Data, Chip Select)
- Uses only 1x 74HC14 and 2x 74HC595
- LED backlight brightness adjustable to 4-bit resolution (via R2R DAC)
- Able to work directly with 3.3/5V systems without any additional buffers
- Ability to expand to control more LCD panels with the same 3 wires (not part of this competition submission)
Schematic:
Operation:
Chip Select pin is first pulled HIGH by external device, causing the shift registers output to be DISABLED at the same time (CS=G=HIGH).
2 bytes of data is then sent to the LCD backpack via SPI (MSB first).
The 2 magic bytes are shifted into 2 74HC595 (shift registers):
Upper byte contains data to be written on LCD's 8-bit interface.
Lower byte contains backlight brightness and HD44870 RS command value.
Upon transfer of 2 bytes, the Chip Select pin is pulled LOW by external device, shift registers output are ENABLED at this momenet (no effect as data during this transient period is not latched to the LCD controller).
RCK pins on the shift registers (connected to CS through 1 inverter) is then pulled HIGH with a propagation delay. The rising edge transition latches the data onto the shift register's output pins.
E pin on LCD (connected to CS through 6 inverters) is pulled LOW with a longer delay. The falling edge transition initaties the LCD controller to read the data on the input pins.
Note: This is possible as RCK (on shift register) and E (on LCD) only respond to rising and falling edge transitions respectively.
LED Backlight:
4 pins on lower byte shift register is connected to the input of a 4-bit R2R DAC, of which the DAC's output is fed directly to the LCD panel's LED backlight. By writing 4-bits according the brightness desired to bit1:3 of lower byte, the brightness can be controlled.
Prototype:
A TI Launchpad with MSP430G2553 is used as the SPI master for this prototype. +5V is supplied to backpack from a powersupply off camera.
Some future developments:
3 bits were reserved on lower byte shift register. A 3-to-8 decoder can be connected to select up to 8 LCD controllers and flip-flops to store the brightness of each individual LCD back light. An AND gate is used to choose which LCD receives the latch signal for loading of information.
A prototype is ready (shown below), but it does not work reliably with different LCD models at the moment.
