Tait TB9100, Service Manual, Page: 79 / 132

TB9100 Reciter Service Manual Network Circuitry 79
© Tait Electronics Limited January 2006
5.7.2 CODEC Digital Interfaces
The CODEC supports two different types of digital interface: an I
2
C
interface for configuration setting and a PCM-type interface for transferring
the digital audio data.
Codec
Configuration
Interface
The I
2
C interface connects to the MPC and is used to set internal control
registers for operating mode, signal routing, gain settings, power control and
tone generation. The transfer of data is similar to an I
2
C access to the
E2PROM (see “Inter-IC (I2C) Bus” on page 58 ) except that CODEC
occupies a different address in the I
2
C address range (see “Appendix A – I2C
Device Addresses” on page 103 ). Refer to the CODEC data sheet
(reference 15) for full details of the internal control registers.
Codec Data
Interface
The PCM interface of the CODEC is connected to the McBSP port 0 of
the DSP using a subset of the McBSP signal lines. Data, in 15-bit linear
coded PCM format, is transferred out of the CODEC on the PCMO
output and into the CODEC on the PCMI input. Data bits are shifted out
of PCMO, msb first, on the positive-going edge of the PCMCLK clock
signal and shifted into PCMI on the negative-going edge.
The PCMCLK clock is provided from the McBSP port transmit clock,
CLKX0; since this is a steady 2.048 MHz, it is also used for the server clock
input (MCLK) to the CODEC for its internal timing. To synchronize the
bit streams the CODEC also requires a frame synchronization signal,
PCMSYNC, which is supplied from the McBSP transmit sync signal, FSX0,
which pulses at the nominal sample rate of 8KHz.
5.7.3 E&M Interface
An E&M (ear and mouth) signalling interface is provided, along with the
audio interface, to implement signalling function that is compatible with
basic telephony signalling circuits. This consists of an output circuit, the M-
lead, and an input circuit, the E-lead. Typically, M-lead function would be
assigned as an RX-GATE output and the E-lead as the TX-KEY input to
the base station.
Normally, in telephone switching circuits, the M-lead would switch to
ground and the E-lead would be connected to the -48V exchange battery
to sense the current flow into the battery. Basic E&M signalling can then be
implemented by connecting the M-lead of one device to the E-lead of
another device, and vice versa.
Actually, for the ASIF, two leads are brought out for each interface to
provide fully floating switch and sensor circuits. This facilitates the
connection of the E&M interface to signalling circuits other than standard
telephony E&M interfaces. This flexibility comes at a price, as the voltage
source for the E-lead must now be wired externally to the ASIF. Any voltage
source from 5 to 50V is suitable, but it will have to be suitably isolated from