c8051t617 Silicon Laboratories, c8051t617 Datasheet - Page 109

• Manufacturer Part Number: c8051t617
• Description: Mixed Signal Byte-programmable Eprom Mcu
• Manufacturer: Silicon Laboratories

12. External RAM
The C8051T61x devices include 1024 bytes of RAM mapped into the external data memory space. All of
these address locations may be accessed using the external move instruction (MOVX) and the data
pointer (DPTR), or using MOVX indirect addressing mode. If the MOVX instruction is used with an 8-bit
address operand (such as @R1), then the high byte of the 16-bit address is provided by the External Mem-
ory Interface Control Register (EMI0CN as shown in SFR Definition 12.1).
For a 16-bit MOVX operation (@DPTR), the upper 6-bits of the 16-bit external data memory address word
are "don't cares.” As a result, the 1024 byte RAM is mapped modulo style over the entire 64 k external
data memory address range. For example, the XRAM byte at address 0x0000 is shadowed at addresses
0x0400, 0x0800, 0x0C00, 0x1000, etc. This is a useful feature when performing a linear memory fill, as the
address pointer doesn't have to be reset when reaching the RAM block boundary.
SFR Definition 12.1. EMI0CN: External Memory Interface Control
Bits 7–2: UNUSED. Read = 000000b. Write = don’t care.
Bits 1–0: PGSEL: XRAM Page Select.
R/W
Bit7
The EMI0CN register provides the high byte of the 16-bit external data memory address
when using an 8-bit MOVX command, effectively selecting a 256-byte page of RAM. Since
the upper (unused) bits of the register are always zero, the PGSEL determines which page
of XRAM is accessed.
For Example: If EMI0CN = 0x01, addresses 0x0100 through 0x01FF will be accessed.
R/W
Bit6
R/W
Bit5
R/W
Bit4
Rev. 0.3
R/W
Bit3
C8051T610/1/2/3/4/5/6/7
R/W
Bit2
R/W
Bit1
PGSEL
R/W
Bit0
SFR Address
00000000
Reset Value
0xAA
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