C8051F321-GMR Datasheet, PDF (114/250 Pages) Silicon Laboratories – Full Speed USB, 16 k ISP FLASH MCU Family

C8051F320/1
12. External RAM
The C8051F320/1 devices include 2048 bytes of on-chip XRAM. This XRAM space is split into user RAM
(addresses 0x0000 - 0x03FF) and USB0 FIFO space (addresses 0x0400 - 0x07FF).
0xFFFF
Same 2048 bytes as from
0x0000 to 0x07FF, wrapped
on 2 kB boundaries
0x0800
0x07FF
0x0400
0x03FF
0x0000
USB FIFOs
1024 Bytes
XRAM
1024 Bytes
Accessed through USB FIFO
registers
Accessed with the MOVX
instruction
Figure 12.1. External Ram Memory Map
12.1. Accessing User XRAM
XRAM can 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 Memory Interface Control Reg-
ister (EMI0CN as shown in Figure 12.1). Note: the MOVX instruction is also used for writes to the Flash
memory. See Section “11. Flash Memory” on page 106 for details. The MOVX instruction accesses XRAM
by default.
For any of the addressing modes the upper 5 bits of the 16-bit external data memory address word are
"don't cares". As a result, the 2048-byte RAM is mapped modulo style over the entire 64k external data
memory address range. For example, the XRAM byte at address 0x0000 is also at address 0x0800,
0x1000, 0x1800, 0x2000, etc.
Important Note: The upper 1k of the 2k XRAM functions as USB FIFO space. See Section 12.2 for
details on accessing this memory space.
12.2. Accessing USB FIFO Space
The upper 1k of XRAM functions as USB FIFO space. Figure 12.2 shows an expanded view of the FIFO
space and user XRAM. FIFO space is accessed via USB FIFO registers; see Section “15.5. FIFO Manage-
ment” on page 147 for more information on accessing these FIFOs. The MOVX instruction should not be
used to load or modify USB data in the FIFO space.
Unused areas of the FIFO space may be used as general purpose XRAM, accessible as described in Sec-
tion 12.1. The FIFO block operates on the USB clock domain; thus the USB clock must be active when
accessing FIFO space. Note that the number of SYSCLK cycles required by the MOVX instruction is
increased when accessing USB FIFO space.
Important Note: The USB clock must be active when accessing FIFO space.
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