read/write operations

[shekhar_dandya]

Hi All,
I wanted to know what is meant by a read-modify-write operation?how is it different from a normal read/write operation?

Another ques:
What is a tri-state?what is the voltage on tri-stated pins?
Thanx
-Shekhar

[Ldanielrosa]

Good afternoon,

Read-modify-write means that the port is read, the bit selected is set or cleared, and the result is written to the port. The disadvantage of this is that the output that is driven high by the output may be overloaded and read as low (or vice-versa) as the output latch is not read only the actual pin. This is why sometimes it is good to use 'shadow' registers to modify, then transfer their contents to the port.

Tri-state means that the output driver is disabled- writing a 0 or 1 to the latch will not affect the external circuitry.

[shekhar_dandya]

what is the advantage of reading if one is going to modify i t?
Is it that the value is read,compared to the one selected for modification then if they are same,the value is not rewritten.if there is a mismatch,then the new value is written?
-Shekhar