Expansion module with external bus for personal digital assistant and design method therefor

Abstract

An expansion module for a Handspring Visor includes a multi-master AMBA Advanced System Bus (ASB). The Springboard bus of the visor is coupled to the ASB bus via Springboard-to-ASB-bus bridge. This bridge includes a protocol translator and a second Arm7 to ASB interface. The protocol translator translates bi-directionally between the Springboard bus protocol and the Arm7TDMI protocol. The translator includes an interface to the Springboard bus and a state machine. The state machine coordinates data transfers between the buses. The state machine also monitors signals indicating when each of the buses begins to treat a data transfer as complete so that the data transfer can be validated or flagged as an error condition. A programmable counter adjusts maximum counts to compensate for different clock frequencies in measuring a write-wait state duration to ensure valid writes from the Visor to the ASB bus.