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I/O Port Configuration in 8051

Each port of 8051 has bidirectional capability. Port 0 is called 'true bidirectional port' as it floats (tristated) when configured as input. Port-1, 2, 3 are called 'quasi bidirectional port'. Port-0 Pin Structure

I/O Port Configuration


Each port of 8051 has bidirectional capability. Port 0 is called 'true bidirectional port' as it floats (tristated) when configured as input. Port-1, 2, 3 are called 'quasi bidirectional port'. Port-0 Pin Structure


Port -0 has 8 pins (P0.0-P0.7).

The structure of a Port-0 pin is shown in fig 4.10.


Port-0 can be configured as a normal bidirectional I/O port or it can be used for address/data interfacing for accessing external memory. When control is '1', the port is used for address/data interfacing. When the control is '0', the port can be used as a normal bidirectional I/O port.


Let us assume that control is '0'. When the port is used as an input port, '1' is written to the latch. In this situation both the output MOSFETs are 'off'. Hence the output pin floats. This high impedance pin can be pulled up or low by an external source. When the port is used as an output port, a '1' written to the latch again turns 'off' both the output MOSFETs and causes the output pin to float. An external pull-up is required to output a '1'. But when '0' is written to the latch, the pin is pulled down by the lower MOSFET. Hence the output becomes zero.


When the control is '1', address/data bus controls the output driver MOSFETs. If the address/data bus (internal) is '0', the upper MOSFET is 'off' and the lower MOSFET is 'on'. The output becomes '0'. If the address/data bus is '1', the upper transistor is 'on' and the lower transistor is 'off'. Hence the output is '1'. Hence for normal address/data interfacing (for external memory access) no pull-up resistors are required.


Port-0 latch is written to with 1's when used for external memory access. Port-1 Pin Structure


Port-1 has 8 pins (P1.1-P1.7) .The structure of a port-1 pin is shown in fig 4.11.


Port-1 does not have any alternate function i.e. it is dedicated solely for I/O interfacing. When used as output port, the pin is pulled up or down through internal pull-up. To use port-1 as input port, '1' has to be written to the latch. In this input mode when '1' is written to the pin by the external device then it read fine. But when '0' is written to the pin by the external device then the external source must sink current due to internal pull-up. If the external device is not able to sink the current the pin voltage may rise, leading to a possible wrong reading.

PORT 2 Pin Structure


Port-2 has 8-pins (P2.0-P2.7) . The structure of a port-2 pin is shown in fig 4.12.

Port-2 is used for higher external address byte or a normal input/output port. The I/O operation is similar to Port-1. Port-2 latch remains stable when Port-2 pin are used for external memory access. Here again due to internal pull-up there is limited current driving capability.


PORT 3 Pin Structure


Port-3 has 8 pin (P3.0-P3.7) . Port-3 pins have alternate functions. The structure of a port-3 pin is shown in fig 4.13.

Each pin of Port-3 can be individually programmed for I/O operation or for alternate function. The alternate function can be activated only if the corresponding latch has been written to '1'. To use the port as input port, '1' should be written to the latch. This port also has internal pull-up and limited current driving capability.


Alternate functions of Port-3 pins are –



1.     Port 1, 2, 3 each can drive 4 LS TTL inputs.


2.     Port-0 can drive 8 LS TTL inputs in address /data mode. For digital output port, it needs external pull-up resistors.


3.     Ports-1,2and 3 pins can also be driven by open-collector or open-drain outputs.


4.     Each Port 3 bit can be configured either as a normal I/O or as a special function bit.

 Reading a port (port-pins) versus reading a latch


There is a subtle difference between reading a latch and reading the output port pin.


The status of the output port pin is sometimes dependant on the connected load. For instance if a port is configured as an output port and a '1' is written to the latch, the output pin should also show '1'. If the output is used to drive the base of a transistor, the transistor turns 'on'.


If the port pin is read, the value will be '0' which is corresponding to the base-emitter voltage of the transistor.


Reading a latch: Usually the instructions that read the latch, read a value, possibly change it, and then rewrite it to the latch. These are called "read-modify-write" instructions. Examples of a few instructions are-


ORL P2, A; P2 <-- P2 or A


MOV P2.1, C; Move carry bit to PX.Y bit.


