MSM80C154S/83C154S/85C154HVS120B = fOSC × 65536-DRCAP21 × 16121 ×where B is the baud rate, fOSC the fundamental oscillator (XTAL1·2) frequency, and DRCAP2the contents of R CAP2L and RCAP2H (expressed in decimal).4.6.3.4.3 Mode 3 transmit operationThe transmit basic clock (TXCLOCK in Figure 4-36) is obtained from a hexadecimal free-runcounter overflow where timer/counter 1 or timer/counter 2 overflow is used as the clock.Transmission is commenced when transmit data is written in SBUF.The start bit, the eight SBUF data bits (with the LSB first), TB8, and the stop bit are transmittedsequentially from the TXD synchronized with the basic clock.As soon as the TB8 output has been completed, the transmit circuit is initialized, and the T1flag is set at the first M1·S3 after the completion of that output.4.6.3.4.4 Mode 3 receive operationThe receive circuit timing is generated by a hexadecimal counter overflow where timer/counter 1 or timer/counter 2 overflow is used as the clock, and the input data received fromthe RXD is bit synchronized. That is, at the same time that reception is started following inputof the start bit, the hexadecimal counter commences to count up, and with one completeround of the hexadecimal counter corresponding to one bit of received data, reception iscontinued by the receive circuit. Therefore, timer/counter 1 must be set so that the period ofa single round of the hexadecimal counter is equal to the reception data baud rate.The RXD change from “1” to “0” is regarded as the beginning of the start bit where receptionis commenced.When this “1” to “0” RXD change is detected, the hexadecimal counter which had beenstopped in reset status commences to count up. When the hexadecimal counter is in state7, 8, and 9, the start bit is sampled, and is accepted as valid if at least two of the three sampledvalues are “0”, thereby enabling data reception to continue. If two or three of the sampledvalues are “1”, the start bit becomes invalid, and the receive circuit is initialized when thehexadecimal counter reaches state 10.The reception data is sampled when the hexadecimal counter is in state 7, 8, and 9, and themore common value of the three sampled values is read sequentially as data into the inputshift register.If the hexadecimal counter is in state 10 during the period of the next bit (that is, the multi-purpose data bit) after the eight bits of data have been received, and if the conditions statedbelow are satisfied, the input shift register data (the LSB being read first) is loaded into SBUF,and the sampled multi-purpose data bit is read into RB8. And when the hexadecimal counteris in state 10 during the period of the next after that (that is, the stop bit) the receive circuitis initialized.The RI flag is set at the first M1·S3 after that.Conditions: (1) RI=“0”(2) SM2=“0”, or SM2=“1” and sampled multi-purpose data bit=“1”
