8051 MICROCONTROLLER: 8-bit microcontroller ● ● ● ● ● ●
8-bit CPU,Reg A,Reg B 15-bit PC and Data Pointer(DPTR) 8-bit PSW 8-bit SP Internal ROM and EPROM (8751) Internal RAM of 256 bytes
Architecture of 8051:
Registers in 8051 Math Register
* indicates that each bit a of the register can be programmed. Interrupt Register
Timer Control Register
Assembler- Assembly level language to Machine code Compiler- High level language to Machine code Simulator- software to simulate the function of a microcontroller Emulator- Combination of software and hardware to simulate function of a microcontroller
Timer Counter Register
8051 Internal Data Memory
It has 256 byte of internal RAM
SFR MAP F8 F0
T2MOD RCAP2L RCAP2H TL2
These n in the 8052 microcontroller.
Addressing Modes: Ex: ADD A, #77 (immediate addressing) A=A + 77(decimal) 1) Immediate Addressing Mode: Where data is available in the instruction itself. 2) Bank Register Addressing: Add A, RO since it has 4 different banks each bank each having 8 bytes and the register RO of which bank is selected by the SFR PSW. PSW: RSI and RSO selects which bank is to be selected.
3)Direct Addressing Mode: Add A,80 A A+(Data of 80 in SFR) ie, A A + PO 4)Indirect Addressing Mode: Registers R0, R1, DPTR r used to store address of the 8-bit and 16-bit. Indirect memory addressing Add A, @ RO A A + R0 5) Register Indirect Addressing Add A, @ DPTR In this address in the DPTR may be internal or external memory address. Ex. MOV A,30H This takes 2 instruction cycles ie, 24 clock cycles. MOV A,@ RO This instruction takes only 1 instruction cycle ie, 12 clock cycles.
External Memory Access:
40 Pin DIP Package of 8051
How to latch 1 to P3 X ? This latching is done in the following process.
That to latch P3 to 1, first write to latch and load 1 into it. Hence the output of the gate is the alternate output function which latches the other functions.
8051 I/O port Structure Port 0 AD0- AD7
This can be used as normal I/O port or address bus Mux switches between normal I/O port and address/data bus Pure bidirectional port
This port is used for normal I/O port only
Quasi bidirectional port
For external memory read operation,SFR PO is over written by FFH
While accessing 16-bit external memory
Port 2 (A8-A15)
Port 3 I/O Port + Alternative function Discussed in previous class
Port can be used as normal I/O port and address bus While addressing 16-bit external memory, P2 SFR value remains unchanged. ALL THE PORTS HAVE LIMITED CURRENT SOURCING CAPABILITY. External Memory Access ROM (Pragramming Memory) ● Always 16-bit address
Always external Memory is accessed If PC > FFFH then ROM is accessed 000-FFFH is internal Memory 1000-FFFFH is external Memory access internal memory
RAM CData 8-bit or 16-bit address If the address is stored in a 8-bit register then it points to 8-bit address MOV A,@RO For 16-bit address MOV A, @ DPTR
How MOV A,@DPTR works
For 8-bit Memory address access, P2 Pins o/p the SFR register contents and helps in memory pages. The higher order 8-bit address is taken the address available in the P2 SFSR and the lower order 8-bit address is the data available in register RO.
Timer/Counter in 8051 8051 has two 16-bit Timer/counter registers. 8052 has these two plus one more:Timer 2. All these can be conﬁgured to operate either as times or event counters. In the ”Timer” function, the register is incremented every machine cycle. Now one can think of it 1 oscillator frequency (∵ one as counting machine cycles (instruction cycles) and the clock rate is 12 instruction cycle = 12 clock cycles). In ”Counter” function, the register is incremented in response to a 1 to 0 transition at its corresponding external input pins, ie. T0, T1 or (in 8052)T2. In this function, the external inputs is sampled during S5P2 of every machine cycle. When the samples show a high in one cycle and a low in the next cycle, the count is incremented. The new count value appears in the register during S3P1 of the cycle following the one in which the transition was detected. Since it takes 2 machine cycles (24 oscillator periods) to recognize a 1-to-0 transition, the maximum count rate is 1/24 of the oscillator frequency. Timer 0 and Timer 1 have 4 operating nodes. Timer 2 in 8052 has 3 modes of operation: ”Capture”, ”auto-reload” and ”bandrate generator”.
