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Because of the glueless nature of the external interfaces, especially the memory interface, it is easy to design hardware in a Rabbit 4000-based system. More details on hardware design are given in the Rabbit 4000 Microprocessor User's Manual.
2.1 Design Conventions
Rabbit-based systems designed using the following conventions will provide a hardware base that is compatible with running Dynamic C applications.
- Include a standard Rabbit programming cable. The standard 10-pin programming connector provides a connection to serial port A and allows the PC to reset and cold boot the target system.
- Connect a static RAM having at least 128 KB to the processor using /CS1, /OE1 and /WE1. It is useful if the PC board footprint can also accommodate a RAM large enough to hold all the code anticipated. Although code residing in some flash memory can be debugged, debugging and program download is faster to RAM.
- Connect a flash memory that is on the approved list and has at least 128 KB of storage to the processor using /CS0, /OE0 and /WE0. Non-approved memories can be used, but it may be necessary to modify several files. Some systems designed to have their program reloaded by an external agent on each powerup may not need any flash memory.
- Install a crystal oscillator with a frequency of 32.768 kHz to drive the battery-backable real-time clock (RTC), the watchdog timer (WDT) and the Periodic Interrupt.
- Install a crystal or oscillator for the main processor clock that is a multiple of 614.4 kHz, or better, a multiple of 1.8432 MHz. These preferred clock frequencies make possible the generation of standard serial baud rates. Common crystal frequencies to use are 7.3728 MHz, 11.0592 MHz, 14.7456 MHz, 22.1184 MHz, 29.4912 MHz or double these frequencies.
NOTE: The internal clock doubler can double these oscillations for a higher operating frequency.
- Digital I/O line PB1 should not be used in the design if cloning is to be used. PB1 should be pulled up with 50K or so pull up resistor if cloning is used. (See "BIOS Support for Program Cloning" on page 63 for more information on cloning.)
2.1.1 Rabbit Programming Connector
The user may be concerned that the requirement for a programming connector places added cost overhead on the design. The overhead is very small--less than $0.25 for components and board space that could be eliminated if the programming connector were not made a part of the system.
The programming connector can also be used for a variety of other purposes, including user applications. A device attached to the programming connector has complete control over the system because it can perform a hardware reset and load new software. If this degree of control is not desired for a particular situation, then certain pins can be left unconnected in the connecting cable, limiting the functionality of the connector to serial communications. Rabbit develops products and software that assume the presence of the programming connector.
2.1.2 Memory Chips
Most systems have one static RAM chip and one or two flash memory chips, but more memory chips can be used when appropriate. Static RAM chips are available in 128K x 8, 256K x 8, and 512K x 8 sizes. They are all available in 3 V versions. Suggested flash memory chips between 128K x 8 and 512K x 8 are given in Chapter 10, "Supported Flash Memories." That chapter also includes instructions for writing your own flash driver. The list of supported flash memories is in Technical Note 226, "Supported Flash Memories."
Dynamic C and a PC are not necessary for the production programming of flash memory since the flash memory can be copied from one controller to another by cloning. This is done by connecting the system to be programmed to the same type of system that is already programmed. This connection is made with the Rabbit Cloning Board. The cloning board connects to the programming ports of both systems. A push of a button starts the transfer of the program and an LED displays the progress of the transfer.
Please visit www.rabbit.com/store/index.shtml to purchase the Rabbit Cloning Board.
2.1.3 Oscillator Crystals
Generally, a system will have two oscillator crystals:
- A 32.768 kHz crystal oscillator to drive the battery-backable timer,
- A crystal that has a frequency that is a multiple of 614.4 kHz or a multiple of 1.8432 MHz. Typical values are 7.3728, 11.0592, 14.7456, 22.1184, and 29.4912 MHz.
These crystal frequencies (except 614.4 kHz and 1.8432 MHz) allow generation of standard baud rates up to at least 115,200 bps. The clock frequency can be doubled by an on-chip clock doubler, but the doubler should not be used to achieve frequencies higher than about 60 MHz on a 3.3 V system. A quartz crystal should be used for the 32.768 kHz oscillator. For the main oscillator, a ceramic resonator that is accurate to 0.5% will usually be adequate and less expensive than a quartz crystal for lower frequencies.
