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The architecture of RoboCube

Overview

RoboCube is build on an open bus system. As shown in the figure below a global bus comprises several sub-buses which are managed from different sources.

Block diagram of the RoboCube architecture

The system is logically divided into four subsystems:

  1. The processor subsystem contains the Motorola MC68332 microcontroller. It provides the address bus, data bus, special timing channels (TP), interrupt lines (/IRQ), several chip selects (/CS), an SPI bus and a RS232 serial bus.
  2. The memory subsystem contains the boot ROM as well as the main memory.
  3. The extension busmaster subsystem provides two serial RS232 buses and two I2C buses for which a broad variety of sensors and actuators ICs are available.
  4. The I/O subsystem contains all the interfaces needed in our current control for the robots.
To get the smallest possible setup which is nevertheless flexible the processor and memory subsystem are placed on the same printed circuit board.

The following subsections describe the different subsystems mentioned in the above item list.

The Motorola MC68332 microcontroller

The MC68332 is a 32-bit integrated microcontroller, combining high-performance data manipulation capabilities with powerful peripheral subsystems. The subsystems work independently from the main CPU32 instruction processing unit which is based on the MC68020 processor, thus allowing for a high overall system performance.

The main blocks of the MC68332 are shown in figure below.

Block diagram of the MC68332 microcontroller

The MC68332 contains intelligent peripheral modules such as the time processor unit (TPU) which provides 16 microcoded channels for performing time related activities from simple input capture or output compare to complicated motor control or pulse width modulation. High speed serial communications are provided by the queued serial module (QSM) with synchronous and asynchronous protocols available. Two kilobytes of fully static standby RAM allow fast two-cycle access for system and data stacks and variable storage with provision for battery backup. Twelve chip selects enhance system integration for fast external memory or peripheral access. These modules are connected on-chip via the intermodule bus (IMB).

The major features of the MC68332 are as follows:

With its up to 16MByte of external RAM and the internally 32-bit architecture this processor offers in addition a powerful processing unit. With the independent and programmable submodules which are specialized to control applications many tasks can be done directly by the processor. This allows for a very compact and powerful design of the control hardware.

The microcontroller offers a synchronous serial bus, the so-called SPI bus. The SPI bus is full duplex and allows multiple bus masters. It has 4 address lines encoding the destination of the transfer. Two additional lines carry the data in each direction (from and to the master). The relatively large number of wires and the fact that only a few devices are available for this bus type make the SPI bus unattractive to us.

Memory subsystem

The system boots out of a 128kByte EPROM which holds a basic input/output operating system (BIOS). A huge part of it is dedicated to the efficient handling of different actuators and sensors. As the EPROM does not allow the same access time as the main memory the BIOS copies itself into the main memory at start-up. Therefore, a certain part of the functionality of the BIOS can be overloaded by the `experienced' user at run time.

The main memory is currently a 256kByte low power SRAM with an access time of 70ns. Thus, no wait states have to be generated and accesses are done with a normal 3 clock bus cycle. With the 24 bit address bus the main memory can be extended up to 16MByte. In a final version of the RoboCube the memory subsystem comprises 2MByte DRAM.

Extension busmasters

A very powerful bus is the inter IC bus from Philips (I2C). It is a synchronous bus that uses only two lines. There are many devices available. The major features are multiple masters, bidirectional operation, hardware bus arbitration, hardware bit and byte level synchronization\footnote{This gives a slow device the opportunity to adjust the transfer speed to its own capabilities and can force wait states during the transfer.}, 7 bit addresses, and maximum 100 kBit/s.

Additionally, there are three standard RS232 serial buses in the system. They allow to bridge long distances and are usually simple to handle, but they offer normally only poor throughput. The main advantage here is, that connecting a board to a computer is very simple. This is especially useful for debugging and downloading software at startup. One of the RS232 connections is dedicated to the radio transmission system, which enables the communication to the board at run time.

Physical layout

One of the most striking features of the RoboCube are its dimensions. With the size of 40mm x 77mm x 86mm it is very small and compact (see figure below).

Physical layout of the complete RoboCube

This layout relies on two special design choices. First, all ICs (except the EPROM with the BIOS) are in surface mounted design (SMD). Second, we use a special stacking connector from AMP which builds the global bus perpendicular to the boards plane. Hence, the system can be easily extended by stacking additional boards on top of the others. Finally, these two connector blocks make the whole layout mechanically very stable and guarantee secure connections.

Data Paths of the RoboCube

Overview | Hardware | BIOS | Connectors | Assembly