Timer mode control (TMOD) Special function Register 7
Timer - 1
Timer - 0
Timer/Counter Control logic osc . freq .
to timer stages Counter T1 /0 Input pin
TR1 /0 Bit in TCON
Gate bit in TMOD
INT1 /0 Input pin
Timer Mode-0 Setting timer x mode bits to 00 in the TMOD register results in using the THX register as a 8-bit counter and TLX as a 5-bit register (lower bits). The upper 3-bits of TLX are indeterminate and should be ignored. The timer overﬂow ﬂag in TCON is set whenever THX goes from FFh to 00h. Pulse Input
TLX 5 bits
THX 8 bits
Timer Mode-1 Mode-1 is similar to mode-0 except TLX is conﬁgured as a full 8-bit counter. When the mode bits are set to 01 in TMOD. Pulse Input
TLX 8 bits
THX 8 bits
The Timer Control (TCON) Special Function Register 7
TF1 → Timer 1 overﬂow ﬂag. Set when timer rolls from all 1’s to all 0. Cleared when processor vectors to execute int. Since routine at 00/Bh TR1 → Timer 1 run control bit. Set to 1 by program to enable time to count. Clear to 0 by program (TRO - for Timer 0) Timer Mode-2 (Auto-reload feature) TLX is used as a 8-bit counter only. THX is used to hold a value that is loaded into TLX everytime TLX overﬂows from FFh to 00h. The time ﬂag is also set when TLX overﬂows. The mode shows auto reload feature where TLX will be initialized to the content of THX after TLX overﬂows. Pulse Input
TLX 8 bits
THX 8 bits
Timer Mode-3 Timer 1 in Mode-3 simply holds its count. The eﬀect is the same as setting TR1=0. Timer 0 in Mode 3 establishes TL0 and TH0 as two sperate counters.
TLO 8 bits
TH0 8 bits
TRI Bit in TCON
Control bit TR1 and TF1 are used by Timer-0
Timer-1 can still be used in Mode-0,1 and 2 but no interrupt will be generated by Timer-1 while Timer-0 is in Mode-3. Timer 2 Like Timer-0 and 1, it can operate either as a timer or as a event counter. T2CON 7
0 CP /RL2
This is selected by bits C/T2 in special function register T2CON. It has there operating modes: ”Capture”, ”auto-load” and ”band rate generator”. RCLK + TCLK
Mode 16-bit Auto-reload
Serial Interface: The serial port is full duplex. SBUF → Special function Register. Mode - 0 Shift register mode. Serial data enters and exists through RXD. 8-bits are transmitted/recieved. Pin TXD is connected to the internal shift frequency pulse source to supply shift pulses to external circuits. The shift frequency or bandrate is ﬁxed at 1/2 of the oscillator frequency.
Mode - 1 Standard UART 10 bits are transmitted (through TXD) or recieved through (RXD), a start bit(0), 8 data bits (LSB ﬁrst), and a stop bit(1). Once recieved, the stop bits goes into RB8 in special function register SCON. The band rate is variable.
Receiver samples data in center of bit times Idle state
Idle state 1
Minimum one stop bit
t Bit time =
Mode - 2 Multiprocessor Mode. 11 bits are transmitted through TXD or recieved through RXD, a start bit (0), 8 data bits (LSB ﬁrst), a programmable 9th bit and a stop bit(1). On transmission, the 9th data bit (TB8 in SCON) can be assigned the value 0 or 1. Or, for example, the parity bit (P in the PSN) could be moved into TB8. On receive, the 9th bit goes into RB8 in SFR SCON, which the stop bit is ignored. The bandwidth is programmable to either 1/32 or 1/64 of oscillator frequency. 2SM OD fosc fband = 64 Mode - 3 11 bits are transmitted through TXD or received through RXD: a start bit, 8 data bits (LSB ﬁrst), a programmable 9th bit, and a stop bit (1). In fact, Mode 3 is same as Mode 2 in all respects except the band rate. The band rate in Mode 3 is variable.
