Cypress CYV15G0404DXB manual 10B/8B Decoder Block, Receive Bist Operation, Bits Detected, Framer

CYV15G0404DXB

through the FRAMCHARx latches through the configuration interface.

The specific bit combinations of these framing characters are listed in Table 6. When the specific bit combination of the selected framing character is detected by the framer, the bound- aries of the characters present in the received data stream are known.

Table 6. Framing Character Selector

FRAMCHARx	Bits Detected in Framer
	Character Name	Bits Detected
0	COMMA+	00111110XX[10]
	COMMA–	or 11000001XX
1	–K28.5	0011111010 or
	+K28.5	1100000101

Framer

The framer on each channel operates in one of three different modes. Each framer is enabled or disabled using the RFENx latches using the configuration interface. When the framer is disabled (RFENx = 0), no combination of received bits alters the frame information.

When the low latency framer is selected (RFMODEx[1:0] = 00), the framer operates by stretching the recovered character clock until it aligns with the received character boundaries. In this mode the framer starts its alignment process on the first detection of the selected framing character. To reduce the impact on external circuits that use the recovered clock, the clock period is not stretched by more than two bit periods in any one clock cycle. When operated with a character rate output clock, the output of properly framed characters may be delayed by up to nine character clock cycles from the detection of the selected framing character. When operated with a half character rate output clock, the output of properly framed characters may be delayed by up to 14 character clock cycles from the detection of the framing character.

When RFMODEx[1:0] = 10, the Cypress-Mode Multi-Byte framer is selected. The required detection of multiple framing characters makes the associated link much more robust to incorrect framing due to aliased SYNC characters in the data stream. In this mode, the framer does not adjust the character clock boundary, but instead aligns the character to the already recovered character clock. This ensures that the recovered clock does not contain any significant phase changes or hops during normal operation or framing, and allows the recovered clock to be replicated and distributed to other external circuits or components using PLL-based clock distribution elements. In this framing mode the character boundaries are only adjusted if the selected framing character is detected at least twice within a span of 50 bits, with both instances on identical 10-bit character boundaries.

When RFMODEx[1:0] = 01, the Alternate-mode Multi-Byte Framer is enabled. Like the Cypress-mode Multi-Byte Framer, multiple framing characters must be detected before the character boundary is adjusted. In this mode, the data stream must contain a minimum of four of the selected framing

Note

characters, received as consecutive characters, on identical 10-bit boundaries, before character framing is adjusted.

10B/8B Decoder Block

The decoder logic block performs two primary functions:

■Decoding the received transmission characters to data and special character codes

■Comparing generated BIST patterns with received characters to permit at-speed link and device testing

The framed parallel output of each deserializer shifter is passed to its associated 10B/8B Decoder where, if the decoder is enabled, the input data is transformed from a 10-bit transmission character back to the original data or special character code. This block uses the 10B/8B decoder patterns in Table 14 and Table 15. Received special code characters are decoded using Table 15. Valid data characters are indicated by a 000b bit combination on the associated RXSTx[2:0] status bits, and special character codes are indicated by a 001b bit combination of these status outputs. Framing characters, invalid patterns, disparity errors, and synchronization status are presented as alternate combinations of these status bits.

When DECBYPx = 0, the 10B/8B decoder is bypassed through the configuration interface. When bypassed, raw 10-bit characters are passed through the receiver and presented at the RXDx[7:0] and the RXSTA[1:0] outputs as 10-bit wide characters.

When the decoder is enabled by setting DECBYPx = 1 through the configuration interface, the 10-bit transmission characters are decoded using Table 14 and Table 15. Received Special characters are decoded using Table 15. The columns used in Table 15 are determined by the DECMODEx latch through the device configuration interface. When DECMODEx = 0 the ALTERNATE table is used and when DECMODEx = 1 the CYPRESS table is used.

Receive BIST Operation

The receiver channel contains an internal pattern checker that can be used to validate both device and link operation. These pattern checkers are enabled by the associated RXBISTx latch using the device configuration interface. When enabled, a register in the associated receive channel becomes a signature pattern generator and checker by logically converting to a Linear Feedback Shift Register (LFSR). This LFSR generates a 511-character or 526-character sequence that includes all data and special character codes, including the explicit violation symbols. This provides a predictable yet pseudo random sequence that can be matched to an identical LFSR in the attached transmitters. When synchronized with the received data stream, the associated Receiver checks each character in the Decoder with each character generated by the LFSR and indicates compare errors and BIST status at the RXSTx[2:0] bits of the Output Register.

When BIST is first recognized as being enabled in the Receiver, the LFSR is preset to the BIST-loop start code of D0.0. This code D0.0 is sent only once per BIST loop. The status of the BIST progress and any character mismatches are presented on the RXSTx[2:0] status outputs.

10.The standard definition of a Comma contains only seven bits. However, since all valid Comma characters within the 8B/10B character set also have the eighth bit as an inversion of the seventh bit, the compare pattern is extended to a full eight bits to reduce the possibility of a framing error.

Document #: 38-02097 Rev. *B Page 17 of 44

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