Apparatus and method for multiplexing bi-directional data onto a low pin count bus between a host cpu and co-processor

Abstract

An apparatus and method for communication between a host CPU and a security co-processor are disclosed, in which a bus having a bi-directional data and command bus, a bi-directional control line, and a uni-directional clock line, is coupled to the CPU and to the co-processor. The bus supports data transfer between the CPU and the co-processor, including read operations and write operations, where each such operation includes a command phase, a data transfer phase, and an error check phase. The CPU and the co-processor have a dual master slave mode wherein either may be master of the bus, while the other is the slave. The bi-directional data and command bus carries command information from the master to the slave during the command phase, and carries data from the master to the slave during the data transfer phase for a write operation, and from the slave to the master for a read operation. The bi-directional control line specifies the start and end of each transfer. The uni-directional clock line synchronously clocks both the bi-directional data and command bus and the bi-directional control line. Data is transferred a packet at a time; each packet consists of an octet of data, which is transferred during 8 clocks. Flow control need only be applied once for each packet of data, and thus, only once per 8 clocks.