AT89LP428 datasheet(16/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

Enhanced CPU

The AT89LP428/828 uses an enhanced 8051 CPU that runs at 6 to 12 times the speed of stan-

dard 8051 devices (or 3 to 6 times the speed of X2 8051 devices). The increase in performance

is due to two factors. First, the CPU fetches one instruction byte from the code memory every

clock cycle. Second, the CPU uses a simple two-stage pipeline to fetch and execute instructions

in parallel. This basic pipelining concept allows the CPU to obtain up to 1 MIPS per MHz. A sim-

ple example is shown in Figure 5-1.

Figure 5-1.

Parallel Instruction Fetches and Executions

The MCS-51 instruction set allows for instructions of variable length from 1 to 3 bytes. In a sin-

gle-clock-per-byte-fetch system this means each instruction takes at least as many clocks as it

has bytes to execute. The majority of instructions in the AT89LP428/828 follow this rule: the

instruction execution time in clock cycles equals the number of bytes per instruction with a

few exceptions. Branches and Calls require an additional cycle to compute the target address

and some other complex instructions require multiple cycles. See “Instruction Set Summary” on

page 107 for more detailed information on individual instructions. Figures 5-2 and 5-3 show

examples of 1- and 2-byte instructions.

Figure 5-2.

Single-cycle ALU Operation (Example: INC R0)

System Clock

nth Instruction

(n+1)th Instruction

Fetch

Execute

Fetch

Execute

Fetch

Tn+1

Tn+2

(n+2)th Instruction

System Clock

Total Execution Time

ALU Operation Execute

Result Write Back

Fetch Next Instruction

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