2.2 ESD Design Guidelines
The following guidelines are recommended for designs incorporating a Rabbit 4000 processor with electrostatic discharge (ESD) sensitivity on VBAT. These guidelines are good recommendations for all Rabbit processors.
- The 1.8 V supply for VBAT should be provided by a regulator with at least 2 kV ESD protection (human body model).
- The 3.3 V supply should have smaller 0.1 µF, 0.01 µF, and 2.2 nF bypass capacitors throughout the layout. In addition, the 3.3 V supply should have a large value bulk capacitor (10 µF).
The power going to VBAT should also be protected by a diode and two resistors. See a schematic for a RabbitCore module based on the Rabbit 4000 for more details.
2.3 Operating Voltages
The operating voltage in Rabbit 4000 based systems will usually be 1.8 V ±10% for the processor core and 3.3 V ±10% for the I/O. The I/O ring can also be run at 1.8 V ±10%.
The maximum computation per watt is obtained in the range of 3.0 V to 3.6 V. The highest clock speed requires 3.3 V. The maximum clock speed with a 3.3 V supply is 54 MHz (26.7264 x 2), but it will usually be convenient to use a 14.7456 MHz crystal, doubling the frequency to 29.4912 MHz. Good computational performance, but not the absolute maximum, can be implemented for a 3.3 V system by using an 11.0592 crystal and doubling the frequency to 22.1184 MHz. Such a system will operate with 70 ns memories. A 29.4912 MHz system will require memories with 55 ns access time. A table of timing specification is in the Rabbit 4000 Microprocessor User's Manual.
2.4 Power Consumption
Various mechanisms contribute to the current consumption of the Rabbit 4000 processor while it is operating, including current that is proportional to the voltage alone (leakage current) and dependent on both voltage and frequency (switching and crossover current).
Table 2-1 shows typical current draw as a function of the main clock frequency. The values shown do not include any current consumed by external oscillators or memory. It is assumed that approximately 30 pF is connected to each address line.
NOTE: VDDCORE = 1.8 V ± 10%, VDDIO = 3.3 V ± 10%, TA = -40°C to 85°C
Table 2-1 Preliminary Current vs. Clock Frequency 2.5 Through-Hole Technology
Most design advice given for the Rabbit 4000 assumes the use of surface-mount technology. However, it is possible to use the older through hole technology and develop a Rabbit 4000 system. One can use a Rabbit 4000-based Core Module, a small circuit board with a complete Rabbit 4000 core that includes memory and oscillators. Another possibility is to solder the Rabbit 4000 processors by hand to the circuit board. This is not difficult and is satisfactory for low production volumes if the right technique is used.
2.6 Moisture Sensitivity
Surface-mount processing of plastic packaged components such as Rabbit microprocessors typically involves subjecting the package body to high temperatures and various chemicals such as solder fluxes and cleaning fluids during solder wave and reflow operations. The plastic molding compounds used for IC packaging (encapsulation) is hygroscopic, that is, it readily absorbs moisture. The amount of moisture absorbed by the package is proportional to the storage environment and the amount of time the package is exposed to the humidity in the environment. During the solder reflow process, the package is heated rapidly, and any moisture present in the package will vaporize rapidly, generating excessive internal pressures to various interfaces in the package. The vapors escaping from the package may cause cracks or delamination of the package. These cracks can propagate through the package or along the lead frame, thus exposing the die to ionic contaminants and increasing the potential for circuit failures. The damage to the package may or may not be visible to the naked eye. This condition is common to all plastic surface-mount components and is not unique to Rabbit microprocessors.
Rabbit microprocessors are shipped to customers in moisture-barrier bags with enough desiccant to maintain their contents below 20% relative humidity for up to 12 months from the date of seal. A reversible Humidity Indicator Card is enclosed to monitor the internal humidity level. The loaded bag is then sealed under a partial vacuum. The caution label (IPC/JEDEC J-STD-020, LEVEL 3) included with each bag outlines storage, handling, and bake requirements.
The requirements outlined on the label only apply to components that will be exposed to SMT processing. This means that completed board-level products that will not be subjected to the solder reflow processing do not have to be baked or sealed in special moisture barrier bags.
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