Pointer Registers
Data Pointer (DPTR – DPL and DPH)
The Data Pointer is a 16-bit Register and is physically the combination of DPL (Data Pointer Low) and DPH (Data Pointer High) SFRs. The Data Pointer can be used as a single 16-bit register (as DPTR) or two 8-bit registers (as DPL and DPH).
DPTR doesn’t have a physical Memory Address but the DPL (Lower Byte of DPTR) and DPH (Higher Byte of DPTR) have separate addresses in the SFR Memory Space. DPL = 82H and DPH = 83H.
The DPTR Register is used by the programmer addressing external memory (Program – ROM or Data – RAM).
Stack Pointer (SP)
SP or Stack Pointer points out to the top of the Stack and it indicates the next data to be accessed. Stack Pointer can be accesses using PUSH, POP, CALL and RET Instructions. The Stack Pointer is an 8-bit register and upon reset, the Stack Pointer is initialized with 07H.
When writing a new data byte into the stack, the SP (Stack Pointer) is automatically incremented by 1 and the new data is written at an address SP+1. When reading data from stack, the data is retrieved from the Address in SP and after that the SP is decremented by 1 (SP-1).
I/O Port Registers (P0, P1, P2 and P3)
The 8051 Microcontroller four Ports which can be used as Input and/or Output. These four ports are P0, P1, P2 and P3. Each Port has a corresponding register with same names (the Port Registers are also P0, P1, P2 and P3). The addresses of the Port Registers are as follows: P0 – 80H, P1 – 90H, P2 – A0H and P2 – B0H.
Each bit in these SFRs corresponds to one physical Pin in the 8051 Microcontroller. All these Port Registers are both Bit Addressable and Byte Addressable. Writing 1 or 0 on a Port Register Bit will reflect as an appropriate voltage (5V and 0V) on the corresponding Pin.
If a Port Bit is SET (declared as 1), the corresponding Port Pin will be configured as Input and similarly if a Port Bit is CLEARED (declared as 0), the corresponding Port Pin is configured as Output. Upon reset, all the Port Bits are SET (1) and hence, all the Port Pins are configured as Inputs.
Peripheral Control Registers
PCON (Power Control)
The PCON or Power Control register, as the name suggests is used to control the 8051 Microcontroller’s Power Modes and is located at 87H of the SFR Memory Space. Using two bits in the PCON Register, the microcontroller can be set to Idle Mode and Power Down Mode.
During Idle Mode, the Microcontroller will stop the Clock Signal to the ALU (CPU) but it is given to other peripherals like Timer, Serial, Interrupts, etc. In order to terminate the Idle Mode, you have to use an Interrupt or Hardware Reset.
In the Power Down Mode, the oscillator will be stopped and the power will be reduced to 2V. To terminate the Power Down Mode, you have to use the Hardware Reset.
Apart from these two, the PCON Register can also be used for few additional purposes. The SMOD Bit in the PCON Register is used to control the Baud Rate of the Serial Port.
There are two general purpose Flag Bits in the PCON Register, which can be used by the programmer during execution.
SCON (Serial Control)
The Serial Control or SCON SFR is used to control the 8051 Microcontroller’s Serial Port. It is located as an address of 98H. Using SCON, you can control the Operation Modes of the Serial Port, Baud Rate of the Serial Port and Send or Receive Data using Serial Port.
SCON Register also consists of bits that are automatically SET when a byte of data is transmitted or received.
Serial Port Mode Control Bits
| SM0 | SM1 | Mode | Description | Baud Rate |
| 0 | 0 | 0 | 8-Bit Synchronous Shift Register Mode | Fixed Baud Rate
( Frequency of oscillator / 12) |
| 0 | 1 | 1 | 8-bit Standard UART
mode |
Variable Baud Rate (Can be set by Timer 1) |
| 1 | 0 | 2 | 9-bit Multiprocessor Comm. mode | Fixed Baud Rate
( Frequency of oscillator / 32 or Frequency of oscillator / 64 |
| 1 | 1 | 3 | 9-bit Multiprocessor Comm. mode | Variable Baud Rate (Can be set by Timer 1) |