In this the latch value of P2 is read, is modified such that P2.1 is the same as Carry and is then written back to P2 latch.


Reading a Pin: Examples of a few instructions that read port pin, are-


MOV A, P0 ; Move port-0 pin values to A


MOV A, P1; Move port-1 pin values to A


Accessing external memory

P3.7 and P3.6).


For external program memory, always 16 bit address is used. For example - MOVC A, @ A+DPTR




Access to external data memory can be either 8-bit address or 16-bit address - 8-bit address- MOVX A, @Rp where Rp is either R0 or R1



16 bit address- MOVX  A,@DPTR



The external memory access in 8051 can be shown by a schematic diagram as given in fig 4.14.

If an 8-bit external address is used for data memory (i.e. MOVX @Rp) then the content of Port-2 SFR remains at Port-2 pins throughout the external memory cycle. This facilitates memory paging as the upper 8 bit address remains fixed.


During any access to external memory, the CPU writes FFH to Port-0 latch (SFR). If the user writes to Port-0 during an external memory fetch, the incoming byte is corrupted. External program memory is accessed under the following condition.


Some typical use of code/program memory access:


External program memory can be not only used to store the code, but also for lookup table of various functions required for a particular application. Mathematical functions such as Sine, Square root, Exponential, etc. can be stored in the program memory (Internal or eternal) and these functions can be accessed using MOVC instruction.

Timers / Counters


8051 has two 16-bit programmable UP timers/counters. They can be configured to operate either as timers or as event counters. The names of the two counters are T0 and T1 respectively. The timer content is available in four 8-bit special function registers, viz, TL0,TH0,TL1 and TH1 respectively.


In the "timer" function mode, the counter is incremented in every machine cycle. Thus, one can think of it as counting machine cycles. Hence the clock rate is 1/12 th of the oscillator frequency.


In the "counter" function mode, the register is incremented in response to a 1 to 0 transition at its corresponding external input pin (T0 or T1). It requires 2 machine cycles to detect a high to low transition. Hence maximum count rate is 1/24 th of oscillator frequency.


The operation of the timers/counters is controlled by two special function registers, TMOD and TCON respectively.


Timer Mode control (TMOD) Special Function Register:

TMOD register is not bit addressable.



Address: 89 H

Various bits  of TMOD are described as follows -


M1 and M0 are mode select bits.


Timer/ Counter control logic:

Fig 4.16 Timer/Counter Control Logic Timer control (TCON) Special function register:


TCON is bit addressable. The address of TCON is 88H. It is partly related to Timer and partly to interrupt.

The various bits of TCON are as follows.


TF1 : Timer1 overflow flag. It is set when timer rolls from all 1s to 0s. It is cleared when processor vectors to execute ISR located at address 001BH.


TR1 : Timer1 run control bit. Set to 1 to start the timer / counter. TF0 : Timer0 overflow flag. (Similar to TF1)


TR0 : Timer0 run control bit.


IE1 : Interrupt1 edge flag. Set by hardware when an external interrupt edge is detected. It is cleared when interrupt is processed.


IE0 : Interrupt0 edge flag. (Similar to IE1)


IT1 : Interrupt1 type control bit. Set/ cleared by software to specify falling edge / low level triggered external interrupt.


IT0 : Interrupt0 type control bit. (Similar to IT1)


As mentioned earlier, Timers can operate in four different modes. They are as follows Timer Mode-0:


In this mode, the timer is used as a 13-bit UP counter as follows.

Timer Mode-2: (Auto-Reload Mode)


This is a 8 bit counter/timer operation. Counting is performed in TLX while THX stores a constant value. In this mode when the timer overflows i.e. TLX becomes FFH, it is fed with the value stored in THX. For example if we load THX with 50H then the timer in mode 2 will count from 50H to FFH. After that 50H is again reloaded. This mode is useful in applications like fixed time sampling.

Timer Mode-3:


Timer 1 in mode-3 simply holds its count. The effect is same as setting TR1=0. Timer0 in mode-3 establishes TL0 and TH0 as two separate counters.

Control bits TR1 and TF1 are used by Timer-0 (higher 8 bits) (TH0) in Mode-3 while TR0 and TF0 are available to Timer-0 lower 8 bits(TL0).



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