Data Bus Buffer D0-D7 : 8-bit data bus used to read or write status, command word or data from or to the A 2. RD : When signal goes low, the MPU either reads a status from the status register or accepts data from data buffer. CLK : Clock input, usually connected to the system clock for communication with the microprocessor. Mode word : Specifies the general characteristics of operation such as baud, parity, number of bits etc.

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The pin diagram of is as shown below: 2. Draw the functional block diagram of The functional block diagram of is shown below: 3. How many different sections does have? How the is programmed? This bit register is divided into two bytes—the first byte corresponds to Mode Instruction Format while the second byte corresponds to Command Instruction Format.

The content of control word register determines synchronous or asynchronous operation, Baud rate, number of bits per character, number of stop bits, nature of parity, etc. What is the function of the Status Word Register of ? The function of the status word register is to check or examine the preparedness of with regard to transmission or reception of data. It contains three buffer registers: z data buffer register z status register. The particular is selected on CS signal going low.

This pin is usually connected to a decoded address bus. When this pin is high, either the control register or status register is selected and when low, data bus buffer is selected. The control register and the status register are distinguished by WR and RD signals, respectively. The figure below shows how the three registers: data buffer register, control register and status register are accessed by making respectively.

Explain the operation of the transmitter section of The transmitter section consists of three blocks—transmitter buffer register, output register and the transmitter control logic block. In the output register, the eight bit data is converted into serial form and comes out via TXD pin. But this happens only if transmitter is enabled and the CTS is low. This clock frequency can be 1, 16 or 64 times the baud.

Explain the operation of the receiver section of The receiver section consists of three blocks — receiver buffer register, input register and the receiver control logic block.

Serial data from outside world is delivered to the input register via RXD line, which is subsequently put into parallel form and placed in the receiver buffer register. This line is then used either to interrupt the MPU or to indicate its own status. MPU then accepts the data from the register. RXC line stands for receiver clock. This clock signal controls the rate at which bits are received by the input register.

The clock can be set to 1, 16 or 64 times the baud in the asynchronous mode. How does the CPU know that the transmitter buffer is empty? How the transmitter is enabled? The transmitter is enabled by setting bit D0 of the command instruction word. What is the status of the start bit on the RXD line for in asynchronous mode only and how does it differentiate between a valid start pulse and transient pulse?

For any data to be received by the receiver, it first checks for a valid start bit which is zero. What happens when a parity error or a framing error occurs in the received data bits in asynchronous mode only? The Parity error and Framing error status bits in the status word are set if there is a parity error or if the stop bit is absent at the end of the received bits respectively.

When the RXRdy line goes high in asynchronous and synchronous mode of operation? In the asynchronous mode, RXRdy line goes high z if the receiver is in the enabled condition this is made so by setting D2 bit of the Command Instruction Word. Whereas in the synchronous mode RXRdy line goes high z if the receiver is enabled.

What is overrun error? If the CPU cannot read the data from the receiver buffer register this happens if the CPU fails to respond to RXRdy line , then on receipt of the next character, the previous data will be written over and the earlier character will be lost. When such is the case D4 bit of the status word is set. This is pin 16 of and stands for sync. This pin is used for detection of SYNC characters in synchronous mode and Break characters in asynchronous mode.

When is programmed to receive two synchronous characters, this output pin goes high at the mid point of the last bit of the second synchronous character. In the input mode—called the external synchronous detect mode—a rising edge on this pin causes to start collecting data characters on the rising edge of the next RXC.

This input signal can be removed once synchronisation is achieved. When external synchronisation is done, the internal synchronisation is disabled. Why modems modulators-demodulators are used in case of digital transmission of data? In a communication environment, two modems are used—one at the transmitting end side and one at the receiving end side. High frequency digital signals require a very wide transmission channel bandwidth which makes the system very costly. However, existing telephone line facilities which carry analog signals in the range of 40 Hz to 4 KHz can be used to transmit such high frequency digital signals.

