PIC16F1459-ESSQTP datasheet(315/418 Pages) MICROCHIP

315 / 418 page

 2012 Microchip Technology Inc.

Preliminary

DS41639A-page 315

PIC16(L)F1454/5/9

TABLE 26-1:

EFFECT OF DTSEN BIT ON ODD/EVEN (DATA0/DATA1) PACKET RECEPTION

26.4.1.3

BDnSTAT Register (SIE Mode)

When the BD and its buffer are owned by the SIE, most

of the bits in BDnSTAT take on a different meaning. The

configuration is shown in Register 26-6. Once the

UOWN bit is set, any data or control settings previously

written there by the user will be overwritten with data

from the SIE.

The BDnSTAT register is updated by the SIE with the

token Packet Identifier (PID), which is stored in the PID

bits of the BDnSTAT register. The transfer count in the

corresponding BDnCNT register is updated. Values

that overflow the 8-bit register carry over to the two

Most Significant digits of the count, BD bits of the

BDnSTAT register.

26.4.2

BD BYTE COUNT

The byte count represents the total number of bytes

that will be transmitted during an IN transfer. After an IN

transfer, the SIE will return the number of bytes sent to

the host.

For an OUT transfer, the byte count represents the

maximum number of bytes that can be received and

stored in USB RAM. After an OUT transfer, the SIE will

return the actual number of bytes received. If the

number of bytes received exceeds the corresponding

byte count, the data packet will be rejected and a NAK

handshake will be generated. When this happens, the

byte count will not be updated.

The 10-bit byte count is distributed over two registers.

The lower 8 bits of the count reside in the BDnCNT reg-

ister. The upper two bits reside in the BC bits of the

BDnSTAT register. This represents a valid byte range

of 0 to 1023.

26.4.3

BD ADDRESS VALIDATION

The BD Address register pair contains the starting RAM

address location for the corresponding endpoint buffer.

No mechanism is available in hardware to validate the

BD address.

If the value of the BD address does not point to an

address in the USB RAM, or if it points to an address

within another endpoint's buffer, data is likely to be lost

or overwritten. Similarly, overlapping a receive buffer

(OUT endpoint) with a BD location in use can yield

unexpected results. When developing USB applications,

the user may want to consider the inclusion of software-

based address validation in their code.

26.4.4

PING-PONG BUFFERING

An endpoint is defined to have a ping-pong buffer when

it has two sets of BD entries: one set for an Even

transfer and one set for an Odd transfer. This allows the

CPU to process one BD while the SIE is processing the

other BD. Double-buffering BDs in this way allows for

maximum throughput to/from the USB.

The USB module supports four modes of operation:

• No ping-pong support

• Ping-pong buffer support for OUT Endpoint 0 only

• Ping-pong buffer support for all endpoints

• Ping-pong buffer support for all other Endpoints

except Endpoint 0

The ping-pong buffer settings are configured using the

PPB bits in the UCFG register.

The USB module keeps track of the Ping-Pong Pointer

individually for each endpoint. All pointers are initially

reset to the Even BD when the module is enabled. After

the completion of a transaction (UOWN cleared by the

SIE), the pointer is toggled to the Odd BD. After the

completion of the next transaction, the pointer is

toggled back to the Even BD and so on.

The Even/Odd status of the last transaction is stored in

the PPBI bit of the USTAT register. The user can reset

all Ping-Pong Pointers to Even using the PPBRST bit.

Figure 26-5 shows the four different modes of opera-

tion and how USB RAM is filled with the BDs. BDs have

a fixed relationship to a particular endpoint depending

on the buffering configuration. The mapping of BDs to

endpoints is detailed in Table 26-2. This relationship

also means that gaps may occur in the BDT if end-

points are not enabled contiguously. This theoretically

means that the BDs for disabled endpoints could be

used as buffer space. In practice, users should avoid

using such spaces in the BDT unless a method of vali-

dating BD addresses is implemented.

OUT Packet

from Host

BDnSTAT Settings

Device Response after Receiving Packet

DTSEN

DTS

Handshake UOWN

TRNIF

BDnSTAT and USTAT Status

DATA0

ACK

Updated

DATA1

ACK

Not Updated

DATA0

ACK

Not Updated

DATA1

ACK

Updated

Either

ACK

Updated

Either, with error

NAK

Not Updated

Legend: x = don’t care

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