Forty Pins Total, 32 Pins I/O
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In this book, we are considering the 40-pin DIP version of the chip. Other, much smaller form factors are also available. Of the 40 pins on the Propeller DIP, 32 are I/O pins. The remaining 8 pins are used for power connections, reset connections, grounding, crystal connection, enabling the system, and other usual and basic housekeeping tasks seen on all microprocessors. All 32 I/O pins can be used for I/O when pins 31 and 32, which are reserved for serial communications to a PC, are not being used for communication. This communication takes place exclusively through pins 31 and 30. Pins 29 and 28 are used for access of external memory when such memory is in use. At other times they can be used for regular I/O connections. Refer back to Figure 2-1 for a pinout diagram of the 40-pin version of the chip.
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When we are connecting to a low-voltage, low-power device such as the Propeller, our goal is to minimize the load on the pins and to thus protect the device from harmful voltages and currents that might be generated by our experimental circuitry (and our attendant unintentional mistakes). The Propeller chip is designed for extremely low-power applications and runs at 3.3 VDC. Though this voltage is adequate for interaction with transistor-transistor logic (TTL) devices for most applications, it is desirable to put the output signals through standard (7404 or 7414) gates to keep the current requirements at the Propeller to a minimum. Connections through gates will isolate the chip and limit the power backfed into the Propeller if we make a wiring mistake. When we use these gates in our designs, they will most probably be inverting gates, meaning that each gate will invert the signal coming into it. We will have to keep this in mind as we write our programs. The advantage of using these gates/buffers is the increased power they provide for driving any attached loads and the isolation/protection of the Propeller chip. Needless to say, a separate 5-VDC power supply has to be provided for these gates and the circuitry beyond them. The ground connection is common. Noninverting gates (7407s) are available if you prefer to use them, but you will find that for most purposes an inverting arrangement is more convenient. Putting a signal through two inverting gates leaves it unchanged in polarity and is a convenient way of getting a signal buffered but unchanged if you need to do so. Figure 2-3 illustrates the use of an inverting buffer to control an LED.
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Figure 2-3 Using an inverting buffer to connect to an LED and a dry contact switch with a pull-up resistor
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The ProPeller ChiP: An overAll DesCriPTion
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The System Counter
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The 32-bit system counter operates at the same rate as the oscillator for the Propeller and is used as the master clock for all timing functions. It provides identical information to all the cogs and can be read simultaneously by all the cogs. Its major purpose is to support the timing of delays and other time-related functions. It does not keep track of the time elapsed or the number of times it has overflowed. If you need that information, you have to create the program functions to do so. All other timing functions can be implemented by observing a time differential based on the counts in the system counter at the two times of interest and knowing the operating frequency of the system meaning that if we have two readings of the system counter and we know how fast the counter is running, we can calculate a time interval.
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Program Storage and Execution
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In this book, we are talking about Spin code execution. Any code examples discussed will be written in Spin. When we move a program we wrote on the PC, in the Propeller Tool environment, to the Propeller chip, the program is moved either to main RAM in the Propeller or to the external serial EEPROM chip connected to the Propeller. Moving it to main RAM is much faster (with the Fn 10 key), and the RAM location is used for most developmental purposes because of this. If the main RAM option is chosen (Fn 10), the program is lost when the Propeller is turned off. The program will have to be reloaded into the Propeller the next time you need to use the program. Whatever you might have stored or saved in your PC will, of course, still be available for reuse. If, on the other hand, the program is moved to EEPROM (with the Fn 11 key), it will be available even after the Propeller system has been turned off. Downloading to the EEPROM takes much longer than downloading to the main RAM. However, programs in EEPROM are not volatile. When the Propeller system is turned on, it looks for a connection to a PC. If there is no connection to a PC, the current program in the Propeller external EEPROM is read, moved to Propeller main RAM, and executed. Therefore, the rule is, Once the application you have in mind has been finalized, store the program to EEPROM. While you are developing the program, use RAM storage. Spin code is always executed from main RAM, by the Spin Interpreter running in the currently selected cog. The Spin Interpreter is copied from main ROM to Cog 0 at startup, and then to any other cog as it is started by the application to run Spin code later on. Cog_0 (an arbitrary name) can launch other cogs after startup; then after that, any cog that is running can start and stop any other cog, as needed, and even stop itself. The Spin Interpreter running in the cog fetches tokens as needed from main RAM