Andy Yeong wrote:
-------------------------------
When I execute this instruction :
mov a, p0 or either mov p0, a caused RD as well as WR signal to assert respectively ?

- - - - - - - - - - - - - - - -
As I understand it, no.

The MOV instruction is only moving between internally mapped registers and latches. When you read or write a port with them, you're just reading the latches from the inside of the microcontroller.

The MOVX commands does involve the RD and WR signal and it only moves between the accumulator and the data bus.

Note that the 8051 supports a very flexible range of possible IO configurations. You can choose to move parallel data in through any port if you aren't using a data bus. You can read in a byte from the port or you can read or test bits individually on that port.
Lots of options.

Bit-Banging as its sometimes called, is programming at the primitive level. You have to tell the microcontroller to take a external IO signal high, and then tell it to take it low. No automatic signals like the data bus configuration offers. This isn't a weakness, though I am certain novices prefer more automated functions. :)

There is a design philosophy that too many options can be bad, because it confuses and slows design down as one compares and selects among the options. This might be one of the initial handicaps of the 8051 but its a strength once you learn to use it.

I saw that philosophy in practice in 1981. The company I worked for designed disk controllers for minicomputers. Our design was based on a 8X300 16 bit microcontroller which had only 8 instructions but it did everything in only 250ns as I recall. It had a separate instruction fetch bus from its data and IO.

We started a new design at the same time as our New England competitors; a super fast drive was to be on the market soon. With our few uC options, We simply drew up the schematics for all required datapaths under the uC control and started PCB layout in a few weeks. If we needed a control line, just use another uC pin; simple. We then started designing the flowcharts and writing code. It took us 6 to 9 months to get to market. Our competitors chose a different path.

Up in New England they chose to use bit-slicing, an elegant choice with too many options (think of it as a uC kit where you design your opcodes and registers). I had a college buddy inside their company and found out that they wasted about a year arguing about the instruction set they were going to use. Six months after that they released a TWO-BOARD set that was expensive for them to build.

That's probably why I like PICs for small tasks. Designing with overkill is not efficient or economical in the long run.

aka j

http://www.geocities.com/mingpzong