3.2
8051 Memory
Figure 3-1 illustrates the two internal EZ-USB RAM regions. 6,976 bytes of general-pur-
pose RAM occupy addresses 0x0000-0x1B3F. This RAM is loadable by the EZ-USB
core or I2C bus EEPROM, and contains 8051 code and data.
The EZ-USB EA (External Access) pin controls where the bottom segment of code
(PSEN) memory is located—inside (EA=0) or outside (EA=1) the EZ-USB chip. If the
EZ pin is tied low, the EZ-USB core internally ORs the two 8051 read signals PSEN and
RD for this region, so that code and data share the 0x0000-0x1B3F memory space. IF
EA=1, all code (PSEN) memory is external.
About 8051 Memory Spaces
The 8051 partitions its memory spaces into code memory and data memory. The 8051
reads code memory using the signal PSEN# (Program Store Enable), reads data memory
using the signal RD# (Data Read) and writes data memory using the signal WR# (Data
Write). The 8051 MOVX (move external) instruction generates RD# or WR# strobes.
PSEN# is a dedicated pin, while the RD# and WR# signals share pins with two IO port
signals: PC7/RD and PC6/WR. Therefore, if expanded memory is used, the port pins
PC7 and PC6 are not available to the system.
1,024 bytes of RAM at 0x7B40-0x7F3F implement the sixteen bulk endpoint buffers. 192
additional bytes at 0x7F40-0x7FFF contain the USB control registers. The 8051 reads and
writes this memory using the MOVX instruction. In the 8-KB RAM EZ-USB version, the
1,024 bulk endpoint buffer bytes at 0x7B40-0x7F3F also appear at 0x1B40-0x1F3F. This
aliasing allows unused bulk endpoint buffer memory to be added contiguously to the data
memory, as illustrated Figure 3-3. The memory space at 0x1F40-0x1FFF should not be
used.
Even though the 8051 can access EZ-USB endpoint buffers at either 0x1B40 or 0x7B40,
the firmware should be written to access this memory only at 0x7B40-0x7FFF to maintain
compatibility with future versions of EZ-USB that contain more than 8 KB of RAM.
Future versions will have the bulk buffer space at 0x7B40-0x7F3F only.
Page 3-2
Chapter 3. EZ-USB Memory
EZ-USB TRM v1.9
CYPRESS [ CYPRESS ]
