tailieunhanh - Hardware and Computer Organization- P6:

Hardware and Computer Organization- P6:Today, we often take for granted the impressive array of computing machinery that surrounds us and helps us manage our daily lives. Because you are studying computer architecture and digital hardware, you no doubt have a good understanding of these machines, and you’ve probably written countless programs on your PCs and workstations. | Chapter 6 In this section we will start from the D-flop as an individual device and see how we can interconnect many of them to form a memory array. In order to see how data can be written to the memory and read from the memory along the same signal path although not at the same instant in time consider Figure . The black box is just a slightly simplified version of the basic D flip-flop. We ve eliminated the S R inputs and Q output. The dark gray box is the tri-state buf Figure Schematic representation of a single bit of memory. The tri-state buffer on the output of the cell controls when the Q output may be connected to the bus. fer which is controlled by a separate OE output enable input. When OE is HIGH the tri-state buffer is disabled and the Q output of the memory cell is isolated Hi-Z state from the data lines DATA I O line . However the Data line is still connected to the D input of the cell so it is possible to write data to the cell but the new data written to the cell is not immediately visible to someone trying to read from the cell until the tri-state buffer is enabled. When we combine the basic FF cell with the tri-state buffer we have all that we need to make a 1-bit memory cell. This is indicated by the light gray box surrounding the two elements that we ve just discussed. The write signal is a bit misleading so we should discuss it. We know that data is written into the D-FF on the rising edge of a pulse which is indicated by the up-arrow on the write pulse W in Figure . So why is the write signal W written as if it was an active low signal The reason is that we normally keep the write signal in a 1 state. In order to accomplish a write operation the W must be brought low and then returned high again. It is the low-to-high transition that accomplishes the actual data write operation but since we must bring the write line to a low state in order to accomplish the actual writing of the data we consider the write signal to be active low.