C8051F986-GUR Silicon Labs, C8051F986-GUR Datasheet - Page 130

• Manufacturer Part Number: C8051F986-GUR
• Description: 8-bit Microcontrollers - MCU 8kB 512B RAM 12b ADC
• Manufacturer: Silicon Labs

Specifications of C8051F986-GUR

• Rohs: yes
• Core: 8051
• Processor Series: C8051
• Data Bus Width: 8 bit

C8051F99x-C8051F98x
11. On-Chip XRAM
The C8051F99x-C8051F98x MCUs include on-chip RAM mapped into the external data memory space
(XRAM). The external memory space may be accessed using the external move instruction (MOVX) with
the target address specified in either the data pointer (DPTR), or with the target address low byte in R0 or
R1. On C8051F99x-C8051F98x devices, the target address high byte is a don’t care.
When using the MOVX instruction to access on-chip RAM, no additional initialization is required and the
MOVX instruction execution time is as specified in the CIP-51 chapter.
Important Note: MOVX write operations can be configured to target Flash memory, instead of XRAM. See
Section “14. Flash Memory” on page 149 for more details. The MOVX instruction accesses XRAM by
default.
11.1. Accessing XRAM
The XRAM memory space is accessed using the MOVX instruction. The MOVX instruction has two forms,
both of which use an indirect addressing method. The first method uses the Data Pointer, DPTR, a 16-bit
register which contains the effective address of the XRAM location to be read from or written to. The
second method uses R0 or R1 in combination with the EMI0CN register to generate the effective XRAM
address. Examples of both of these methods are given below.
11.1.1. 16-Bit MOVX Example
The 16-bit form of the MOVX instruction accesses the memory location pointed to by the contents of the
DPTR register. The following series of instructions reads the value of the byte at address 0x1234 into the
accumulator A:

The above example uses the 16-bit immediate MOV instruction to set the contents of DPTR. Alternately,
the DPTR can be accessed through the SFR registers DPH, which contains the upper 8-bits of DPTR, and
DPL, which contains the lower 8-bits of DPTR.
11.1.2. 8-Bit MOVX Example
The 8-bit form of the MOVX instruction uses the contents of R0 or R1 to determine the 8-bits of the
effective address to be accessed. The following series of instructions read the contents of the byte at
address 0x0034 into the accumulator A.
130
MOV
MOVX
MOV
MOVX
DPTR, #0034h
A, @DPTR
R0, #34h
a, @R0
; load DPTR with 16-bit address to read (0x0034)
; load contents of 0x0034 into accumulator A
; load low byte of address into R0 (or R1)
; load contents of 0x0034 into accumulator A
Rev. 1.1