generated, SPA bit is set in register ICRA. After 8 CNTB pulses , the data in the shift register is dumped into the SERIALB Data Register (SDRB) and an interrupt is generated, SPB bit is set in register ICRB.

In the output mode, TIMER A is used for the baud rate generator of serial port A, Timer C for serial port B. Data is shifted on an SP pin at half the underflow rate of the TIMER used. The maximum baud rate possible is ClMHz divided by four, but the maximum useable baud rate will be determined by line loading and the speed at which the receiver responds to input data. Transmission will start following a write to Serial Data Register (provided the proper TIMER used is running and in continuous mode). The clock signal derived from TIMER A would appear as an output on the CNTA pin; the one from TIMER C would appear otn the CNTB pin. The data in the Serial Data Register will be loaded into its corresponding shift register then shift out to the SPA or SPB pin when a CNTA or CNTB pulse occurs, respectively. Data shifted out becomes valid on the falling edge of its CNT clock and remains valid until the next falling edge. After 6 CNT pulses, an interrupt is generated to indicate more data can be-sent. If the Serial Data Register was loaded with new information prior to this interrupt, the new data will automatically be loaded into the shift register and transmission will continue. If the microprocessor stays one byte ahead of the shift register, transmission will be continuous. If no further data is to be transmitted, after the 8th CNT pulse, CNT will return high and SP will remain at the level of the last data bit transmitted. SDR data is shifted out MSB first and serial input data should also appear on this format.

The bidirectional capability of each of the Serial Ports and CNT clocks allows many 4510 to be connected to a common serial communication bus on which one Serial Port would act as a master, sourcing data and shift clock, while the other Serial Port (and all other ports from other 4510 devices) would3ect as slaves. All the CNT and SP outputs are open drain to allow such a common bus. Protocol for master/slave selection can be transmitted over the serial bus, or via dedicated handshaking lines.