The MicroStamp11 module is a very small (1 by 1 inch) printed circuit board (PCB) built around the Motorola 68HC11 micro-controller integrated circuit (IC). A picture of the MicroStamp11 is shown in figure 4.
A labelled drawing of the MicroStamp11 will be found in figure 5. In this figure, we can identify the major components comprising the MicroStamp11. In the center of the module's front side, there is a large square IC. That IC is the Motorola 68HC11D3 micro-controller. This IC is the heart of the module. In order to function, however, the micro-controller needs a power source. The 68HC11D3 requires a direct-current (DC) 3 or 5 volt power supply. On the front of the module, you'll find a voltage regulator that step down applied DC voltages to the required 5 volt level.
Micro-controllers are synchronous digital devices. By synchronous we mean that all instructions are executed in sync with a hardware clock and by digital we mean that all voltage levels in the device are TTL-logic voltages of either 0 (low) or 5 (high) volts. On the front of the MicroStamp11 you'll find a small cylindrical crystal that helps the micro-controller maintain a steady real-time clock tick.
Finally, the 68HC11D3 micro-controller has a limited amount of internal memory. It has 256 bytes of random access memory (RAM) and no read-only memory (ROM). RAM is generally used for program variables and ROM is used as program memory. because the 68HC11D3 has no internal program memory, the chip must be interfaced to external memory chips. Figure 5 shows where the additional memory IC is located on the module and it also identifies some additional IC's used to help interface the micro-controller to its external memory.
In examining figure 5, you'll also find a couple of switches on the module. The MicroStamp11 is generally operated in one of two modes. There is the boot mode and the run mode. Boot mode is used to download programs into the module's program memory. These programs are written and compiled on your laptop. You download your compiled programs into the MicroStamp11 over a serial cable connecting the MicroStamp11 and your laptop. For this to work, however, the MicroStamp11's switches must be positioned so the module is in boot mode. After your program has been loaded in the MicroStamp11, then you may start executing your program by switching the module into run mode. Once again, the module's switches must be positioned so the module is in run mode. Once in run mode, your program will start executing as soon as the module has been reset. The Micro-controller can be reset by pulling its RESET pin low, or by simply cycling power to the unit.
As discussed earlier, the 68HC11 interacts with the outside world through its pins. In the MicroStamp11 module,however, many of the 68HC11 pins are used by the module. For example, the pins associated with the clock logic are connected to the crystal. The pins on PORTB and PORTC are connected to the external memory IC. So there are fewer pins on the MicroStamp11 module than will be found on the 68HC11 chip. The pinout for the MicroStamp11 is shown below in figure 6.
The MicroStamp11 has 20 pins. Pins 1-8 are associated with
PORTA on the 68HC11. The nth bit on PORTA is denoted as
PAn. In particular pin 1-8 on the MicroStamp11 have
the names PA7 t PA0, respectively. MicroStamp11 pin 9 is
tied to the RESET pin on the 68HC11. When this pin's
TTL-logic level is pulled low (zero volts), then the 68HC11
restarts itself. Pulling this pin low is similar to
pushing CTRL-ALT-DEL on a Windows machine. MicroStamp11
pin 10 corresponds to VIN on the 68HC11.
The MicroStamp11 gets its power through pins 11 and 12. MicroStamp11 pin 11 (GND) is tied to zero volts and MicroStamp11 pin 12 (VDC) is tied to +5 volts. The MicroStamp11 pins 13 and 14 are tied to the interrupt logic pins IRQ and XIRQ, respectively.
The 68HC11's PORTD is tied to MicroStamp11 pins 15-20. From
figure 6, you'll note that these pins have
two names. This is because the PORTD pins not only serve
as I/O pins but they can also be used by the serial
interface subsystems SPI and SCI. The logical names for
the nth bit on PORTD is
PDn. So we see that
MicroStamp11 pins 15-20 are tied to
PA0, respectively. Note that there are only 6 pins
from PORTD available on the MicroStamp11. This is because
two of PORTD's pins are used to interface the 68HC11 to its
external memory chip. MicroStamp11 pins 19-20 are also
used by the SCI interface. MicroStamp11 pins 15-18 are
tied to the SPI interface.
As a final bit of information, figure 7
illustrates the MicroStamp11's memory map. As you can see
from the figure, RAM is located in low memory below address
0x00FF. EEPROM memory is located above this