
SMSC DS – FDC37N958FR
Page 131
Rev. 09/01/99
Clock Source
External Clock Signal
The X1K clock source is from a 14.318MHz TTL
compatible clock. In “SLEEP” mode, the
external clock signal on X1K is not loaded by the
chip.
Internal Clock Signal
The 8051 may program itself to run off of an
internal ring oscillator having a frequency range
between 4 and 12MHz. This is not a precise
clock, but is meant to provide the 8051 with a
clock source when VCC2 is shut down in the
system.
8051 Memory Map
The 8051 can address 256B of internal Scratch
ROM and 32K of external ROM. The nEA pin is
used to enable access to the 256B of internal
Scratch ROM or External program ROM. The
FDC37N958FR also contains 256 bytes of
internal on-chip RAM.
When nEA=0, all the ROM is addressed as the
external ROM. It can support up to 32K bytes of
external code memory addressed as 00h to
7FFFh (the addresses from 8000h to FFFFh
wrap to the same addresses as 00h to 7FFFh).
This 32K can be mapped to any of the eight 32K
memory blocks in the 256K external ROM by the
KMEM register. At initial power-up (VCC1 POR)
the chip will execute from the block selected by
the default value of the KMEM register.
The 8051 can access upto 32K bytes of external
RAM addressed from 0-7FFFh. Refer to Table
54 for a list of the implemented on-chip memory
mapped registers. External memory addressed
from 8000h-FFFFh will access the 32K bytes of
program memory (8000-FFFFh) selected by the
KMEM register.
The 256 bytes of RAM from 7E00h-7EFFh as
well as the 256 bytes of scratch RAM from
7D00h-7DFFh are powered by VCC1. These
are
general
purpose
available to the 8051. The scratch RAM may be
converted into scratch ROM by setting the
Memory Map control bit.
read/write
registers
Memory Map Configuration Control Bit
The Configuration Register 0, an 8051 memory
mapped register at address 7FF4h includes a bit
called the Memory Map Control bit (MMC). The
MMC bit is bit-3 of this register and defaults to
zero on VCC1 POR. When MMC=0 the 8051
memory map will contain an additonal 256 bytes
of external scratch RAM in the address range
7D00h through 7DFFh. When MMC=1 the
scratch RAM at 7D00h-7DFFh becomes scratch
ROM at 00h-0FFh.
The Configuration Register 0 register is
described in the 8051 Control Register Section.