Serial Port Control Register SCON (MSB )
(LSB ) SM1
SM0 SMI MODE 0 0 0 0 1 1 1 0 2 1 1 3
Description Band rate shift register fosc /12 8-bit UART Variable 9-bit UART f /32, f /64 9-bit UART Variable
SM2 Enables multiprocessor communication in Mode 2 and 3. REN Enables serial reception. TB8 9th data bit that will be transmitted in Mode 2 and 3. RB8 9th data bit that was received. In mode-1, a SM2=0, RB8 is was received. In mode-0, RB8 is not used. T1 Transmit interrupt ﬂag R1 Receive interrupt ﬂag
the stop bit that
The Power Mode Control (PCON) Special function reg. 7
2SM OD fosc = × 32 12 × [256 − T H1]
Timer-1 is used to generate band rate for mode-1 using overﬂow ﬂag of the timer to determine the band frequency. Typically Timer-1 is used in mode-2 as an auto load 8-bit that generates the band frequency. If Timer-1 is not run in timer mode-2 then the band rate is 2SM OD × timer 1 overﬂow frequency Fband = 32 fclock = 11.0592 M Hz fband = 9600 20 11.0592 × 106 T H1 = 256 − × = 253 = 0F DG 32 12 × 9600
Multiprocessor Communication Mode 2 and 3 have a special provision for multiprocessor communication. In this mode, 9 data bits are received/transmitted. The port can be programmed such that when the stop bit is received, the serial interrupt will be activated only if RB8=1. This feature is enabled by setting bit SM2 in SCON. A way to use this feature is given here. Rx Tx
Rx Tx Master
Master communication with one slave at a time
When a master processor wants to send a data byte to a slave, the master sends the address of the slave ﬁrst. There may be many slave processors. An address byte diﬀers from a data byte in that the 9th bit is 1 in an address byte and 0 in a data byte. An address byte interrupts all slaves when SM2=1. But a data byte does not interrupt the slaves if they have SM2=1. The address byte is checked by each slave and the target/addressed slave clears its SM2 so that it can receive the data
byte. The slaves that are not addressed leave their SM2’s set and go on with their business, ignoring the incoming data bytes. SM2 has no eﬀect in mode 0, and in mode 1, it can be used to check the reliability of the stop bit. In mode 1, if SM2=1, then receive interrupt will not be activated unless a valid stop bit is received.
Interrupts 8051 provides 5 interrupt sources. The 8052 provides 6. 0 IE0
Interrupt 0 IE1
INT1 IT1 TF1
IN T 0 and IN T 1 are external interrupts and can be level triggered or edge triggered (negative) depending upon IT 0/IT 1 in TCON SFR. IT 0 Set → falling edge triggered for IN T 0 IT 0 Cleared → low level triggered for IN T 0 IE0/1E1 Interrupt 0/1 edge ﬂag. Set by hardware when external interrupt edge detected. Cleared when interrupt processed. Timer 0 and Timer 1 interrupts are generated by TF0 and TF1, which are set by a rollover in their respective Timer/Counter register (except Timer 0 in Mode 3). When a timer interrupt is generated, the ﬂag that generated it is cleared by on chip hardware when the interrupt service routine is vector to. The serial port interrupt is generated by the logical OR of RI and TI. Neither of these ﬂags is cleared by hardware when the interrupt service routine is vectored to. These have to be cleared by software. All of the bits that generate interrupts can be set or cleared by software, with the same result as though it had been cleared by hardware. That is, interrupts can be generated or pending interrupts can be canceled in software. Each of these interrupts can be individually enabled or disabled by setting or clearing bit in the SFR IE. IE contains a global disable bit, EA which disable all interrupts at once.
