Bus Interface

On top of the on-die RAM, the AS2650v2 also provides an interface for attaching memory and IO devices externally, though an address/data bus.

16-bit addresses and 8-bit data is multiplexed on the ADB pins for this purpose. The control lines LE_LO, LE_HI, OE, WE, IOC and IOD control any address latches or memory and IO devices connected to this bus.

Bus transactions can be split into two categories: memory transactions and IO transactions. Memory transactions are used to access any byte as addressed by a 16-bit address which is multiplexed out of the processor. IO transactions do not emitt any address. Instead, the IOD and IOC pins are used to indicate if the transferred byte is a command or regular data. The intended way to use these is to latch the byte sent using IOC and use it as a separate 8-bit IO address before IOD is used to actually interface with the addressed IO device.

Every memory transaction begins with the target address being latched one byte at a time using LE_LO and LE_HI while the relevant part of the full address is presented on the ADB lines. Of note is that the processor internally keeps track of the state the external address latches should be in, and only emits LE_HI if the most-significant byte of the address has indeed changed. LE_LO is always generated.

After the address is set, OE and WE indicate if the transfer is from or to a memory device. IO transactions skip directly to this step, asserting either IOC or IOD and then OE or WE.

Memory Read transaction

Memory Write transaction

IO Read transaction

IO Write transaction