Rainbow Electronics AT89LP216 - Manual (Page 9)

9
3621A–MICRO–6/06
AT89LP216 [Preliminary]
8. Enhanced CPU
The AT89LP216 uses an enhanced 8051 CPU that runs at 6 to 12 times the speed of standard
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 simple
example is shown in
Figure 8-1 .
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 AT89LP216 follow this rule: the instruc-
tion 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 59. for more detailed information on individual instructions. Figures 8-2 and 8-3 show
examples of 1- and 2-byte instructions.
Figure 8-1. Parallel Instruction Fetches and Executions
Figure 8-2. Single-cycle ALU Operation (Example: INC R0)
System Clock
n th Instruction
(n+1) th Instruction
Fetch Execute
Fetch Execute
Fetch
T
n
T
n+1
T
n+2
(n+2) th Instruction
System Clock
Total Execution Time
Register Operand Fetch
T
1
T
2
T
3
ALU Operation Execute
Result Write Back
Fetch Next Instruction