Cypress Semiconductor FX2LP Informações Técnicas Página 147

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Chapter 8. Access to Endpoint Buffers Page 8-11
These registers do not affect full-speed (12 Mbps) operation; full-speed isochronous transfers are
always fixed at one packet per frame.
Table 8-6. Isochronous IN Packets per Microframe, High-Speed Only
8.6.2.3 EP2CS, EP4CS, EP6CS, EP8CS
Because the four large FX2 endpoints offer double, triple or quad buffering, a single BUSY bit is
not sufficient to convey the state of these endpoint buffers. Therefore, these endpoints have multi-
ple bits (NPAK, FULL, EMPTY) that can be inspected in order to determine the state of the end-
point buffers.
Multiple-buffered endpoint data must be read or written
only
at the buffer addresses given in
Table 8-3. The FX2 automatically switches the multiple buffers in and out of the single addressable
buffer space.
NPAK[2:0] (EP2, EP6) and NPAK[1:0] (EP4, EP8)
NPAK values have different interpretations for IN and OUT endpoints:
OUT Endpoints: NPAK indicates the number of packets received over USB and ready for
the firmware to read.
IN Endpoints: NPAK indicates the number of IN packets committed to USB (i.e., loaded
and armed for USB transfer), and thus unavailable to the firmware.
The NPAK fields differ in size to account for the depth of buffering available to the endpoints. Only
double buffering is available for EP4 and EP8 (two NPAK bits), and up to quad buffering is avail-
able for EP2 and EP6 (three NPAK bits).
FULL
While FULL and EMPTY apply to transfers in both directions, “FULL” is more useful for IN trans-
fers. It has the same meaning as “BUSY”, but applies to multiple-buffered IN endpoints. FULL=1
means that all buffers are committed to USB, and none are available for firmware access.
For IN transfers, FULL=1 means that all buffers are committed to USB, so firmware should not
load the endpoint buffer with any more data. When FULL=1, NPAK will hold 2, 3 or 4, depending
on the buffering depth (double, triple or quad). This indicates that all buffers are in use by the USB
INPPF1 INPPF0 Packets
0 0 Invalid
0 1 1
1 0 2
1 1 3
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