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Chapter 15. Registers Page 15-35

By default, FLAGA is the Programmable-Level Flag (PF) for the endpoint currently pointed to by

the FIFOADR[1:0] pins. For EP2 and EP4, the default endpoint configuration is BULK, OUT, 512,

2x, and the PF pin asserts when the entire FIFO has greater than/equal to 512 bytes. For EP6 and

EP8, the default endpoint configuration is BULK, IN, 512, 2x, and the PF pin asserts when the

entire FIFO has less than/equal to 512 bytes.

In other words, the default-configuration PFs for EP2 and EP4 assert when the FIFOs are half-full,

and the default-configuration PFs for EP6 and EP8 assert when those FIFOs are half-empty.

In the first example below, bits 5-3 have data that is required to complete the transfer. In the sec-

ond example, bits 5-3 do not matter - those bits are don’t cares because PKTSTAT=1:

Example 1:

Assume a Bulk IN transfer over Endpoint 2 and PKTSTAT=0:

EP2FIFOPFH = 0001 0000

• b6=0 (or PKTSTAT=0): this indicates that the transfer will include packets (as defined

by bits 5, 4, and 3) plus bytes (the sum in the flag low register)

• b5b4b3=010 binary (or 2 decimal): this indicates the number of packets to expect dur-

ing the transfer (in this case, two packets…)

EP2FIFOPFL = 0011 0010

•…plus 50 bytes in the currently filling packet

(the sum of the binary bits in the EP2FIFOPFL register is 2 +16 + 32 = 50 decimal)

DECIS=0, thus PF activates when less than 2 PKTS+50 bytes.

Example 2:

To perform an IN transfer of a number over the same endpoint, set PKTSTAT=1 and write a value

into the EP2FIFOPFL register:

EP2FIFOPFH = 01xxx000

EP2FIFOPFL = 75

Setting PKTSTAT=1 causes the PF decision to be based on the byte count alone, ignoring the

packet count. This mode is valuable for double-buffered endpoints, where only the byte count of

the currently-filling packet is important.

DECIS=0, thus PF activates when less than 75 bytes in the current PKTS.

Bit 1-0 PFC9:8 PF Threshold

Bits 1-0 of EP2FIFOPFH are bits 9-8 of the byte count register.

1 2 ... 350 351 352 353 354 355 356 357 358 359 360 ... 459 460

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