Interrupt Enable Register(IE)
(MSB ) EA
(LSB ) ET2
EA = 0 no interrupt is acknowledged. = 1 each int source is individually enabled or disabled ET2 = 0 disables Timer 2 overﬂow int = 1 enables Timer 2 overﬂow int ES = 0 Serial port int is disabled = 1 Serial port int is enabled ET1 = 0 Timer 1 overﬂow int is disabled = 1 Timer 1 overﬂow int is enabled EX1 = 0 External int 1 (IN T 1) is disabled. = 1 External int 1 (IN T 1) is enabled. ET0 = 0/1 Disables Timer 0 OF int EX0 = 0/1 Disables/enables external int (IN T 0) Priority structure Each interrupt source can be individually programmed to one of two priority levels by setting or cleaning a bit in special function register IP. A low priority interrupt can itself be interrupted by a high priority interrupt, but not by another low priority interrupt. If two requests of diﬀerent priority levels are received simultaneously, the request of higher priority level is serviced. If requests of same priority level are received simultaneously, an internal polling sequence determines which request has to be serviced. Thus within each priority level, there is a second priority structure determined by the polling sequence, as follows: Source Priority within level 1. IE0 (Highest) 2. TF0 ↓ 3. IE1 ↓ 4. TE1 ↓ 5. RI+TI ↓ 6. TF2+EXF2 (Lowest) How the interrupts are handled Interrupt ﬂags are sampled in S5P2 of each instruction cycle. The samples are polled during the following instruction cycle (machine cycle). If one of the ﬂags was
in a set condition at S5P2 of the preceding cycle, the polling will ﬁnd it and the interrupt system will generate an LCALL to the appropriate service routine, provided this hardware generated LCALL is not blocked by any of the following conditions: 1. An interrupt of equal or higher priority level is already in progress. 2. The current polling is not the ﬁnal machine cycle of the instruction in progress 3. The instruction in progress is RET1 or any write to IE or IP registers. If an interrupt ﬂag is a active but not being responded to for one of the above conditions, if the ﬂag is not still active when the blocking condition is removed, the denied interrupt will not be serviced. This is because the interrupt ﬂag once active but not serviced is not remembered. Every polling cycle is new C1 C2 C3 C4 C5 C6
Int . goes active
Interrupt Service Routine
Long call to interrupt vector address Int are polled
After a interrupt is vectored to, some interrupt ﬂags are cleared and some are not by hardware. For example: Serial port and Timer 2 interrupt ﬂags are not cleared automatically. This has to be done by user’s software. IEU and IE1 are cleared if the interrupts are transition activated. TF0 and TF1 are cleared by hardware generated LCALL pushes PC into the stack but not PSW. Source Vector address IE0 TF0 IE1 TF1 RI+TI TF2+EXF2
−→ −→ −→ −→ −→ −→
0003H 000BH 0013H 001BH 0023H 002BH
External Interrupt IN T O and IN T 1 Level triggered or Transition triggered (Low) (1-to-0 transition) IT0/IT1=0 IT0/IT1=1 Low or high should be maintained at the pin for at least 12 clock cycles (1 machine cycle). Special function Register (IP) Interrupt Priority Register (IP) (MSB )
(LSB ) PT2
PT2 → Timer 2 1-high priority 0-Low priority PS → Serial Port Interrupt 1-high priority 0-Low priority PT1 → Timer 1 Interrupt 1-high priority 0-Low priority PX1 → External Int. 0 1-high priority 0-Low priority PT0 → Timer 0 Int. 1-high priority 0-Low priority PX0 → External Int 1 1-high priority 0-Low priority Software generated interrupts When any interrupt ﬂag is set to 1 by any means, an interrupt is generated unless blocked. This means that the program itself can cause interrupts of any kind to be generated by simply setting the desired interrupt ﬂag to 1 using program instruction. Example of interrupt Use Single-Step operation The following program enables simple step operation JNB P3.2, FDH JB P3.2, FDH RETI Reset Non-maskable Interrupt Holding RST pin high for at least 2 machine cycles while the osc is running. After RST is made low PC ← 0000H SP ← 07H Postlatches ← FFH SBUF ← XX All other SFR ← 00 RAM content is not changed.
