EZ-USB Datasheet, PDF (175/334 Pages) Cypress Semiconductor – The EZ-USB USB Integrated Circuit

2. Code the jump table with jump instructions to each individual USB interrupt ser-
vice routine. This table has two important requirements, arising from the format of
the AVEC byte (zero-based, with 2 LSBs set to 0):
• It must begin on a page boundary (address 0xNN00).
• The jump instructions must be four bytes apart.
• The interrupt service routines can be placed anywhere in memory.
• Write initialization code to enable the USB interrupt (INT2), and Autovector-
ing.
0043
0044
0045
8051 USB
Interrupt
Vector
LJMP
04
(00)2C
AVEC
USB core
2C
USB_Jmp_Table:
0400
042C
042D
042E
LJMP EP2OUT_ISR
01
19
EP2OUT_ISR:
0119
Figure 9-7. The Autovector Mechanism in Action
Figure 9-7 illustrates an ISR that services endpoint 2-OUT. When endpoint 2-OUT
requires service, the EZ-USB core activates the USB interrupt request, vectoring the 8051
to location 0x43. The jump instruction at this location, which was originally coded as
“LJMP 04-00” becomes “LJMP 04-2C” due to the EZ-USB core substituting 2C as the
Autovector byte for Endpoint 2-OUT (Table 9-3). The 8051 jumps to 042C, where it exe-
cutes the jump instruction to the endpoint 2-OUT ISR shown in this example at address
0119. Once the 8051 takes the vector at 0043, initiation of the endpoint-specific ISR takes
only eight 8051 cycles.
Page 9-12
Chapter 9. EZ-USB Interrupts
EZ-USB TRM v1.9