Student "edition" found at {csi dot journalspace dot com}.

Maybe I shouldn't have started this blog now, not with everything that's been going on.

In the meeting of my Integrated Computer Systems lecture class for the fifth week of the second term, we started talking about the three-port programmable interface chip, the 8255, the largest integrated circuit they will yet encounter so far with 40-inputs, as opposed to the 16 they are used to.

First I asked them was the CS pin means, and even though these students just took up their Switching Theory and Digital Design last term, they didn't know or remember that it means Chip Select for activating an I C.

I gave them the three modes of operation of the chip, which is either simple I/O, strobed (one or two way) handshaking, or bidirectional (one port) handshaking.

I also gave them the two bit addressing of the data port, which is either to one of the three outputs, or to the control byte within the chip.

Next I gave them the meaning of each bit of the control byte. The most significant bit is mode select when it has a status of 1, and bit select when it has a status of 0.

In mode select... mode, the least significant bits will determine the use of Port B and the lower 4-bit part of Port C, which can either be in the first or second mode of operation. The next four bits will determine the assignment for Port A and the upper part of Port C, which - unlike the first “group” - can be in set to any of the three modes.

In bit select mode, only five pins are used, the most significant, obviously set to zero. The least significant bit will be the value that will be assigned to a particular bit of Port C, and the next three bits after it will determine which of the eight bits will be set or reset in value.

I gave them some examples of determining the various modes to which to set the 8255. Afterwards, I gave them the pin assignments for choosing mode 1 or 2, where one pin has to be “ready to send”, “ready to receive”, “data on line”, “data received” and “stopped sending”, a strobed communication needing a minimum of five bits to work.

Session 1357 can't copy what's on the board and listen to the teacher lecture at the same time. Class dismissed.