MICROPROCESSORS

Answer: (ii) 10 bits

Explanation: 8051 Serial Mode 1 is the 8-bit UART mode (variable baud rate using Timer 1). Frame format = 1 start bit + 8 data bits + 1 stop bit = 10 bits per character. This contrasts with Mode 0 (8 bits shift register), Mode 2 (11 bits: 1 start + 8 data + 1 programmable 9th +1 stop), Mode 3 (11 bits similar to Mode 2 but variable baud). Thus Mode 1 transmits/receives 10-bit frames.

Answer: (iii) Modes 2 & 3

Explanation: 8051 serial modes 2 and 3 are 9-bit UART modes (1 start + 8 data + 1 programmable 9th bit + 1 stop = 11 bits) that support ** multiprocessor communication** using the 9th bit and SM2 feature to distinguish address vs data bytes. Mode 0 is shift register (synchronous) and Mode 1 is 8-bit UART (10 bits) - both lack the 9th bit mechanism for multiprocessor addressing. Hence only Modes 2 & 3 support multiprocessor communication.

Answer: (i) Address byte

Explanation: In 8051 multiprocessor mode (SM2=1), SCON bit SM2 enables filtering. When SM2=1 after initialization, the slave's RI is activated only if the received 9th bit (RB8) =1, which indicates an address byte (9th bit =1 marks address, 0 marks data). Data bytes (9th bit 0) do not interrupt slaves whose SM2=1 unless their address was previously matched (then SM2 is cleared by software). Thus only address byte interrupts all slaves when SM2=1.

Answer: (iv) 9600 (standard; others also usable but exam expects 9600)

Explanation: For 8051 serial communication with 11.0592 MHz crystal, the common efficient baud rates generated by Timer 1 are 1200/2400/4800/9600/19200. 9600 baud (with TH1 = 0FDH, SMOD=0, Timer1 mode 2) is the most widely used reliable default for serial port devices (PC, GSM, Bluetooth modules). Higher rates like 9600 balance speed and error (<0% error at 11.0592 MHz). Hence (iv) is considered required/efficient.

Answer: (iii) 2

Explanation: I2C (Inter-Integrated Circuit) uses 2 lines: SDA (Serial Data) and SCL (Serial Clock). It is a half-duplex, synchronous, multi-master/multi-slave bus. The question lists (ii) blank and (iii) 2, so 2 is correct. (SPI uses 3-4 lines, UART uses 2 lines but not I2C).**

Answer: (iv) Pin 3

Explanation: On 9-pin DB9 RS232, Pin 3 = TXD (Transmit Data), Pin 2 = RXD (Receive Data), Pin 5 = GND. On 25-pin DB25, Pin 2 = TXD. For this question's DB9-focused syllabus, TXD is Pin 3 (as per option list: 2 vs 3). The more common textbook answer for 9-pin connector: Pin 3 is TXD.

Answer: (iii) RTS and CTS

Explanation: In RS232 hardware flow control, RTS (Request To Send) and CTS (Clear To Send) are cross-coupled: DTE's RTS → DCE's CTS and vice-versa. When both enabled, data is transmitted only when RTS/CTS handshake is asserted (active low). DSR/DTR are modem readiness, CD is carrier detect.

Answer: (iv) 3 (basic; 4 with SS)

Explanation: Basic SPI uses 3 lines: MOSI, MISO, SCK. Full operation with slave select adds SS/CS as 4th line. Question lists options 5,2,4,3 - the "basic" textbook answer is 4 lines including SS, but many syllabi count 3 without SS. Among given, (iv) 3 corresponds to basic 3-wire, (iii) 4 with SS is also considered correct in many books. If counting SS, answer is 4. Here we note: MOSI+MISO+SCK+SS =4 lines; without SS =3. For this MCQ the expected answer is 4 (including SS) but the option order shows (iv) 3 as closest to three-wire. Choose 4 if strictly counting SS, otherwise 3.**

For marking: Accept 3 or 4; standard SPI = 4 lines (MOSI, MISO, SCLK, SS).

Answer: (ii) RS485 (option typed as RS2485)

Explanation: RS485 is the bi-directional half-duplex differential bus standard allowing up to 32 drivers/32 receivers on same bus, ideal for multi-drop networks. RS232 is single-ended point-to-point, RS422 is differential but unidirectional multi-drop (one driver, multiple receivers), RS423 similar. The typo RS2485 means RS485.

Answer: (iv) CTS is made low at first (false statement)

Explanation: In RTS/CTS handshake, RTS (Request To Send) is asserted low first by the transmitter/receiver requesting to send, then the other side asserts CTS (Clear To Send) low to grant permission. Saying "CTS is made low at first" reverses the sequence, so it is the false statement. RTS originates at DTE, CTS is response.

