Initial command to switch transistors disconnecting keys from microphone line

摘要

An audio accessory key detection system (40) includes a host circuit (2-3) coupled to communicate via a microphone line (7) with an accessory circuit (3-3) in either a MSFT mode or a digital communication mode. Depletion mode transistors (44-1,2,3) in the accessory circuit are coupled between keys (15-1,2,3) of the accessory circuit, respectively. The depletion mode transistors are allowed to remain conductive for MSFT mode operation. For digital communications mode operation, the host circuit sends a command via the microphone line to a key detector and controller circuit (29A) in the accessory circuit. In response, a voltage is generated to turn the depletion mode transistors off so as to allow digital communications mode operation between the accessory circuit and the host circuit.

主权项

1. An audio accessory key detection system comprising:
(a) a host circuit including a host controller, and a bias circuit coupled by a first resistor to a microphone out line; (b) a host transceiver in the host circuit coupled to the microphone out line and the host controller; (c) an analog to digital converter in the host circuit for digitizing an analog signal level on the microphone out line and conveying a digital representation of the analog signal level to the host controller during an analog mode of operation; and (d) an accessory circuit including: (i) a microphone in line coupled to the microphone out line; (ii) a microphone and bias circuit coupled to the microphone in line; (iii) an energy storage circuit coupled between the microphone in line and ground and having a supply voltage conductor; (iv) an accessory transceiver having a line coupled with the microphone in line, a data in line, and a data out line; (v) a key detector and controller circuit having a data input coupled to the data in line, a data output coupled to the data out line, a power input coupled to the supply voltage conductor, an enable output, a first key conductor input, and a second key conductor input; (vi) a first key connected between the first key conductor input and ground; (vii) a second key connected between the second key conductor and ground through a first resistor; (viii) a first switch transistor coupled from the microphone in line to between the first key conductor input and the first key and having a control input; (ix) a second switch transistor coupled from the microphone in line to between the second key conductor input and the second key and having a control input; and (x) a charge pump having an enable input coupled to the enable output, a supply voltage input coupled to the supply voltage conductor, and an output coupled to the control inputs of the switch transistors.