
xr
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
XRT72L52
REV. 1.0.1
117
This R/W bit allows the user to loopback data presented to the HDLC block prior to D3/E3framing. When this
bit is set to “1” loopback is enabled, when “0” this loopback path is disabled.
2.4
T
HE
L
OSS
OF
C
LOCK
E
NABLE
F
EATURE
The timing for the Microprocessor Interface section, originates from a line rate (e.g., either a 34.368MHz or
44.736 MHz) signal that is provided by either the TxInClk[n] or the RxLineClk[n] signals. However, if the Fram-
er device experiences a Loss of Clock signal event such that neither the TxInClk[n] nor the RxLineClk[n] signal
are present, then the Framer Microprocessor Interface section cannot function.
The Framer device offers a Loss of Clock (LOC) protection feature that allows the Microprocessor Interface
section to at least complete or terminate an in-process Read or Write cycle (with the local μP) should this Loss
of Clock event occur. The LOC circuitry consists of a ring oscillator that continuously checks for signal transi-
tions at the TxInClk[n] and RxLineClk[n] input pins. If a Loss of Clock Signal event occur such that no transi-
tions are occurring on these pins, then the LOC circuitry will automatically assert the RDY_DTCK signal in or-
der to complete (or terminate) the current Read or Write cycle with the Framer Microprocessor Interface sec-
tion.
The user may enable or disable this LOC Protection feature by writing to Framer I/O Control Register, Bit 7
(Disable TxLOC), as depicted below.
Writing a "1" to this bit-field disables the TxLOC Protection feature. Writing a "0" to this bit-field disables this
feature.
N
OTE
:
The Ring Oscillator can be a source of noise, within the Framer chip. Hence, there may be situations where the
user will wish to disable the LOC Protection feature.
2.5
U
SING
THE
PMON H
OLDING
R
EGISTER
The Microprocessor Interface section consists of an 8-bit bi-directional data bus. As a consequence, the local
μP will be able to read from and write to the Framer on-chip registers, 8 bit per (read or write) cycle. Since
most of the Framer on-chip registers contain 8-bits, communicating with the local μP over an 8-bit data bus is
not much of an inconvenience. However, all of the PMON registers contain 16 bits. Consequently, any reads
of the PMON registers, will require two read cycles. To make matters potentially more complicated, these
PMON registers are Reset-upon-Read registers. Therefore, the contents of both the MSB and LSB registers
(of the READ PMON register) are reset to zero upon the first of these two read cycles.
Fortunately, the XRT72L52 Framer IC includes a feature that will make reading a PMON register a slightly less
complicated task. The Framer chip address space contains a Read-Only register known as the PMON Holding
register, which is located at 0x6C. Whenever the local μP reads in an 8-bit value of a given PMON registers
(e.g., either the upper-byte or the lower byte value of the PMON register), the other 8-bit value of that PMON
register will automatically be loaded into the PMON Holding register. As a consequence, the other 8-bit value
of the PMON register is accessible by reading the PMON Holding register.
Hence, anytime the local μP is trying to read in the contents of a PMON register, the first read access must be
made directly to one of the 8-bit values of the PMON registers (e.g., for example: the PMON LCV Event Count
Register - MSB, Address = 0x50). However, the second read must always be made to a constant location in
system memory, the PMON Holding Register.
2.6
T
HE
I
NTERRUPT
S
TRUCTURE
WITHIN
THE
F
RAMER
M
ICROPROCESSOR
I
NTERFACE
S
ECTION
The XRT72L52 Framer device is equipped with a sophisticated Interrupt Servicing Structure. This Interrupt
Structure includes an Interrupt Request output, Int, numerous Interrupt Enable Registers and numerous Inter-
FRAMER I/O CONTROL REGISTER (ADDRESS = 0X01)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
TxLOC
Disable
LOC
Disable
RxLOc
AMI/Zero Sup
Unipolar/
Bipolar
TxLine
Clk Invert
RxLine
Clk Invert
Reframe
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
1
0
1
0
0
0
0
0