Loop start

Abstract: A power-conserving, loop start signalling, dual-voltage line-feed circuit that maintains ac longitudinal balance on the loop conductors and which limits average and peak loop current not only when the loop is in the off-hook transmission state but also during part-time on-hook transmission. The current-limiting circuitry employs feedback so that a virtual ground is presented toward the line feed transformer at voice frequencies to reduce battery noise and potential crosstalk that might otherwise couple into the circuit because of non-zero battery impedance. The circuit provides for injecting a voltage-controlled feed current into the current regulator to maintain sufficient drive current so the current regulator can continue to maintain the virtual ground when the loop is on hook. The circuit employs a battery switch and switch hook state detector which controls the connection of a higher voltage battery to the loop when the loop is on-hook and a lower voltage battery when the loop is off-hook to conserve power. To reduce the likelihood of oscillation between on and off-hook indications, the switchhook status detector responds to a loop resistance threshold that is lower when the high voltage battery is being used than when the low voltage battery is being used.

Abstract: There is disclosed, for use in a digital signal processor, an apparatus for dynamically sizing a hardware loop that executes a plurality of instruction sequences forming a plurality of instruction loops. The apparatus comprises: 1) N pairs of loop start registers and loop end registers, each loop start register for storing a loop start address and each loop end register for storing a loop end address; 2) N comparators, each of the N comparators associated with one of the N pairs of loop start registers and loop end registers, wherein each of the N comparators compares a selected one of a first loop start address and a first loop end address to a fetch program counter value to detect one of a loop start hit and a loop end hit; and 3) fetch address generation circuitry for detecting the loop start hit and the loop end hit and fetching from an address in a program memory an instruction associated with one of the loop start hit and the loop end hit and loading the fetched instruction into the hardware loop.

Abstract: A switchable common mode reference circuit for tip conductor and ring conductor line feed amplifiers provides a tip-ring common mode feedback reference for loop-start operations and a common mode fixed bias for ground-start operations. The functioning of the line feed arrangement is enhanced by using separately-sensed tip and ring current signals to control the switching function and by switching the line feed amplifiers between a current source mode for loop-start operation and a voltage source mode for ground-start operation.

Abstract: Conditional hardware branching employs zero overhead loop logic and writing to a loop count register within a program loop. The zero overhead loop logic includes a program counter (701), loop end registers (711, 712, 713), loop start registers (721, 722, 723), loop counter registers (731, 732, 733), comparators (715, 716, 717) and loop priority logic (725). Normally the program counter (701) is incremented each cycle. The comparators (715, 716, 717) compare the address stored in the program counter (701) with respective loop end registers (711, 712, 713). If the address in the program counter (701) equals a loop end address, loop priority logic (725) decrements the loop count register and loads the program counter with the loop start address in loop start register. Hardware looping involves loading a loop count register during program loop operation. An arithmetic logic unit operation generates a status bit loaded into a status register or is split into sections and generates a status bit for each section stored in a multiple flags register (211) used to load the loop. count register. If this is zero then the loop priority logic reloads the program counter (701) with the loop start address and repeats the loop. If this is nonzero, program counter (701) may increment normally or will be loaded with the loop start address of a higher priority loop.