Embedded Systems September 2000 Vol13_10

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JEAN J. LABROSSE New Twists on the zao Despite being a quarter-century old, the 8-bit Z80 and its derivatives continue to be popular with embedded system designei'S. With two new derivatives emerging within the last year, it's a good time to take a fresh look at the architecture. This article considers the architectural choices from the perspective of an RTOS implementer or user. ntroduced by ZiLOG in the early '70s, the Z80 was an impres- sive processor for its time. The original Z80 (a 2MHz part) was a souped-up version of the the n-popular Intel 8080 processor. While compatible with the 8080, the Z80 offered many new fea- tures: new instructions, new addressing modes, two index regis- ters, faster execution, high performance peripheral chips, and a clever interrupt scheme. Soon, Z80s were available in higher speed grades ( 4MHz and 6MHz) . For many years, the Z80 was the king of 8-bit processors. In 1976, ZiLOG introduced the Z280, which was an enhanced Z80, but it turned out to be a big failure. Around 1985, Hitachi introduced the 64180 microprocessor, which was an improved Z80. ZiLOG second-sourced the processor under the Z180 name. This 64-pin DIP processor executed most instructions with fewer clock cycles than the Z80 and contained a number of on-chip peripherals such as a pair of DARTS, a clocked serial I/ 0 port, an on-chip interrupt controlle1~ two DMA channels, two 16-bit timers, and an on-chip memory management unit allowing the '180 class p.-ocessor to extend the address space from 64K to 1,024K Although caJled an MMU, this device only implemented memory banking (described later). In the mid '90s, ZiLOG announced the Z380 but, like the Z280, it never really went anywhe re. Today, a number of companies are selling ZSO- derived processors, including AB-Semicon, Kawasaki , LSI, NEC, Toshiba, and VAutomation. Towards the end of last year, ZiLOG announced a new generation of

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