Power-on Reset When power is switched on (Vcc ), the capacitor behaves as a short circuit and RST pin remains high for considerable amount of time to enable the micro controller to go into RESET mode. Vcc
10 µ F Vcc RST 8051
8 .2 k Ω Vcs
Power-saving modes of operation Often, power saving becomes important for microcontroller-based applications.The CHMOS version of 8051 provides reduced power modes of operation as a standard feature. CHMOS → (Complementary High density MOS)- A chip with high density of CMOS transistors. CHMOS Power reduction modes CHMOS version has two power-reducing modes, idle and power down. The internal circuitry which implements these features is given below.
Interrupt Internal Port TImer Port
Idle and power down modes are activated by setting the corresponding bits (IDL and PD respectively) in PCON special function register. (MSB ) PCON 87H
(LSB ) GF1
PD - Power down bits. Setting this bit activates power down operation IDL - Idle mode bit. Setting this bit activates idle mode operation.
Not bit addressable
Idle Mode An instruction that sets P CON0 causes that to be the last instruction executed before going into the idle mode. In the idle mode, the internal clock signal is gated oﬀ to the CPU, but not to the interrupt, Timer and Serial port functions. The CPU status is preserved entirely. SP,PC,PSW,Acc, and all other registers maintain thier data during the idle mode. The port pins hold their logical status they had at the time idle was activated. ALE and P SEN are hold at logic high levels. There are two ways to terminate idle. Activation of any enabled interrupt will cause PCON0 to be cleared by hardware, terminating the idle mode. After RET1 is executed, the next instruction starts from the one following the instruction which enabled the idle mode. The ﬂag bits GF0 and GF1 can be used to give an indication if an interrupt occurred during normal operation or during the idle mode. For example, an instruction that activated idle can also set and or both ﬂags. When idle is terminated by an interrupt, the ISR can examine the ﬂag bits. The other way of terminating the idle mode is with a hardware reset.(Reset should be high for two machine cycle or 24 clock cycles). The signal at the RST pin clears IDL bit directly and asynchronously. At this time the CPU resumes the program execution from where it left oﬀ; that is , at the instruction following the one that involved idle mode. Two to three machine cycles should be executed before the internal reset algorithm takes control. Power down mode An instruction that sets PCON.1 causes that to be the last instruction executed before going into the power down more. In the power down mode, the on-chip oscillator is stopped. With the clock frozen, all functions are stopped, but the on-chip RAM and special function register are held. The port pins output the values held by thier respective SFRs, ALE and P SEN output. The only exit from Power down mode of operation in by a hardware reset. Reset redeﬁnes all SFRs, but does not change the on-chip RAM. Vcc may be reduced to as low as 2V during Power down mode. However Vcc should be restored to the rated value and allow clock to stabilize (≤ 10ms)before the Power down mode is exited, ie, hardware Reset is pressed.
8051 Instruction Set Some notes Rn Register R0-R7 of the currently selected register bank. direct 8-bit internal data location address (Internal data with address 0-127 or SFR) @Ri 8-bit internal Data RAM location addressed data 8-bit constant included in instruction (immediate 8-bit data) data 16 16-bit immediate data included in the instruction. add 11 11-bit destination address. Used by ACALL and AJMP. The branch will be within the same 2k byte page of Program Memory as the ﬁrst byte of the following instruction. addr 16 16-bit destination address. Used by LCALL and LJUMP. A branch anywhere within 64 kbyte of Program Memory space. rel Signed (2’s complement) 8-bit oﬀset byte used by SJUMP and all conditional jumps. Range is -128 to +127 bytes relative to ﬁrst byte of the following instruction bit Direct addressed bit in internal RAM or SFR.
