
are not internally generated are obtained from the external interface. 
Section 2.3.1
, 
Section 2.3.2
, 
Section 2.3.3
, and 
Section 2.3.4
 describe internally 
generated octets.
In STS-1 mode, there are four clock pulses on TOVH_CLK for each row in 
the framing format (a total of 36 clock pulses per frame). The Synchronous 
Payload Envelope (SPE) starts immediately after the row 1 overhead (the J1 octet 
follows the C1 octet). TMRKR is high during row 1 of the framing format (octets 
A1, A2, C1, J1). STS-3c/STM-1 mode has the same format except there are 10 
clock pulses for each row for a total of 90 clock pulses per frame. 
In STS-1/STS-3c/STM-1 modes, the content of octets D1, D2, and D3 is from 
the internal HDLC formatter, if enabled. These octets can also be provided via the 
TXOVH[7:0] input.
In G.832 E3 mode, there is a total of 7 clock pulses per frame and TMRKR is 
high during the FA1 and FA2 octets. In G.832 E4 mode, there is a total of 16 clock 
pulses per frame, and TMRKR is again high during the FA1 and FA2 octets. The 
TXOVH[7:0] inputs should be connected to ground if all 0s octet data is desired 
for octets that are not internally generated.
CN8223
2.0 Functional Description
ATM Transmitter/Receiver with UTOPIA Interface
2.3 Overhead Generation
100046D
Conexant
2-17
2.3.5  Transmit Framing Overhead Interface
An octet interface is available for external insertion of certain framing overhead 
in STS-1/STS-3c/STM-1 and G.832 E3/E4 framing modes. The interface consists 
of an output clock on TOVH_CLK, an output marker on TMRKR, and an 8-bit 
input bus for overhead octets. 
The timing for this interface is 
illustrated
 in 
Figure 2-12
.
There is a clock pulse on the TOVH_CLK output for each overhead octet that 
appears in the framing format and that is provided on the bus input. The bus input 
is sampled on the falling edge of the TOVH_CLK signal. TMRKR is high on a 
particular octet in each mode to synchronize external circuitry.
All overhead octets can be provided by external insertion if Enable External 
Overhead [bit 15] in CONFIG_2 [0x01] is set. If this bit is not set, only octets that 
Figure 2-12.  Transmit Framing Overhead Interface Timing
TOVH_CLK
TMRKR
A1
A2
C1
J1
TXOVH[7:0]
8