tonyw wrote:
I'm building an external Flash ROM reader/writer/programmer to plug into an A500 (look, I've got one already, OK?). Most of the bus signals are pretty mundane, but what about the UDS and LDS signals? LDS means Low Data Select (bits 7:0), UDS means Upper Data Select (bits 15:
.
When do I need to use them? Is it during a Write (Byte) operation, or a Read (Byte) operation, or when?
tony
Hi Tony,
Not sure if I'll explain this very well.
The R/W line determines whether a byte or word is read or written, as you probably know.
The UDS and LDS lines 'Upper Data Strobe' and 'Lower Data Strobe' is used to make the selection between a byte (8 bits) and a 'word' (16 bits).
Now, the address bus always word-addresses memory, but during byte access the processor will simply transfer half a word (i.e. a byte) during byte access, and this will either be the upper half or the lower half of the word, which is signalled through UDS or LDS - one of them will be set to 9 whilst the other will remain at 1 [UDS and LDS are active when low]. To access a 'word' both lines will be set.
