DATA TRANSFER SCHEMES OF 8085

1. 8051 Microcontroller Data Transfer
Schemes
PREPAREDBY
B.SARAVANAMANIKANDAN
ASSISTANTPROFESSOR
Kongunaducollegeof engineeringandtechnology

2. SERIAL INTERFACE
• Data transfer between memory and register, register and port and
port to outside unit in these data transfers, a byte is transferred
between two units.
• Thus 8 bits are transferred in parallel.
• This type of data transfer is mostly used for short distance, data
transfer.
• When data becomes more than few meters the parallel data transfer
suffers from phenomenon called bit skewing.
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3. ASYCHRONOUS and SYNCHRONOUS
• In synchronous communication the transmission from one device
and receipt at other devices is synchronized by a common clock,
thus as one bit/byte is sent out from A to B.
• Another variation of synchronous communication is when two units
are quite apart, they are driven by individual clocks but these clocks
are synchronized.
• In Asynchronous Communication the transmission and reception
are not synchronized.
• A device can send its own and other device may receive it. In this
case byte is enveloped in a data frame will have
Start bit
stop bit
parity bit
• The baud rate data transmission speed in terms per second also
must be predefined.
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4. • Serial data transfer in bit from received to parallel data in byte form
for storage and processing.
• Parallel data in memory to serial data ready to be transmitted
• Such devices working in asynchronous mode are called USART.
• The 8051 has on chip for serial communication.
• RXD
• TXD
Serial Communication Modes
• MODE 0
• MODE 1
• MODE 2
• MODE 3
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6. MODE 0:
• Data is transmitted as well as received through the RxD pin.
• The shift clock is supplied through the TxD pin.
• 8 bits of data are transmitted and received.
• The LSB is transmitted first and similarly while receiving.
• The baud rate is 1/12 of a oscillator frequency.
• Transmit Interrupt flag (TI) is set when data bytes are transmitted and
Receive Interrupt(RI) flag is set after receiving bits.
• The TI and RI flags are noting but SCON register. This mode
corresponds to SM0=0 and SM1=0.
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7. MODE 1:
• Data is sent through the TxD pin and received through the RxD pin. A
10 bit frame containing 8 bit data (SM0=0), a start bit (=0) and a stop
bit (=1) are used.
• Thus there is a clear demarcation between end of one data frame
and start of another data frame, once the stop bit is received.
• The Stop bit is loaded into a bit cell RB8 in the SCON register.
• As mode 0 here also an interrupt is generated once all the bit frames
are received or transmitted.
• Baud rate varies here the baud rate varied by changing preset count
in the Timer1.
• The start bit and stop bit are automatically added by the hardware by
transmitting data.
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8. MODE 2:
• 11 bits are transmitted or received. This 11 bit frame is specified as:
1 bit for start
8 bits for data
1 bit can be programmed
1 bit for stop bit
• The 9 data bit programmable. This bit TB8 bit in SCON register, this is effectively
used while transmitting data.
• With little software overhead, this bits can be used to send the parity of the byte
that is to be transmitted.
• Transmitting:
If we want to use 9 th bit as a parity we must load the TB8 with the parity of
the byte that is to be transmitted. ACC has the byte to be transmitted.
• Receiving:
on reception of frame the 9 th bit goes to RB8 of the SCON register. The stop
bit is ignored the baud rate in this mode is programmable either 1/32 or 1/64 of the
oscillator frequency
By making SMOD = 0 baud rate can be set to 1/64 of oscillator frequency, SMOD =
1 baud rate can be set to 1/32 of oscillator frequency. SMOD is 7 th bit PCON after
SFR.
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9. MODE 3:
• This mode is same as the mode 2 expect for baud rate.
• Here the baud rate is variable as in case of mode 1.
• The baud rate depends on overflow rate of timer 1.
• By changing preset count in Timer 1 baud rate can be varied.
Baud rate calculation = Timer 1 overflow rate
n
Where n is an integer and its value is either 32 or 16
If SMOD = 1 then n =16 else n = 32
Overflow rate = rate at which the counting in done /256-TH1
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11. PCON
• Power control register through which certain power control functions
in CMOS version of the 8051 implemented.
• In the HMOS version all the bits of PCON except 7 bit are dummy.
• Bit 7 is SMOD and is used in both CMOS and HMOS versions to
double baud rate in mode 1,2 and 3.
• PCON is not bit addressable.
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