A modem converts a digital signal into audio tone frequencies at the transmitting end side and reconverts this audio frequencies into h.

The inverse operation is done at the receiving end side. The block diagram is shown below: Digital data is delivered at the DTE may be in parallel form, which is then converted into serial form and sent to DCE via RS cable. The DCE a modem output is an audio signal carried through a telephone line. At the receiving end side, the opposite process is carried out to retrieve the original data. Draw the data loading sequence and explain the same.

The data loading sequence of is shown below: The mode control is specified first, which indicates the general operating conditions. This is followed by loading the command instruction format.

It is followed by command instruction and data in that order which is repeated all over again. These two bits determine the baud rate factor. Bits D2 D3 determine the character length which may be 5 to 8 bits in length -depending on the content of the se two bits. Bits D6 and D7 determine the number of stop bits. There can be 1, 1 y.

Then the data bits are transmitted, followed by stop bit s. The data bits start with LSB of the serial output register. All these bits start bit, data bits, stop bit s are shifted out on the falling edge of TXC transmitter clock. In case when no data is transmitted, TXD output remains high.

The receive format is identical to transmit format. Data reception starts with RXD receive data pin line going low—it indicates the arrival of start bit. This circuit then samples the RXD line half-a-bit time later to ensure the presence of a genuine start bit. If this sampling results in a low on RXD line, it indicates a valid start bit. The bit counter is started on the second sampling—hence each subsequent data bit is sampled at the middle of each bit period.

The bit counter thus samples the data bits, parity bit and lastly the stop bit. The receiver needs only one stop bit—but the transmitter is affected by the number of stop bits. For any error during receiving of data with regard to Parity, Framing or Overrun—the corresponding flags in the status word are set.

This is done to avoid any possibility of a false start bit detection due to a transient noise pulse. Bits D2 D3 indicates the character length. In the transmission format, either one or two synchronous characters are sent, followed by data characters.

The number of synchronous characters i. The characters are shifted out of the serial output register on the falling edge of the TXC transmitter clock , and at the same rate as the TX C. Once transmission commences, it is the duty of the CPU to replenish the transmitter buffer register in response to TXRdy. In such a case, TXE Transmitter Empty pin becomes high to indicate that the transmitter buffer is empty. The receiver buffer register content is compared at every bit boundary with the SYNC character previously loaded till a match occurs.

This is resetted once status read is over. The SYNDET pin gets set in the middle of the parity bit if the parity is enabled; otherwise in the middle of the last data bit. Show the Command Instruction Format and explain the same.

The Command Instruction Format is shown below: The command instruction format controls the functioning of What happens when a power is switched on b the system is resetted?

On powering on the system, either enters into SYNC or command instruction format. On resetting the system, returns to the mode instruction format from the command instruction format. Draw the status word format and explain the same. The CPU, for its proper operation, needs various informations. These are provided by the status word.

It should be borne in mind that the status word is continuously updated by , but not while the CPU reads it. What are the modem control pins associated with ? Describe the functioning of these pins. Out of these, the first and third are input pins input to and the rest two are output pins. All these pins are active low.


Universal Asynchronous Receiver Transmitter

It takes data serially from peripheral outside devices and converts into parallel data. After converting the data into parallel form, it transmits it to the CPU. Similarly, it receives parallel data from microprocessor and converts it into serial form. After converting data into serial form, it transmits it to outside device peripheral. The data transmission is possible between and CPU by the data bus buffer block. It controls the overall working by selecting the operation to be done. The operation selection depends upon input signals as: In this way, this unit selects one of the three registers- data buffer register, control register, status register.


Microprocessor | 8251 USART

A few additional control lines are provided for modem-control and efficient handshaking or interrupts. The bus-interface consists of the bidirectional 8-bit data-bus lines D Four additional lines, provide modem-control capabilities. The nCTS input signal also directly controls the transmitter of the chip.

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