• In this module, we will be discussing the
MCS-51 family of microcontroller, in particular the 8051, which is the generic IC representative of this family.

• 4K bytes ROM
• 128 bytes RAM
• Four 8-bit I/O ports
• Two 16-bit timers
• Serial interface
• 64K external code memory space
• 64K data memory space

FIGURE 2
–1
8051 block diagram

• The 8051 is a 40 pin device, but out of these
40 pins, 32 are used for I/O.
• 24 of these are dual purpose, i.e. they can operate as I/O or a control line or as part of address or date bus.

FIGURE 2
–2
8051 pinouts

• Port 0 is a dual purpose port, it is located from pin 32 to pin 39 (8 pins) and is labeled in the fig.2-2 as AD0 to AD7.
• Port 1 is a dedicated I/O port from pin 1 to pin 8. It is generally used for interfacing to external device thus if you need to connect to switches or LEDs, you could make use of these 8 pins.

• Like port 0, port 2 is a dual-purpose port. It can be used for general I/O or as the high byte of the address bus for designs with external code memory.
• Port 3 is also dual purpose but designers generally avoid using this port unnecessarily for I/O because the pins have alternate functions which are related to special features of the 8051. Indiscriminate use of these pins may interfere with the normal operation of the 8051.

_____
• PSEN (Program Store Enable)
– This is a dedicated control line on pin 29 and is used to enable external program (code) memory. This pin usually connects to an
EPROM’s Output Enable (OE) pin.
– This is a logic low pin as represented by the bar above the word PSEN, this means that during a fetch stage involving an instruction stored in external memory, the pin will be pulsed
‘LOW’.

• ALE ( Address Latch Enable)
– This pin is used to demultiplex the address and data lines.
– Remember that port 0 has 2 functions. As the low byte of the address bus and as the data bus.
In designs with external memory, port 0 is connected to both the address and data lines of the external RAM thus during the part of the fetch cycle where the address is supplied, the
ALE is pulsed to enable the G (gate) control pin of the latch IC thus the data goes to RAM and is interpreted as an address. (see fig 2-10 and 2-
11).

___
• EA (External Access)
– If you need to connect to external ROM then this pin must be tied LOW (0V).
– This pin must be tied high (+5V) if the programs executes from internal ROM.

RST (Reset)
• This is pin 9 of the IC and is used as the master reset for the 8051. In order for the
8051 to recognise that a reset has occurred, this pin must be brought HIGH for at least two machine cycles. During normal operation, this pin must be at logic LOW.
This will be discussed in more detail later.

Oscillator ( clock) Input
• The 8051 is typically driven by a crystal oscillator connected to pin 18 and 19 as shown in fig.2-3.
• The words XTAL is short for crysTAL.

FIGURE 2
–3
Driving the 8051 from a TTL oscillator

• Power Connections
– The 8051 requires a +5V input on its Vcc input
(pin 40) and Vss connection is on page 20.

• The internal circuitry for the I/O port is shown in fig 2-4.
• If you want to read in from a pin, you must first give a logic ‘1’ to the port latch to turn off the FET otherwise the data read in will always be logic ‘0’.
• When you write to the port you are actually writing to the latch e.g. a logic 0 given to the latch will be inverted and turn on the FET which cause the port pin to be connected to gnd (logic 0).

FIGURE 2
–4
Circuitry for I/O ports

• 12 clock cycles make one machine cycle as shown in fig 2-5.
• E.g. if we use a 12 MHz oscillator, each clock cycle will have a time period of
1/12MHz. Twelve of these make one machine cycle so 12 x (1/12 MHz) = 1 microsecond. That’s the time of 1 machine cycle.

FIGURE 2
–5
Relationship between oscillator clock cycles, states, and the machine cycle

• While most microprocessors implement a shared memory space for data and code (programs), microcontrollers has limited memory and the program is usually stored in ROM.
• In the 8051, both code and data may be internal but they are stored in separate memories, namely the internal ROM and RAM. Expandable to a max of 64K using external memory.
• The next page shows the 8031 which has no internal ROM.

FIGURE 2
–6
Summary of the 8031 memory spaces

FIGURE 2
–7
Summary of the 8051 on chip data memory

• 4 Register Banks — Bank0, Bank1, Bank2 and Bank3
• Each Bank consists of — R0, R1, R2, R3,
R4, R5, R6, R7
• Bank 0 is the default upon power up of the microcontroller
• Other banks can be selected by programming PSW register.

