
DS26522 Dual T1/E1/J1 Transceiver
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8.9.16 T1 Programmable In-Band Loop Code Detection
The DS26522 can generate and detect a repeating bit pattern from one to eight bits or 16 bits in length. This
function is available only in T1 mode.
Table 8-26. Registers Related to T1 In-Band Loop Code Detection
REGISTER
FRAMER
ADDRESSES
FUNCTION
Receive In-Band Code Control Register
082h
Used for selecting length of receive in-
band loop code register.
Receive Up Code Definition Register 1
0ACh
Receive up code definition register 1.
Receive Up Code Definition Register 2
0ADh
Receive up code definition register 2.
Receive Down Code Definition Register 1
0AEh
Receive down code definition register 1.
Receive Down Code Definition Register 2
0AFh
Receive up code definition register 2.
Receive Spare Code Register 1 (
T1RSCD1)
09Ch
Receive spare code register 1.
Receive Spare Code Register 2 (
T1RSCD2)
09Dh
Receive spare code register 2.
Receive Real-Time Status Register 3 (
RRTS3)0B2h
Real-time loop code detect.
Receive Latched Status Register 3 (
RLS3)
092h
Latched loop code detect bits.
Receive Interrupt Mask Register 3 (
RIM3)
0A2h
Mask for latched loop code detect bits.
Note: The addresses shown are for Framer 1. The address for Framer 2 can be calculated by adding 200 hex to the framer address.
The framer has three programmable pattern detectors. Typically, two of the detectors are used for “loop-up” and
“loop-down” code detection. The user programs the codes to be detected in the Receive Up Code Definition
T1RDNCD2). The length of each pattern is selected via the Receive In-Band Code Control register (
T1RIBCC).
registers. When detecting a 16-bit pattern, both receive code definition registers are used together to form a 16-bit
register. For 8-bit patterns, both receive code definition registers are filled with the same value. Detection of a 1-,
2-, 3-, 4-, 5-, 6-, and 7-bit pattern only requires the first receive code definition register to be filled. The framer
detects repeating pattern codes in both framed and unframed circumstances with bit-error rates as high as 10E-2.
The detectors can handle both F-bit inserted and F-bit overwrite patterns. Writing the least significant byte of the
receive code definition register resets the integration period for that detector. The code detector has a nominal
integration period of 48ms. Thus, after about 48ms of receiving a valid code, the proper status bit (LUP, LDN, and
LSP) is set to 1. Note that real-time status bits, as well as latched set and clear bits, are available for LUP, LDN,
and LSP
(RRTS3 and
RLS3). Normally codes are sent for a period of 5 seconds. It is recommended that the
software poll the framer every 50ms to 100ms until 5 seconds has elapsed to ensure that the code is continuously
present.