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8.5.3.1
Transmit Error Insertion
Errors can be inserted into the generated pattern one at a time or at a rate of one out of every 10
n bits. The value of
n is programmable (1 to 7 or off) in the
BERT.TEICR:TEIR[2:0] configuration field. Single bit error insertion is
enabled by setting
BERT.TEICR:BEI and can be initiated from the microprocessor interface or by the manual error
insertion pin (GPIOB2). See Section
8.7.5 for more information about manual error insertion.
8.6 Loopbacks
Each LIU has three internal loopbacks. See
Figure 2-1. When only the hardware interface is enabled (
IFSEL = 000
and
HW = 1), loopbacks are controlled by the
LBn[1:0] and
LBS pins. When a microprocessor interface is enabled
≠ 000), loopbacks are controlled by the LB[1:0] and LBS fields in the
PORT.CR3 register.
Analog loopback (ALB) connects the outgoing transmit signal back to the receiver’s analog front end. During ALB
the transmit signal is output normally on
TXP/TXN, but the received signal on
RXP/RXN is ignored.
Line loopback (LLB) connects the output of the receiver to the input of the transmitter. The LLB path does not
include the B3ZS/HDB3 decoder and encoder so that the signal looped back is exactly the same as the signal
received, including bipolar violations and code violations. During LLB, recovered clock and data are output on
Diagnostic loopback (DLB) connects the TCLK, TPOS/TDAT and TNEG pins to the
RCLK,
RPOS/RDAT, and
RNEG/RLCV pins. During DLB (with LLB disabled), the signal on
TXP/TXN can be the normal transmit signal or an
AIS signal from the AIS generator. DLB and LLB can be enabled simultaneously to provide simultaneous remote
and local loopbacks.
8.7 Global Resources
8.7.1 Clock Rate Adapter (CLAD)
The CLAD is used to create multiple transmission-quality reference clocks from a single transmission-quality
(
±20ppm, low jitter) clock input on the
REFCLK pin. The LIUs in the device need up to three different reference
clocks (DS3, E3, and STS-1) for use by the CDRs and jitter attenuators. Given one of these clock rates or any of
several other clock frequencies on the
REFCLK pin, the CLAD can generate all three LIU reference clocks. The
internally generated reference clock signals can optionally be driven out on pins
CLKA, CLKB, and
CLKC for
external use. In addition a fourth frequency, either 77.76MHz or 19.44MHz, can be generated and driven out on the
CLKD pin for use in Telecom Bus applications.
When only the hardware interface is enabled (
IFSEL = 000 and
HW = 1), the CLAD is controlled by the
CLADBYPpin, and the
REFCLK frequency is fixed at 19.44MHz. When the
CLADBYP pin is high all PLLs in the CLAD are
become inputs, and the DS3, E3, and STS-1 reference clocks, respectively, are sourced from these pins.
Transmission-quality clocks (
±20ppm, low jitter) must be provided to these pins for each line rate required by the
LIUs. When
CLADBYP is low, all four PLLs in the CLAD are enabled, and the generated DS3, E3, STS-1, and
When a microprocessor interface is enabled
(IFSEL≠ 000), the CLAD clock mode and the
REFCLK frequency are
set by the
GLOBAL.CR2:CLAD[6:4] bits, as shown in
Table 8-11. When CLAD[6:4] = 000, all PLLs in the CLAD are
become inputs, and the DS3, E3, and STS-1 reference clocks, respectively, are sourced from these pins.
Transmission-quality clocks (
±20ppm, low jitter) must be provided to these pins for each line rate required by the
LIUs. CLAD[6:4] = 000 is equivalent to pulling the
CLADBYP pin high. When CLAD[6:4]
≠ 000, the PLL circuits are
enabled as needed to generate the required clocks, as determined by the CLAD[6:0] bits and the LIU mode bits
(
PORT.CR2:LM[1:0]). If a clock rate is not required as a reference clock, then the PLL used to generate that clock
is automatically disabled and powered down. The CLAD[3:0] bits are output enable controls for
CLKA, CLKB,
CLKC and
CLKD, respectively. Configuration bit
GLOBAL.CR2:CLKD19 specifies the frequency to be output on the
CLKB and
CLKC pins and lock status for the CLAD.
Each LIU block indicates the absence of the reference clock it requires by setting its
LIU.SR:LOMC bit.