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Tag Archives: lcd
SPI data transfers
I use SPI on my micromouse both to talk to the Nokia graphical LCD and to talk to the LS7366 quadrature encoders. A large number of devices can be connected to the SPI data lines, MOSI and MISO. Each device needs its own select line. This all appear very simple and friendly. There is, however a potential problem due to the flexibility of the SPI configuration.
Aside from the device select line, three wires are used to transfer data between a a master and a slave. MOSI is the Master-Out-Slave-In line while MISO is the Master-In-Slave-Out. There is also a clock signal SCK. Data transfers occur simultaneously on the data lines and the clock edges are used to shift and latch the data at each device. Now rather than have a single standard for which clock edges do what, there are four possibilities:
Mode 0:
The clock is active high, data latched on the rising edge, data clocked out on the falling edge.
Mode 1:
The clock is active high, data latched on the falling edge, data clocked out on the rising edge.
Mode 2:
The clock is active low, data latched on the falling edge, data clocked out on the rising edge.
Mode 3:
The clock is active low, data latched on the rising edge, data clocked out on the falling edge.
Note that some microcontrollers may not be able to set up their SPI peripheral in all of these modes and may be limited to just one – probably Mode 0
Confused yet? So am I.
Let’s have a look at the data sheet for the LS7366. On the very first page it says
Received bytes are shifted in [latched] on MOSI with the leading edges (high transition) of SCK. Output data is shifted out on MISO with the trailing edges of the SCK clocks.
Figure 2 of the datasheet has a timing diagram and the statement that SCK is idle low. This then is operating in Mode 0.
In itself, this is not a big deal but if you have other devices using the same SPI port, it will be important to know if they also use Mode 0 or will require reconfiguration of the port for use with each device.
The Nokia display uses the same protocol as the PCD8544 chip on the Nokia 3310 displays. The data sheet for this says, at the beginning of section 8, ‘SDIN is sampled at the positive edge of SCLK’ . So far then, it could be either Mode 0 or Mode 3. Fortunately, Figure 12:
Makes it reasonably clear we are dealing with Mode 0 again. Note that the Nokia display only accepts data and cannot communicate back to the master device.
Having both devices use Mode 0 will make it all that bit easier to talk to them.
Find out more here:
Introduction to Serial Peripheral Interface [embedded.com]
SPI – Serial Peripheral Interface [mct.net]
LCD testing for Decimus
Having installed the LCD, it was time to get the code working for it. First I realised that it may not have been so clever to put the LCD on the same SPI port as the encoders. This will mean that I have to take care that the encoders are not being read when the LCD is active. Since the encoders are sampled every interrupt, it means turning them off while writing to the screen. That, in turn, means not writing to the screen while moving, or holding position. I may end up having to patch the board to put the LCD on the other SPI port. In fact, I wish I had done that in the first place as it would just share lines with the UART and I could easily switch between the UART and the SPI for writing debugging messages. Ah well, live and learn.Turns out I wired up the LCD module incorrectly and destroyed it. Fortunately I had several spares so I built up another. These things are cheaper than text displays so it is no great loss. Attaching wires to the LCD display is easier than you might think. As supplied, the Nokia 3410 LCD display comes as part of a replacement panel that looks like this:
Prise off the aluminium surround and the actual display element will come free. It is attached with a small amount of adhesive or double-sided tape. Sprung contacts serve to make the connection between the display and the phone main board. These too are attached with adhesive and can be removed to reveal a row of small pads to which you can solder wires. [See warning in comment below before removing this connector]. I used polyurethane coated copper wire from a Verowire pen (about 0.15mm diameter). The ends of the wires are stripped by simply heating with the soldering iron set to about 400 degrees Fahrenheit:
These wires are then taken to pin headers on a matrix board so that the entire assembly can be plugged into the mouse when needed. once the connections have been tested, the LCD is held onto the carrier with some double-sided foam adhesive pads.
It does not weigh much and could probably be left in place when running. The mouse looks a lot better with it off though. For now, a simple ‘hello’ is enough to show that it works.
LCD screen for Decimus
Probably the last part to be added to Decimus will be the Nokia 3410 LCD screen. Nearly all of my mice have these either permanently fitted or attached by a simple socket so they can be removed. With a resolution of 96×64 pixels, this screen is an ideal debugging aid. The display is relatively simple to drive if you have an SPI port available. The more commonly available 3310 screen has a lower resolution and cannot easily display a picture of the entire maze. With the 3410 model, it is possible to either show the maze or to have 8 lines of 16 characters of data displayed.
Because Decimus is the most compact micromouse I have yet built, the display sits on a couple of small riser boards, made from veroboard, to allow it to clear the motors and batteries. Not only is the graphical LCD smaller than the more common 16×2 text display, it is cheaper and lighter. With a bit of care, the display can replace many, if not all, the practical functions of a real-time debugger.
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