Interface Description, PCM Interface, Overview

Interface Description

PCM Interface

Overview

PCM Pulse Code Modulation is a standard method used to digitise human voice

Four-wire, full duplex serial interface used to transfer a single voice channel to an external audio codec

Maximum of one SCO connection is possible using the PCM interface

Further SCO channels must use the HCI protocol layer

Data format is 13 bit linear PCM

Description

Pulse Code Modulation (PCM) is the standard method used to digitise human voice patterns for transmission over digital communication channels. Through its PCM interface, SieMo provides hardware support for continual transmission and reception of PCM data, thus reducing processor overhead for wireless headset applications.

SieMo offers a bi-directional digital audio interface that routes directly into the baseband layer of the on-chip firmware (it does not pass through the HCI protocol- layer). Hardware on SieMo allows the data to be sent to and received from a SCO connection. Only one SCO connection can be supported by the PCM interface at any one time. Any additional SCO connections must receive and transmit their data over the HCI protocol layer. The data format is 13-bit linear PCM. SieMo can operate either as a master (with an output clock of 256kHz) or as a slave (with the input clock varying between 128kHz and 512kHz). When in slave mode and using higher clock rates SieMo can support Motorola's Synchronous Serial Interface (SSI) standard that allows multiple bi-directional audio channels to be multiplexed onto a single physical connection.

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Generic PCM Interface

For a generic PCM interface there is one master and one slave device. The master generates the clock and synchronisation signals. The sync signal identifies the start of the sample data and has an 8kHz period. There are two types of frame sync: long and short. In long frame sync mode PCM_SYNC going high indicates the first (and most significant) bit of the sample. It must remain high for at least two clock cycles, but this can be longer. In short frame sync MSB start is signalled by sync going low (normally it only goes high for one clock cycle).

The clock runs at a higher rate than sync: at least 8 x bits_per_sample MHz, although higher rates are common. The sample resolution is 13 bits/sample, uncompressed. Several Motorola CODECs allow their output gain to be controlled via the addition of three extra data bits after the audio data. SieMo supports this feature, effectively raising the bits per sample to 16. Data from both the master and slave is clocked out on the rising clock edge and sampled on the falling edge. Master mode is the default setting. In master mode SieMo generates a 256kHz clock signal (PCM_CLK) and the 8kHz, long format synchronisation signal (PCM_SYNC). Short frame sync is not supported. See PCM Timing Diagrams for more information.

Slave mode is selected by setting a Persistent Store value. In slave mode SieMo clocks output data on the rising edge of the received clock signal and samples incoming data on the falling edge. The incoming clock frequency should be between 128kHz and 512kHz. (Note that 128kHz is 8 x 16 kHz, therefore the absolute minimum possible frequency for the 8ksamples/sec and 16bits/sample (13 audio data plus three gain data). The frame sync must be long format. Short format is not supported (see SSI Mode and Timing Diagrams in this section for more information).

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