Arithmetic Instructions Mnemonics ADD A,Rn ADD A,direct ADD A,@Ri ADD A,data ADDC A,Rn ADDC A, direct ADDC A,@Ri ADDC A,data SUBB A,Rn SUBB A,direct SUBB A,@Ri SUBB A,data INC A INC Rn INC direct INC @Ri DEC A DEC Rn DEC direct DEC @Ri INC DPTR MUL AB DIV AB DA A
Description A ← A + Rn A ← A + Direct A ← A + @Ri A ← A+data A ← A+ Rn+C bit A ← A+(direct)+C bit A ← [email protected]
+C bit A ← A+data+C-bit A ← A-Rn-C-bit A ← A-(direct)-C-bit A ← [email protected]
A ← A-data-C-bit A ← A+1 Rn ← Rn+1 direct ←(direct)+1 @Ri ←@Ri+1 A ← A-1 Rn ← Rn-1 direct ←(direct)-1 @Ri ← @Ri-1 DPTR ← DPTR+1 A ← lowbyte (A*B) B ← highbyte (A*B) A ← quotient (A/B) B ←remainder (A/B) decimal adjust acc
Bytes Instruction Cycles 1 1 2 1 1 1 1 1 1 1 2 1 1 1 2 1 1 1 2 1 1 1 2 1 1 1 1 1 2 1 1 1 1 1 1 1 2 1 1 1 1 2 1 4 1
Logical Instructions Mnemonics ANL A,Rn ANL A,direct ANL A,@Ri ANL A,data ANL direct,A ANL direct, data ORL A,Rn ORL A,direct ORL A,@Ri ORL A,data ORL direct,A ORL direct,data XRL A,Rn XRL A,direct XRL A,@Ri XRL A,data XRL direct,A XRL direct,data CLR A CPL A RL A RLC A RR A RRC A SWAP A
Description A ← A.Rn A ← A.Direct A ← [email protected]
A ← A.data (direct) ← (direct)A (direct) ← (direct).data A ← A+Rn A ← +(direct) A ← [email protected]
A ← A+data (direct) ← (direct)+A (direct) ← (direct)+data A ← A⊕ Rn A ← A⊕ (direct) A ← A⊕ @ Ri A ← A⊕ data A ← direct ⊕ A direct ← direct⊕ data A ← 00H A←A Rotate Left Rotate left through carry Rotate right Rotate right through carry Swap nibbles within the ACC
Bytes Instruction Cycles 1 1 2 1 1 1 2 1 2 1 3 2 1 1 2 1 1 1 2 1 2 1 3 2 1 1 2 2 1 1 2 1 2 1 3 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Data transfer Instructions Mnemonics Description MOV A, Rn MOV A, direct MOV A, @Ri MOV A, data MOV Rn, A MOV Rn, direct MOV Rn, data MOV direct, A MOV direct, Rn MOV direct, direct MOV direct, @Ri MOV direct, data MOV direct, A MOV @ Ri, A MOV @ Ri, direct MOV @ Ri, data MOV DPTR,data16 A MOV A, @A+DPTR Code byte move to A relative to DPTR MOV A, @A+PC Code byte move to A relative to PC MOVX A, @Ri External data memory move to A (8-bit addr) MOVX A, @DPTR External data memory move to A (16-bit addr) MOVX @Ri, A Move A to ext data memory (8-bit addr) MOVX @DPTR, A Move A to ext data memory (16-bit addr) PUSH direct POP direct XCH A, Rn XCH A, direct XCH A, @Ri XCHD A, @Ri
Bit Operations / Jump instructions Mnemonics CLR C CLR bit SETB C SETB bit CPL C CLR bit ANL C,bit ANL C, /bit ORL C,bit ORL C,/bit MOV C,bit MOV bit, C JC rel JNC rel JB bit, rel JNB bit, rel JBC bit, rel ACALL addr11 LCALL addr16 RET RET1 AJUMP addr11 LJUMP addr16 SJMP rel JMP @ A+ DPTR JZ rel JNZ rel CJNE A, direct, rel CJNE CJNE CJNE DJNZ DJNZ
Description Carry ← 0 bit ← 0 Carry ← 1 bit ← 1 C←C bit ← bit C ← C.bit C ← C.bit C ← C+bit C ← C + bit C ← bit bit ← C Jump if C=1 Jump if C=0 Jump if bit = 1 Jump if bit = 0 Jump if b=1 and b ←0 Absolute jump Long jump Return from subroutine Return from interrupt Absolute jump Long jump Short jump Jump relative to DPTR
Compare with Acc and jump if not equal
A, data, rel Rn,data, rel @Ri,data, rel Rn, rel Decrementreg and jump if not 0 direct, rel
