Intel 8051 manual Serial Frame, Transmission Rate Generation, Uart Error Conditions

8051 Architectural Specification and Functional Description

1.The hardware In each slave'sserial port begins by listening for an addre88. Receipt of an addre88 frame will force an Interrupt If the slave'sSM2 bit Is set to one (1) to enable "Interrupt on addre88 frame only".

2.The master then transmits a frame con- taining the 8-blt address of the slave that Is to receive the subsequent commands and data. A transmitted addre88 frame has Its ninth data bit (TB8) set equal to one (1).

3.When the addre88 frame Is received, each s'ave'sserial port Interrupts its CPU. The CPU then compares the addre88 sent to Its own.

4.The 8051 stave which has been addressed then resets Its SM2 bit to zero (0) to receive all subsequent transmissions. All other 8051'sleave their SM2 bits at a one (1) to Ignore transmissions until a new address arrives.

5.The master device then sends controllnfor- matlon and data, which In tum Is accepted by the previously addressed 8051 [i.e., the one that had set Its SM2 bit to zero (0)].

Figure 2.50. Protocol for Multl- Processor

Communications

2.12.3 The Serial Frame

A frame is a string of bits. The frame transmitted and received in Mode 0 is 8 bits in length. The data bits of the frame are transmitted SBUF.O first and SBUF.7 last.

The frame transmitted and received in Mode 1 is ten bits in length. The frame transmitted and received in Modes 2 and 3 is eleven bits in length. These frames consist of one start bit, eight or nine data bits and a stop bit. Data bits 0- 7 are loaded into SBUF.O-SBUF.7 respectively, and data bit 8 into RB8 (receive) or TB8 (transmit). With nominal software overhead, the last data bit can be made a parity· bit, as shown in Figure 2.51.

110BAUDTTV

ISTART IDATA .01 .1 1·21 .31 .41 .51 .61 PARITY ISTOP ISTOP I

TYPICAL ASCII TERMINAL

ISTART IDATA .01 .1 1·21 .31 .41 .51 .61 PARITY 1 STOP I

OR

ISTART 1DATA.O 1 .1 I.21 .31 .41 .51 .6 I.7 1PARITY j STOP I

OR

ISTAAT IDATA.oj .1 1.21.31.41.51.&1.71.81 STOP I

Figur.e 2.52. Typical Frame Formats

2.12.4 Transmission Rate Generation

The proper timing for the serial I/O data is provided by a transmission-rate generator. On-board the 8051, three different methods of transmission rate generation are provided. The transmission-rate achievable is dependent upon the operating mode of the serial port.

In the I/O expansion mode (Mode 0) the oscillator frequency is simply divdied by 12 to generate the transmission rate. This produces a transmission rate of 1M bits per second at 12 MHz. If Modes lor 3 are being used, the transmission rate can be generated from the oscillator frequency or from an external reference fre- quency. In these modes, either one-twelfth the oscillator

frequency or the TI input frequency is divided by 256- minus-the-value-in-TH I (counter I must be configured in auto-reload mode by software) and then divided by 32 to generate the transmission rate. When the oscillator frequency input (rather than TI) is selected, this method produces a transmission rate of 122 to 31,250 bits per second (including start and stop bits) at 12 MHz. The TI external input is selected by setting the C;Tbit to one (1). When Mode 2 is used, the oscillator frequency is simply divided by 64 to generate the transmission rate. This produces a transmission rate of 187,500 bits per second (including start and stop bits) at 12 MHz.

MOV C, P ; Parity moved to carry (byte already in A).

MOV TB8, C ; Put carry into Transmitter Bit 8 MOV SBUF, A; Load Transmit Register

Figure 2.51. Generating Parity and

Transmitting Frame

Figure 2.52 shows some typical frame formats for different applications. The data bits of the frame are transmitted least significant bit first (SBUF.O) and TB8 last.

2.12.5 UART Error Conditions

There are two UART error conditions that should be accounted for when designing systems that use the serial channel.

First, the 805I's serial channel provides no indication that a valid stop bit has been received. However, since a start bit is detected as a high-level to low-level transition, the UART will not receive additional frames if a stop bit is not received.

Second, the RI flag is set and SBUF and RB8 are loaded from the receiver's input shift register when the received

AFN-01488A-36

32