
DS2156
221 of 262
30.
The DS2156 contains an on-chip clock synthesizer that generates a user-selectable clock output on the
BPCLK pin, referenced to the recovered receive clock (RCLK). The synthesizer uses a phase-locked loop
to generate low-jitter clocks. Common applications include generation of port and backplane system
clocks. The CCR2 register is used to enable (CCR2.0) and select (CCR2.1 and CCR2.2) the clock
frequency of the BPCLK pin.
Register Name:
CCR2
Register Description:
Common Control Register 2
Register Address:
71h
Bit #
7
6
5
4
Name
TRPA4
TRPA3
TRPA2
TRPA1
TRPA0
Default
0
0
0
0
Bit 0/Backplane Clock Enable (BPEN)
0 = disable BPCLK pin (pin held at logic 0)
1 = enable BPCLK pin
Bits 1, 2/Backplane Clock Selects (BPCS0, BPCS1)
BPCS1
BPCS0
BPCLK Frequency (MHz)
0
0
16.384
0
1
8.192
1
0
4.096
1
1
2.048
Bits 3 to 7/UTOPIA Port Address (TRPA0 to TRPA4).
See
Register Definitions
in Section 24.7.
31.
FRACTIONAL T1/E1 SUPPORT
31.1 TDM Backplane Mode
The DS2156 can be programmed to output gapped clocks for selected channels in the receive and
transmit paths to simplify connections into a USART or LAPD controller in fractional T1/E1 or ISDN-
PRI applications. The receive and transmit paths have independent enables. Channel formats supported
include 56kbps and 64kbps. This is accomplished by assigning an alternate function to the RCHCLK and
TCHCLK pins. Setting CCR3.0 = 1 causes the RCHCLK pin to output a gapped clock as defined by the
receive fractional T1/E1 function of the PCPR register. Setting CCR3.2 = 1 causes the TCHCLK pin to
output a gapped clock as defined by the transmit fractional T1/E1 function of the PCPR register. CCR3.1
and CCR3.3 can be used to select between 64kbps and 56kbps operation. See Section 5 for details about
programming the per-channel function. In T1 mode no clock is generated at the F-bit position.
When 56kbps mode is selected, the LSB clock in the channel is omitted. Only the seven most significant
bits of the channel have clocks.
PROGRAMMABLE BACKPLANE CLOCK SYNTHESIZER
3
2
1
0
BPCS1
0
BPCS0
0
BPEN
0
0