Direct bit addressing Values between 0 and 127 (00H and 7FH) deﬁne bits in a block of 16 bytes of on-chip RAM between addresses 20H-2FH. They are numbered consecutively from the lowest-order bytes lowest order bit through the highest order bit. Bit addresses between 128 and 255 (80H and 0FFH) correspond to bits in a number of special function registers mostly used for I/O or peripheral device control. These positions are numbered with a diﬀerent scheme than RAM. The ﬁve high-order address bits match those of the registers own address
while the three low-order bits identiﬁes the bit position within that register.
External Addressing using MOVX and MOVE Read
A Reg MOVX @ RT
R0 or R1 DPTR DPTR + A PC + A
Int & Ext RAM
MOVX @ DPTR MOVC A, @ A + DPTR MOVC A,@ A + PC
Jump and Call Program Range Relative Range: Jump that replaces the program counter content with a new address that is greater than the address of the instruction following the jump by 127 or less than the address of the instruction following jump by 128 are called relative jumps. The address following the jump is used to calculate the relative jump because the PC is incremented to the next instruction before the current instruction is extended. Relative jump has two advantages. First, only 1 byte of data (2’s complement) need to be speciﬁed for jumping ahead(positive range 0-127) or jumping back (negative range -128). Specifying only 1 byte saves program bytes and speeds up program execution. Second, the program that is written using relative jumps can be relocated anywhere in the program namely without reassembling the code to generate absolute addresses. The disadvantage of relative jump is the short jump range (-128 to 127). This can be problematic in large programs where multiple relative jump may be require if higher jump range is required. Instructions using relative range jump are SJMP rel, and all conditional jumps.
Short Absolute Range: Short Absolute range makes use of the concept of dividing memory into logical divisions called pages. Program memory may be regarded as one continuous stretch of addresses from 0000H to 0FFFFH or it can be divided into a series of pages of any convenient binary size. The 8051 program memory is arranged on 2k byte pages giving a total of 32 (20H) pages. The
hexadecimal address of each page is shown in the following table. Page 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
Address Range 0000 - 07FF 0800 - 0FFF 1000 - 17FF 1800 - 1FFF 2000 - 27FF 2800 - 2FFF 3000 - 37FF 3800 - 3FFF 4000 - 47FF 4800 - 4FFF 5000 - 57FF 5800 - 5FFF 6000 - 67FF 6800 - 6FFF 7000 - 77FF 7800 - 7FFF
Page 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
Address Range 8000 - 87FF 8800 - 8FFF 9000 - 97FF 9800 - 9FFF A000 - A7FF A800 - AFFF B000 - B7FF B800 - BFFF C000 - C7FF C800 - CFFF D000 - D7FF D800 - DFFF E000 - E7FF E800 - EFFF F000 - F7FF F800 - FFFF
It can be seen that the upper 5 bits of the program counter hold the page number and the lower 11 bits of the program counter hold the address within each page. Thus an absolute address is formed by taking page number of the instruction following the branch and attaching the absolute page range address of 11 bits to it to form the 16-bit address. Diﬃculty is encountered when the next instruction (the instruction following the jump instruction) starts at X800H or X000H. This places the jump or call address on the same page as the next instruction. This does not give rise to any problem on forward jump, but results in error if the branch is backward in the program. This should be checked by assembler and the user should be instructed to relocate the program suitably. Short absolute range jump is also relocatable as the relative jump. Instructions using short absolutes range are ACALL addr 11 AJMP addr 11