• The general purpose RAM area is from address 30H to 7FH. The locations from address 20H to 2FH can also be used as general purpose RAM although these addresses have very specific role given in the next section.

• The 8051 contains 210 bit-addressable locations of which 128 are at byte address
20H through 2FH as shown in fig 2-7.
• This is the powerful feature of most microcontroller because individual bits can be set, cleared, ANDed, ORed etc. with a single instruction instead of having to read a byte and modify

• Example
– we could issue a simple instruction
SETB 67H
This would set the bit at address 67H to logic HIGH.
Bit 67H is bit 7(most significant bit) of byte location
2CH
In order to achieve the same result, a microprocessor would need to do this:
MOV A, 2CH
ORL A,#10000000B
MOV 2CH,A

• Above 7FH, there is another block of memory 80H to 0FFH in all the version of
MCS51 uP
• this 128 bytes of memory are reserved for
Special Function Register (SFR). There are
21 SFRs. Refer to fig 2-7.

• SFR are usually addressed by name
• Memory location 0F0H is given a name called
Register B, similarly 80H is called P0.
• Not all memory location has a name
– memory location 35H has no name
• Some locations between the SFRs have no names as well e.g. 91H. Such locations should not be used to store any data. If you do it then your data may be lost.
• Some important or commonly used SFRs will be discussed while others will be explained when you need to use them in your projects.

• This is a very important register because it contains status bits which indicates the current state of the cpu.
• PSW.7
CarrY(CY)
• PSW.6
Aux Carry (AC)
• PSW.5
Flag 0 (F0)
• PSW.4
Register Bank Select 1
• PSW.3
Register Bank Select 0
• PSW.2
Overflow(OV)
• PSW.1
reserved
• PSW.0
Even Parity Flag (P)

• Accumulator, it has two names, A and ACC.
Many instruction make use of the accumulator, eg: mov A,R0, push acc
• SP, always pointing to the top of the stack, increasing by 1 before write to stack, decreasing by 1 after read from stack

Fig 3-0 B
Pin Configuration
1
P1.0
2
P1.1
3
P1.2
4
P1.3
5
P1.4
6
P1.5
7
P1.6
8
P1.7
9
10
RST
P3.0 RXD
11
P3.1 TXD
12
P3.2 INT0
13
P3.3 INT1
14
P3.4 T0
15
P3.5 T1
16
P3.6 WR
17
P3.7 RD
18
XTAL2
19
XTAL1
20
VSS
VCC
AD0 P0.0
AD1 P0.1
AD2 P0.2
AD3 P0.3
80C31
AD4 P0.4
AD5 P0.5
AD6 P0.6
AD7 P0.7
EA/VPP
ALE/PROG
PSE
A14 P2.6
A13 P2.5
A12 P2.4
A11 P2.3
A10 P2.2
A9 P2.1
A8 P2.0
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
• In the SFR, register P0,
P1, P2, P3 are connected to the physical pin on the uP
PORT 0 addressed by name function

• If the design involves external code memory, both P0 and P2 should not be used as general purpose I/O since P2 is now the
Higher address bus while P0 is the multiplexed Lower address bus and the data bus.
_____
• As stated earlier, the PSEN pin must be used. See fig 2-9

FIGURE 2
–8
Multiplexing the address bus (low-byte) and data bus

FIGURE 2
–9
Accessing external code memory

• Fig.2-10 shows what happens during an opcode fetch for a 2 byte instruction that has a time of 1 machine cycle.

FIGURE 2
–10
Read timing for external code memory

• For designs with external RAM, a typical connection is shown in fig.2-12. Note the control lines that must be used.
• MOVX instruction is used to indicate that the external RAM is involved.
• e.g. MOVX A, @dptr ( a read operation)
• e.g. MOVX @dptr, A (a write operation)

• The timing diagram for a read operation is shown in fig 2-11.

FIGURE 2
–11
Timing for MOVX instruction

FIGURE 2
–12
Interface to 1K RAM

• To reset the 8051, the RST pin must be held high for at least 2 machine cycles.
• This can be achieved upon power–up using an RC network.
• Fig.2-16 shows 2 circuits for achieving this, one is a manual reset, the other is a poweron reset.
• How does the 2 circuit works?
– Try to remember capacitor is open during steady-state.

FIGURE 2
–16
Two circuits for system reset. (a) Manual reset (b) Power-on reset.