Sonic Alert msm85c154hvs, msm83c154s Mode Outline, Mode 2 baud rate, Mode 2 transmit operation

INTERNAL SPECIFICATIONS

4.6.3.3Mode 2

4.6.3.3.1 Outline

Mode 2 is an 11-bit frame UART mode (with one start bit, eight data bits, one multipurpose data bit, and one stop bit) where the baud rate is 1/64th or 1/32nd of the fundamental oscillator (XTAL1·2) frequency.

A block diagram of the serial port in mode 2 is shown in Figure 4-33, and the operational timing chart is given in Figure 4-34.

4.6.3.3.2 Mode 2 baud rate

Since the fundamental oscillator frequency divided by two serves as the baud rate clock source in mode 2, the baud rate is determined by the SMOD value according to the following equations.

B = fOSC ⋅	1	⋅	1	⋅	1	(SMOD=0)
			2		16
2
B = fOSC ⋅	1	⋅	1			(SMOD=1)
			16
2

where B is the baud rate and fOSC the fundamental oscillator (XTAL1·2) frequency.

4.6.3.3.3 Mode 2 transmit operation

The transmit basic clock (TXCLOCK in Figure 4-34) is obtained from a hexadecimal free-run counter overflow where the frequency of 1/2XTAL1·2 (fundamental oscillator frequency divided by 2) divided by 2 (when SMOD=0) or the 1/2XTAL1·2 frequency (when SMOD=1) 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 transmitted sequentially from the TXD synchronized with the basic clock.

As soon as the TB8 output has been completed, the transmit circuit is initialized, and the T1 flag is set at the first M1·S3 after the completion of that output.

4.6.3.3.4 Mode 2 receive operation

The receive circuit timing is generated by a hexadecimal counter overflow where the frequency of 1/2XTAL1·2 (fundamental oscillator frequency divided by 2) divided by 2 (when SMOD=0) or the 1/2XTAL1·2 frequency (when SMOD=1) is used as the clock, and the input data received from the RXD is bit synchronized. That is, at the same time that reception is started following input of the start bit, the hexadecimal counter commences to count up, and with one complete round of the hexadecimal counter corresponding to one bit of received data, reception is continued by the receive circuit. Therefore, the reception data baud rate must be equal to the period of a single round of the hexadecimal counter.

The RXD change from “1” to “0” is regarded as the beginning of the start bit where reception is commenced.

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