Long Absolute Jump: Address that can access the entire program from 0000H to FFFFH use long-range addressing. Long range addresses require more bytes of code to specify and relocatable only at the beginning of 64 K byte pages. Since the normal code memory is only 64k bytes, the program must be reassembled every time a long-range address changes and then branches are not generally relocatable. Instructions using long absolute range are LCALL addr 16 LJMP addr 16 JMP @ A+DPTR
Example of Conditional Jump Org 0100H Loop: ADDA:
MOV A, 10H MOV RO, A ADD A, RO JNC ADDA
MOV A, 10H ADDR: ADD A,RO JNB 0D7H, ADDR JBC 0D7H, LOOP Example of External Data ORG O MOV RO,043H MOV A, 12 MOVX @RO, A MOVX A, 34 MOVX A, @RO
Character transmission using a time delay A program called Senddata takes the character in A register, transmit it, delays for transmission time, and then returns to the calling program. Time 1 must be used to set the bandrate, which is 2400 10 sec = 0.00416s or 4.16msec. The software band in this program. The delay for one character is 2400 delay of 5msec is used. Time 1 generates a bandrate close to 2400. Using a 12 MHz crystal, the reload value is 256 − 12 × 106 /(32 × 12 × 2400), which is 242.98 or 243. This gives rise to an actual bandrate of 2404. SMOD is programmed to be 0. ; Send data, using a 12 Mhz Crystal for VART timing. ; 2400 nominal band rate ﬁr an actual band rate of 2404. ; Delay between characters for 5 msec. EQU EQU EQU EQU ORG ANL ANL ORL MOV SETB MOV
BAUDNUM, 0F3H DELAY, 0A6H DLYLSB, 05H DLYMSB, 00H 0000H PCON, 7F H TMOD, 3F H TMOD, 20H THI, BAU DN U M TRI SCON, 40H
MOV SBUF, ’A’ ACALL XMITTIME SJMP XMIT
MOV A, DLY LSB MOV B, DLY M SB ACALL SOFTIME RET
PUSH 07H PUSH ACC ORL A,B CJNE A, 00H, OK POP ACC SJMP DONE POP ACC
No on chip program memory 128 KB of on chip RAM No ROM, 256KB on chip RAM 4K byte of Masked ROM (factory programmable) 128 KB of on chip RAM Upto 12 or 16 MHz 8K byte of ROM, 256Kb RAM 4K byte of EPROM (UV Erasable) Upto 12 or 16 MHz
8052: 8751: ATMEL AT 89C51:
4K byte of Reprogrammable ﬂash memory 128K byte of RAM(on-chip) From 0-24MHz Interrupt driven data reception BAUDNUM ORG SJUMP
EQU 0000H OVER
CLR MOV PI, RETI
ANL PCON, 7FH ; Set SMOD=0 ANL TMOD, 0FH ; Alter Timer=1 conf only ORL TMOD, 20H ; Timer 1 in mode -2 MOV TH1, BAUDNUM ; Reload value SETB TR1, ; Run Timer - 1 MOV SCON, 40H ; Serial Mode - 1 SETB REN, ; Enable serial reception ORL IE, 90H ; Enable Interrupt WAIT: SJMP WAIT, ; Wait for recieving data END Interrupt driven character transmission BAUDNUM EQU 0F3H ORG 0000H SJMP OVER
CLR MOV RET1 ANL ANL ANL ORL MOV SETB MOV ORL MOV SJMP END
TI SBUF,’A’ PCON,7FH PCON,7FH TMOD,0FH TMOD,20H TH1,BAUDNUM TR1, SCON,40H IE,90H SBUF,’A’ WAIT,
; ; ; ; ; ; ; ; ;
SMOD=0 SMOD=0 Timer-1 in mode - 2 Reload value Rum Timer-1 Serial mode-1 Enable serial int Send data