Datasheet

DS21Q50

49 of 87

12. PER-CHANNEL LOOPBACK

The DS21Q50 has per-channel loopback capability that can operate in one of two modes: remote per-

channel loopback or local per-channel loopback. PCLB1/2/3/4 are used for both modes to determine

which channels are looped back. In remote per-channel loopback mode, PCLB1/2/3/4 determine which

channels (if any) in the transmit direction should be replaced with the data from the receiver or, rather, off

the E1 line. In local per-channel loopback mode, PCLB1/2/3/4 determines which channels (if any) in the

receive direction should be replaced with the data from the transmit direction. If either mode is enabled,

then transmit and receive clocks and frame syncs must be synchronized. There are no restrictions on

which channels can be looped back or on how many channels can be looped back.

PCLB1, PCLB2, PCLB3, PCLB4

Per-Channel Loopback Registers

2B Hex, 2C Hex, 2D Hex, 2E Hex

Bit # 7 6 5 4 3 2 1 0

Name CH8 CH7 CH6 CH5 CH4 CH3 CH2 CH1

Name CH16 CH15 CH14 CH13 CH12 CH11 CH10 CH9

Name CH24 CH23 CH22 CH21 CH20 CH19 CH18 CH17

Name CH32 CH31 CH30 CH29 CH28 CH27 CH26 CH25

NAME BIT FUNCTION

CH1 to CH32 PCLB1.0 to 4.7

Per-Channel Loopback Control Bits

0 = do not loopback this channel

1 = loopback this channel

13. ELASTIC STORE OPERATION

The DS21Q50 contains a two-frame (512 bits) elastic store for the receive direction. The elastic store is

used to absorb the differences in frequency and phase between the E1 data stream and an asynchronous

(i.e., not frequency locked) backplane clock that can be 2.048MHz for normal operation or 4.096MHz,

8.192MHz, or 16.384MHz when using the IBO. The elastic store contains full-controlled slip capability.

If the receive elastic store is enabled (RCR.4 = 1), the user must provide a 2.048MHz clock to the

SYSCLK pin. If the IBO function is enabled, a 4.096MHz, 8.192MHz, or 16.384MHz clock must be

provided at the SYSCLK pin. The user can either provide a frame/multiframe sync at the RSYNC pin

(RCR.5 = 1) or have the RSYNC pin provide a pulse on frame/multiframe boundaries (RCR.5 = 0). If the

user wishes to obtain pulses at the frame boundary, RCR1.6 must be set to 0. If the user wishes to have

pulses occur at the multiframe boundary, RCR1.6 must be set to 1. If the elastic store is enabled, either

CAS (RCR.7 = 0) or CRC4 (RCR.7 = 1) multiframe boundaries are indicated through the RSYNC output.

See Section 19.1

for timing details. If the 512-bit elastic buffer either fills or empties, a controlled slip

occurs. If the buffer empties, a full frame of data (256 bits) is repeated at RSER, and the SR1.4 and RIR.3

bits are set to 1. If the buffer fills, a full frame of data is deleted, and the SR1.4 and RIR.4 bits are set to 1.