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PXAG49KBBD Datasheet(PDF) 26 Page - NXP Semiconductors |
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PXAG49KBBD Datasheet(HTML) 26 Page - NXP Semiconductors |
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26 / 42 page  Philips Semiconductors Preliminary data XA-G49 XA 16-bit microcontroller family 64K Flash/2K RAM, watchdog, 2 UARTs 2001 Jun 27 26 Serial Port Control Register The serial port control and status register is the Special Function Register SnCON, shown in Figure 16. This register contains not only the mode selection bits, but also the 9th data bit for transmit and receive (TB8_n and RB8_n), and the serial port interrupt bits (TI_n and RI_n). TI Flag In order to allow easy use of the double buffered UART transmitter feature, the TI_n flag is set by the UART hardware under two conditions. The first condition is the completion of any byte transmission. This occurs at the end of the stop bit in modes 1, 2, or 3, or at the end of the eighth data bit in mode 0. The second condition is when SnBUF is written while the UART transmitter is idle. In this case, the TI_n flag is set in order to indicate that the second UART transmitter buffer is still available. Typically, UART transmitters generate one interrupt per byte transmitted. In the case of the XA UART, one additional interrupt is generated as defined by the stated conditions for setting the TI_n flag. This additional interrupt does not occur if double buffering is bypassed as explained below. Note that if a character oriented approach is used to transmit data through the UART, there could be a second interrupt for each character transmitted, depending on the timing of the writes to SBUF. For this reason, it is generally better to bypass double buffering when the UART transmitter is used in character oriented mode. This is also true if the UART is polled rather than interrupt driven, and when transmission is character oriented rather than message or string oriented. The interrupt occurs at the end of the last byte transmitted when the UART becomes idle. Among other things, this allows a program to determine when a message has been transmitted completely. The interrupt service routine should handle this additional interrupt. The recommended method of using the double buffering in the application program is to have the interrupt service routine handle a single byte for each interrupt occurrence. In this manner the program essentially does not require any special considerations for double buffering. Unless higher priority interrupts cause delays in the servicing of the UART transmitter interrupt, the double buffering will result in transmitted bytes being tightly packed with no intervening gaps. 9-bit Mode Please note that the ninth data bit (TB8) is not double buffered. Care must be taken to insure that the TB8 bit contains the intended data at the point where it is transmitted. Double buffering of the UART transmitter may be bypassed as a simple means of synchronizing TB8 to the rest of the data stream. Bypassing Double Buffering The UART transmitter may be used as if it is single buffered. The recommended UART transmitter interrupt service routine (ISR) technique to bypass double buffering first clears the TI_n flag upon entry into the ISR, as in standard practice. This clears the interrupt that activated the ISR. Secondly, the TI_n flag is cleared immediately following each write to SnBUF. This clears the interrupt flag that would otherwise direct the program to write to the second transmitter buffer. If there is any possibility that a higher priority interrupt might become active between the write to SnBUF and the clearing of the TI_n flag, the interrupt system may have to be temporarily disabled during that sequence by clearing, then setting the EA bit in the IEL register. |
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