C8051F352 Datasheet, PDF (113/234 Pages) Silicon Laboratories – 8 k ISP Flash MCU Family

C8051F350/1/2/3
13. Prefetch Engine
The C8051F350/1/2/3 family of devices incorporate a 2-byte prefetch engine. Because the access time of
the Flash memory is 40 ns, and the minimum instruction time is 20 ns, the prefetch engine is necessary for
full-speed code execution. Instructions are read from Flash memory two bytes at a time by the prefetch
engine, and given to the CIP-51 processor core to execute. When running linear code (code without any
jumps or branches), the prefetch engine allows instructions to be executed at full speed. When a code
branch occurs, the processor may be stalled for up to two clock cycles while the next set of code bytes is
retrieved from Flash memory. The FLRT bit (FLSCL.4) determines how many clock cycles are used to read
each set of two code bytes from Flash. When operating from a system clock of 25 MHz or less, the FLRT
bit should be set to ‘0’ so that the prefetch engine takes only one clock cycle for each read. When operat-
ing with a system clock of greater than 25 MHz (up to 50 MHz), the FLRT bit should be set to ‘1’, so that
each prefetch code read lasts for two clock cycles.
SFR Definition 13.1. PFE0CN: Prefetch Engine Control
R
R
R/W
R
R
R
R
PFEN
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bits 7–6: Unused. Read = 00b; Write = Don’t Care
Bit 5: PFEN: Prefetch Enable.
This bit enables the prefetch engine.
0: Prefetch engine is disabled.
1: Prefetch engine is enabled.
Bits 4–1: Unused. Read = 0000b; Write = Don’t Care
Bit 0: FLBWE: Flash Block Write Enable.
This bit allows block writes to Flash memory from software.
0: Each byte of a software Flash write is written individually.
1: Flash bytes are written in groups of two.
R/W
Reset Value
FLBWE 00100000
Bit0
SFR Address: 0xE3
Rev. 1.1
113