AS2507T

Preliminary Data Sheet AS2507 ® Austria Mikro Systeme International AG Single Chip 2-Wire Intercom CMOS Integrated Circuit General Description The AS2507 is a CMOS integrated circuit that contains all the functions needed to build a 2wire intercom network. The device incorporates 2/4-wire conversion (hybrid), soft clipping for high speech quality, FSK modem and a simple interface to a microcontroller. The signalling mode is selectable between FSK modulation and burst mode. The low standby current (typ. 2 mA) allows several devices to listen to the 2-wire line. The speech circuit is designed for compatibility with commonly used handset (150 Ω earpiece and electret microphone) with receive gain of -6 dB and transmit gain of 32 dB (relative to line). Key Features
Line/speech circuit and signalling on one 14









pin CMOS chip Only 2 wires needed for power supply, signalling and speech Soft clipping to avoid harsh distortion Fully integrated 2/4 wire conversion Side tone cancellation Low noise Signalling with FSK modem Low standby power consumption allows parallel operation of up to 25 terminals on a bus pair with central supply Parallel operation of up to 70 terminals if supplied locally Controllable via simple µC interface Very few external components Application Entrance telephone system, intercom and data transmission, alarm systems, toy phone Package Available in 14 pin DIP and 16 pin SOIC Block Diagram 1/9 July 1998 Preliminary Data Sheet AS2507 Pin description Pin # DIP 14 7 6 2 4 Pin # SO 16 7 6 2 4 Symbol Function M1 M2 RI RO Microphone Input 1 Differential input for the microphone (electret) Microphone Input 2 Differential input for the microphone (electret) Receive Input Input for ac-separated receive signal Receive Output to Handset Output for driving a dynamic earpiece with an impedance from 150Ω to 300Ω 14 9 8 11 12 5 15 16 9 10 8 12 13 5 VDD VSS AGND OSC1 OSC2 PD Voltage Drain Drain Positive Power Supply Voltage Source Source Negative Power Supply Analogue Ground Signal ground for the internal amplifiers Oscillator Output 1 Output to ceramic resonator 3.58MHz. Oscillator Input 2 Input for ceramic resonator 3.58MHz. Power Down Input Active high, i.e. a high level on this pin will power down the analogue signal path. Receive Data Output Output of the FSK demodulator Transmit Data Input Input for the FSK modulator 10 13 1 11 14 1 RxD TxD TE Transmit Enable Input for enabling transmit data 2/9 July 1998 Preliminary Data Sheet AS2507 Functional Description The AS2507 is a CMOS integrated circuit that incorporates a speech circuit and a FSK modem. It is intended to be used as linepowered interface on a 2-wire intercom bus. FSK Modulator Two signalling modes are provided, namely FSK and burst mode. The signalling mode and the signalling can be controlled by a microcontroller using the TE and TXD inputs as follows: TE 0 0 1 1 TXD 0 1 0 1 MODE IDLE, no transmission TEST, for testing only SPACE ("0") 18.645 kHz MARK ("1") 22.375 kHz Standby Condition During standby operation (PD=High) only the FSK demodulator is active to provide the companion microcontroller with all signalling information on the 2-wire bus. The low power consumption (typical 2mA) and a high input and output impedance in standby mode allow the parallel operation of many terminals on each bus pair. 2/4-Wire Conversion The AS2507 has a built-in side tone cancellation circuit. The transmit signal is attenuated by 6 dB over the 600Ω resistor (ac impedance) and subtracted from the receive signal at the receive input (RI). This configuration allows the selection of the required ac impedance and yet maintaining a good side tone cancellation. AC Impedance The ac impedance is determined by an external resistor at the TO output (typical 600Ω). Transmit Path The gain of the transmit path from the microphone inputs (M1/M2) to the transmit output (TO) is set to 32 dB. The soft clip level is set to 0.5VPEAK at TO. In standby the output impedance is 60kΩ. Receive Path The gain of the receive path from the receive input (RI) to the receive output (RO) is set to -6dB. The receive input is internally biased to AGND with a 500kΩ resistor. FSK Demodulator Also the demodulator provides two modes for detection as follows: Signal on line IDLE, no transmission SPACE ("0") 18.645 kHz MARK ("1") 22.375 kHz RXD 1 0 1 The demodulator consists out of a bandpass filter to attenuate interfering speech signals and a period counter. The RxD output is updated after each valid period or a counter overflow. Transmission Protocol In order to assure a safe data transmission, data framing is recommended. Each frame should consist out of a preamble (e.g. FFhex), a header (e.g. AAhex), data bytes and checksum. Independence of bus polarity Centrally supplied terminals can be connected to the 2-wire bus independent from polarity. In this case a rectifier bridge, known from telephony, is recommended since the DCsupply current will bias the diodes. 3/9 July 1998 Preliminary Data Sheet AS2507 Typical 2-Wire Intercom Application 4/9 July 1998 Preliminary Data Sheet AS2507 Electrical characteristics Electrical charateristics are measured with the Test Circuit application. Typical mean values will not be tested. Absolute maximum ratings Positive Supply Voltage Input Current Analogue Input Voltage Digital Input Voltage Electrostatic Discharge (HBM 1.5k Ω-100pF) Storage Temperature -0.3V