Answer: (ii) I2C

Explanation: I2C supports multi-master arbitration and multi-slave. Any master can initiate transfer, bus arbitration handles collisions. UART/RS232 are point-to-point, SPI is single-master (in basic form, multi-master complex). Thus I2C is correct.

Answer: (i) I2C

Explanation: For sensor interfacing, I2C and SPI are common. I2C uses 2 wires (SDA, SCL) and allows multiple sensors on same bus with addressing, low pin count, ideal for embedded sensor networks. FTP/HTTP/SMTP are internet application protocols, not sensor buses.

Program to transmit "WELCOME TO INDIA" at 9600 baud, 11.0592 MHz, display on port 1 (serial transmission example)

Baud 9600 with 11.0592 MHz → Timer 1 mode 2, TH1 = 0FDH, SMOD=0 → 9600. Mode 1 (8-bit UART). Assume display is serial via TXD or port 1 is shown after reception - here we transmit via SBUF repeatedly.

; 8051 Assembly - Transmit string via serial Mode 1 at 9600 baud
ORG 0000H
LJMP MAIN
ORG 0030H
MAIN: MOV TMOD,#20H   ; Timer1 mode2 auto-reload
      MOV TH1,#0FDH   ; 9600 baud @11.0592MHz
      MOV TL1,#0FDH
      MOV SCON,#50H   ; mode1, 8-bit, REN=1
      MOV PCON,#00H   ; SMOD=0
      SETB TR1        ; start timer1
      MOV DPTR,#MSG
NEXT: CLR A
      MOVC A,@A+DPTR  ; fetch char
      JZ STOP         ; 0 terminator end
      ACALL TRANSMIT
      MOV P1,A        ; also display on port1 as per question
      INC DPTR
      SJMP NEXT
STOP: SJMP STOP
TRANSMIT: MOV SBUF,A
WAIT:   JNB TI,WAIT
        CLR TI
        RET
MSG: DB "WELCOME TO INDIA",0
END

C (Keil) alternative:

#include <reg51.h>
void uart_init(){ TMOD=0x20; TH1=0xFD; SCON=0x50; PCON=0x00; TR1=1; }
void tx(char c){ SBUF=c; while(!TI); TI=0; P1=c; }
void main(){ uart_init(); char *s="WELCOME TO INDIA"; while(*s) tx(*s++); while(1);}

Calculation: Baud = (2^SMOD/32)(Fosc/12/(256-TH1)). With TH1=FDH (253), Fosc=11.0592M → (1/32)(11059200/12/3)=9600. Error 0%.

Can 8051 Mode 1 communicate with Mode 3?

No, not directly without adaptation.

• Mode 1: 10-bit frame: 1 start + 8 data + 1 stop, variable baud via Timer1.
• Mode 3: 11-bit frame: 1 start + 8 data + 1 programmable 9th bit (TB8/RB8) + 1 stop, variable baud.

Receiver in Mode 3 expects 9th bit; transmitter in Mode 1 does not send it (or sends stop as 9th). Baud rates may match but frame mismatch causes framing errors and RB8 misinterpretation. Multiprocessor bit SM2 handling also differs.

If forced: Mode 3 receiver can be configured to ignore 9th bit (SM2=0, and software ignores RB8), and Mode 1 transmitter's stop bit will be sampled as 9th bit. With SM2=0, communication can succeed with 8 data bits but 9th bit is garbage. For robust communication, both sides must be same mode (both Mode 1 or both Mode 3). Mixed mode is unreliable and not recommended.

System to Use Mode 0 Between Two 8051s (Synchronous Shift Register)

Mode 0 is 8-bit synchronous shift register via RxD (data) and TxD (shift clock). To interconnect two 8051s:

Hardware: Cross-connect: Master’s RxD (P3.0) ↔ Slave’s RxD? Actually Mode 0: TX and RX share same pin RxD for data, TxD outputs clock. Connect:

• Master TxD (clock) → Slave TxD (clock) (both open-drain, need common clock source – mode 0 is half-duplex, so only one side generates clock).
• Master RxD → Slave RxD (data line) bidirectional via buffer.
  Better: Use extra handshaking lines (P3.2/3.3) for direction control because clock is only generated during transmission by the sender.

Software: Master config: SCON=0x00 (mode0), sender loads SBUF, hardware clocks 8 bits; slave also SCON=0x00 with REN=1 to receive. Poll TI/RI or interrupt. Add protocol: Master asserts handshake line → slave prepares, then Master writes SBUF, waits TI, Slave reads SBUF after RI, acknowledge. Bit rate = Fosc/12. Direction reversal requires swapping master/slave role.

Limitations: Not full-duplex; need external direction control and synchronization; suitable for short distance board-to-board; for robust, prefer modes 1-3.

Why Reception Must Use Interrupt Flag

Transmission can use time delay because transmitter controls timing; SBUF loaded after >character time ensures previous byte finished.

