The coder (Figure 9) scans two of the micro-controller's output ports (namely Port B and Port C as shown later) for the address and data bits and sends to the serial output a frame containing these bits packed with synchronization bit blocks.
Figure: The coder.
All the circuits required to perform this task (oscillator, divider, transmission gate circuit, counter, trinary detector, data selector, buffer and synchronization circuit) are contained in the HT648 (Holtek) CMOS integrated circuit. It has the same features as the matching decoder HT640 regarding power consumption, operating voltage and noise immunity. Its built-in oscillator is set to perform at the same frequency as the decoder's one in order to get the synchronization required by a valid data transmission. Upon receipt of a transmission enable command, the coder begins a 3 word transmission cycle. This cycle is repeated as long as the transmission enable input is held high. After the transmission enable falls low, the coder output completes its final cycle and then stops as shown in Figure 10.
Figure: The coder transmission cycle.
Figure: An information word is composed of 4 periods.
Figure: The coder activity flowchart.
An information word is composed of 4 periods as shown in Figure 11 and the coder activity flowchart is presented in Figure 12.