8.6
Fast Transfers
EZ-USB has a special fast transfer mode for applications that use external FIFOs con-
nected to the EZ-USB data bus. These applications typically require very high transfer
speeds in and out of EZ-USB endpoint buffers.
EZ-USB
Registers
(Addressed
DPTR
as external
RAM)
movx a,@dptr
movx @dptr,a
Accumulator
Figure 8-7. 8051 MOVX Instructions
The 8051 transfers data to and from EZ-USB registers and RAM using the MOVX (move
external) instruction (Figure 8-7). The 8051 loads one of its two 16-bit data pointers
(DPTR) with an address in RAM, and then executes a MOVX instruction to transfer data
between the accumulator and the byte addressed by DPTR. The “@” symbol indicates
that the address is supplied indirectly, by the DPTR.
The EZ-USB core monitors MOVX transfers between the accumulator and any of the six-
teen isochronous FIFO registers. If an enable bit is set (FISO=1 in the FASTXFR regis-
ter), any read or write to an isochronous FIFO register causes the EZ-USB core to connect
the data to the EZ-USB data bus D[7..0], and generate external read/write strobes. One
MOVX instruction thus transfers a byte of data in or out of an endpoint FIFO and gener-
ates timing strobes for an outside FIFO or memory. The 2-cycle MOVX instruction takes
2 cycles or 333 ns. Figures 8-8 and 8-9 show the data flow for fast writes and reads over
the EZ-USB data bus.
EZ-USB TRM v1.9
Chapter 8. EZ-USB CPU
Page 8-9
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