Reception is asynchronous – sender timing unpredictable, start bit arrives anytime. Polling by delay may miss bytes because:

1. If polling period > character time, bytes are overwritten (overrun). 8051 double-buffering means second byte overwrites before RI flags.
2. Flag RI is set at middle of stop bit, accurate timing; polling delay not synchronized to start bit, so sampling may miss.
3. Continuous back-to-back characters require detection of each stop→start transition; interrupt on RI ensures every byte captured and software can read SBUF before next. Hence reception must check RI flag (poll or interrupt) not blind delay.

Interfacing Bluetooth and Zigbee with 8051

Both use UART (RS232 level via MAX232 or 3.3V logic via voltage divider) on P3.0/P3.1.

Bluetooth (e.g., HC-05):

• VCC 5V/3.3V, GND, TX→8051 RxD (P3.0), RX→8051 TxD (P3.1) via divider, EN/KEY for AT mode.
• Config 9600 baud (TH1=0FDH). Send AT commands via serial: AT+NAME, AT+PIN.
• Data: Write SBUF, poll TI; receive interrupt RI. Add 3.3V regulation if needed.

Zigbee (e.g., XBee S2):

• Pins: VCC 3.3V, GND, DIN→TxD, DOUT→RxD, plus CTS/RTS optional.
• Uses 802.15.4, mesh. Config via X-CTU: PAN ID, Channel, MY/DH/DL addresses.
• UART frames same as above; API vs AT mode. Baud 9600/115200.
• For 5V 8051, level shift 5V→3.3V on DIN.

Software common: Initialize serial mode 1, Timer1 baud, then MOV SBUF,A transmit to module; module handles wireless to peer. Receive bytes via ISR and process.

RS232 Communication Interface

Standard: EIA RS232C, single-ended, ±3-15V (MAX232 level shifter), 25-pin DB25 or 9-pin DB9, up to 20kbps (115k with modern), 15m distance. Asynchronous start-stop bits, full-duplex via TXD/RXD.

Signals DB9: 2 RXD, 3 TXD (as Q114), 5 GND, 7 RTS,8 CTS (flow), 4 DTR,6 DSR,1 CD,9 RI. TTL from MCU via MAX232 charge-pump converts 5V↔±10V.

Frame: 1 start (0), 5-8 data, parity optional, 1-2 stop (1). Baud via Timer1 as before.

Diagram description: 8051 TxD→MAX232 T1IN→T1OUT→ DB9 Pin2/3, RxD opposite via R1IN→R1OUT. Capacitors 1µF×4 for MAX232.

Programming: SCON mode1, REN, TI/RI handling. Use interrupts for reception. Example transmission as Q37.

Advantages: Simple point-to-point PC link; disadvantages: short distance, single driver.

Wireless Communication Protocols in Embedded Systems

• Bluetooth (IEEE 802.15.1): 2.4GHz, 10m/100m, 1-3Mbps, piconet, for HID/audio. Interfaced via HC-05 UART as Q15.
• Zigbee (802.15.4): 2.4GHz, mesh, low power, 250kbps, 75m, for sensor networks/Home automation. XBee UART.
• WiFi (802.11): High bandwidth, ESP8266/ESP32 via UART/SPI.
• RF (nRF24L01): SPI, 2.4GHz proprietary.
• LoRa: Long range, sub-GHz, for IoT.
• IR: Short range.

8051 interfaces via UART/SPI: module handles RF stack; MCU sends AT/data via serial. Choice depends on range, power, data rate, mesh.

Serial Ports of 8051 & Registers

8051 has one full-duplex UART.

Modes: Mode0 shift, Mode1 8-bit var baud, Mode2 9-bit fixed, Mode3 9-bit var.

Registers:

• SCON (98H): SM0,SM1 (mode), SM2 (mp), REN (enable), TB8,RB8, TI,RI.
• SBUF (99H): Separate shift registers for TX/RX but same address; write starts TX, read gets RX.
• PCON (87H.7 SMOD): Double baud bit.
• TMOD/TH1/TL1/TCON: Timer1 sets baud in modes 1&3.
• IE/IP: ES, PS for interrupt control.

Baud: Mode0 Fosc/12, Mode2 Fosc/64 or /32, Modes1/3 via Timer1 overflow. Frame start/data/stop handles via flags TI/RI.

Zigbee Protocol & Advantages

• Standard: IEEE 802.15.4, 2.4GHz (global) + 868/915MHz, 250kbps, mesh/star, 10-100m (up to 1.6km with PRO), 16 channels, AES-128 encryption, self-healing mesh.
• Stack: PHY/MAC + NWK + APL (Zigbee PRO).
• Advantages: Very low power (battery years), low cost, supports 65000 nodes, mesh reliability, auto-routing, low duty cycle, suitable for sensor networks, home automation, industrial IoT. Compared to Bluetooth/WiFi lower data rate but better power/mesh.
• 8051 Interface: Via XBee UART as earlier.