ZX-jen screen shot

Here is a screenshot from the "ZX-jen" project, or the Drake-TV mentioned earlier in this blog. The cursor is '>' and the screen continues at the top after it ends at the bottom, instead of scrolling. The square block is the cursor and indicates the current position. Before that is the executing of the program, which is:

10 print "Hello world!"
20 let a%=3

As you can see the result of the execution is:

pHello world!
l%

The "Hello world!" is the actual output, the "p" and "l" are debug indicators: they are the first letter of the command being executed, so "print" and "let". The % sign is the type of variable being read, in this case an integer, which indicates that the let statement was correctly interpreted. The only other command that is currently available is "goto". The "Esc" key can be used to interrupt a program. The next step will be to allow printing of a%, although for that to happen the assignment should actually store the value in memory. After this various simple operations, like + and - should be implemented. After this I will consider the "for" and "next" statements.

As you can see the system supports "run" and "list" commands, but also "new" to wipe the memory. The program is stored in an external 24LC512, so even if the power fails the program will be available. Program lines can be entered by just preceding something with a line number, and deleted by providing a line number with no data. Overwriting is possible too.

I2C EEPROM microcontroller connection

Hooking up an I2C device, like an EEPROM, is relatively simple. I2C uses a 2-pin bus, with labels SCL (clock) and SDA (data). In case your microcontroller has a built in I2C controller, it is wise to hook these two lines to that port. However, I generally write it in software, because the hardware is slightly more complicated than needed (it handles clock skewing, master/slave configurations and multiple devices) in this case. It also does not give you the nice three byte buffer, which the UART (serial port input) does. If you do write it in software, it is best to hook the data input to the so-called "open collector" port. On most older PIC microcontrollers, like the 16F628(A), this is RA4 (port A bit 4). Note that both lines need pull up resistors. The values of these resistors depend on the speed that you wish to transmit data over. The disadvantage of low values is the current drain on the system. 1k Ohm works well. Note that the 24LCxxx (xxx indicates the memory in bits, a 512 module therefore has 64K of memory) needs its own decoupling capacitor, which as usual I chose to be 0.1uF. The write protect line and the address lines can all be grounded, unless you plan to have more than 64K of memory: you will need to configure different addresses for each of the EEPROM in that case.

Reading the EEPROM runs at 400 kHz. Not very fast, but suitable (and it seems most of them allow you to go up to 700 kHz, if needed). Writing is slower: you can send up to 256 bytes to the EEPROM without any delay, but after that a wait is needed (so-called ack-polling) until the EEPROM is available again for other operations.

Camera GPS

Another project I work(ed) on was the camera GPS. The idea is that when you take a picture, your camera tends to send a signal to the flash port, which could trigger the flash if necessary. A very simple flash port just has an on/off switch, which could be hooked up to a GPS to record the current date, time and place so that you can later associate the picture with this information. The board shown here actually used to be a prototype for the servo controller featured elsewhere in this blog, but was modified to communicate with a gps. On the board you can see a 7-segment LED display in green, an on/off LED, a 24LC512 external EEPROM to store the results, a PIC16F628A microcontroller, an RS-232 input port that would hook up to the GPS, the usual 6-pin ICSP and a 74HC125, which seems useless but was used to hook up a smaller GPS module that worked on 3V. There is also the obvious 7805 power supply with 9V clip. This, as well as the software for the GPS camera controller, will all be explained in more detail in later posts, first another picture of the prototype hooked up to a camera (a Minolta X700 in this case, which worked really well).