Data Acquisition

the control register of the 8255. The 8255 then operates in the specified mode until it isreset or new configuration information is written to the control register. The format of thisregister is shown below, followed by the format of the data registers and description onhow to use the chip in the different modes.C.1 DIO0CTRL – control register of the 8255This register has two functions: to set the operating modes of the three ports in the chip,and to set and reset individual bits in port C. The function and bit names of the registertherefore depend on the setting of bit 7. The layout of the register is shown below.Figure C.2DIO0CTRL 8255 control register layoutBit 7, function select: If this bit is set when the register is written to, then the register isin configuration mode. If the bit is 0, then the register is in bit set/reset mode (see below).The functions of the remaining bits are described below; the functions depend on thesetting of bit 7.This allows the mode to be set for each of the two groups of the 8255 and the direction setfor the individual ports. Note that the direction of port C is independently programmablein two 4-line nibbles.Bits 6–5, group A mode select: These two bits set the mode of the group A ports. Theseare port A and the upper four lines of port C. The bit combinations are as follows: