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586 Part 3 ½ Computer Classes Section 1½ Monolithic Microcomputers


cheaper package and less board area) or to a smaller program storage (e.g., because the application requires only a small control program leading to less ROM and a higher chip-fabrication yield).

Another approach to microcomputer families is to take an existing ISP and implement it in as few chips as possible. Future technology growth will eventually allow the total system to be placed on a single chip. This approach simplifies software development, since the existing software is directly usable. Care must be taken, however, that at least some members of the family have expansion capability to handle growth of the larger applications.

The Texas Instruments TM S9900 series is an example of a family in transition (see Table 3). Based on the TI-990 minicomputer ISP, the family offers three microcomputers with varying speed. The TM59980A is a reduced-complexity chip. The TMS9985 offers a small amount of on-chip memory. Finally the TMS9940 is a single-chip computer system with on-chip RAM/ROM and 32 programmable I/O lines instead of the traditional I/O bus for memory and peripherals.



Conclusion

The monolithic microcomputer class is already well populated with examples. This computer class will increase in importance as technology makes more complex systems feasible.



References

Gregoretti [1978]; Hayn, McDonogh, and Bellay [1981]; Simpson et al. [1978]; Wakerly [1979].

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