
141
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
FDC37C957FR 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 Read/Write registers available to the
8051. The Scratch RAM may be converted into scratch ROM by setting the MMC bit.
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 of this
specification.