PEB 20560
Operational Description
Semiconductor Group
3-4
2003-08
devices to be connected together for concentrator functions. If the output driver
capability of the EPIC-1 should prove to be insufficient for a specific application, an
external driver controlled by the TSC# can be connected.
The
PCM-standby function
makes it possible to switch all PCM-output lines to high
impedance with a single command. Internally, the device still works normally. Only the
output drivers are switched off.
The number of time-slots per 8-kHz frame is programmable in a wide range (from 4
to 128). In other words, the
PCM-data rate can range between 256 kbit/s up to
8.192 Mbit/s
. Since the overall switching capacity is limited to 128 time-slots per
direction, the number of PCM-ports also depends on the required number of time-slots:
in case of 32 time-slots per frame (2.048 Mbit/s) for example, four highways are
available, in case of 128 time-slots per frame (8.192 Mbit/s), only one highway is
available.
The partitioning between number of ports and number of bits per frame is defined by the
PCM-mode
. There are four PCM-modes.
The timing characteristics at the PCM-interface (data rate, bit shift, etc.) can be varied in
a wide range, but they are the same for each of the four PCM-ports, i.e. if a data rate of
2.048 Mbit/s is selected, all four ports run at this data rate of 2.048 Mbit/s.
The PCM-interface has to be clocked with a
PCM-Data Clock (PDC)
signal having a
frequency equal to or twice the selected PCM-data rate. In
single clock rate
operation,
a frame consisting of 32 time-slots, for example, requires a PDC of 2.048 MHz. In
double clock rate
operation, however, the same frame structure would require a PDC
of 4.096 MHz.
For the synchronization of the time-slot structure to an external PCM-system, a
PCM-
Framing Signal (PFS)
must be applied. The EPIC-1 evaluates the rising PFS edge to
reset the internal time-slot counters. In order to adapt the PFS-timing to different timing
requirements, the EPIC-1 can latch the PFS-signal with either the rising or the falling
PDC- edge. The PFS-signal defines the position of the first bit of the internal PCM-frame.
The actual position of the external upstream and downstream PCM-frames with respect
to the framing signal PFS can still be adjusted using the
PCM-offset function
of the
EPIC-1. The offset can then be programmed such that PFS marks any bit number of the
external frame.
Furthermore it is possible to select either the rising or falling PDC-clock edge for
transmitting and sampling the PCM-data.
Usually, the repetition rate of the applied framing pulse PFS is identical to the frame
period (125
μ
s). If this is the case, the
loss of synchronism indication function
can
be used to supervise the clock and framing signals for missing or additional clock cycles.
The EPIC-1 checks the PFS-period internally against the duration expected from the
programmed data rate. If, for example, double clock operation with 32 time-slots per
frame is programmed, the EPIC-1 expects 512 clock periods within one PFS-period. The
synchronous state is reached after the EPIC-1 has detected two consecutive correct