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Address Bit Address FF F0 F7 F6 F5 F4 F3 F2 F1 F0 B E0 E7 E6 E5 E4 E3 E2 E1 E0 ACC D0 D7 D6 D5 D4 D3 D2 - D0 PSW B8 - - - BC BB BA B9 B8 IP B0 B7 B6 B5 B4 B3 B2 B1 B0 P3 A2 AF - - AC AB AA A9 A8 IE A0 A7 A6 A5 A4 A3 A2 A1 A0 P2 99 Not bit Addressable SBUF 98 9F 9E 9D 9C 9B 9A 99 98 SCON 90 97 96 95 94 93 92 91 90 P1 8D Not bit Addressable TH1 8C Not bit Addressable TH0 8B Not bit Addressable TL1 8A Not bit Addressable TL0 89 Not bit Addressable TMOD 88 8F 8E 8D 8C 8B 8A 89 88 TCON 87 Not bit Addressable PCON 83 Not bit Addressable DPH 82 Not bit Addressable DPL 81 Not bit Addressable SP 80 87 87 85 84 83 82 81 80 P0 13. ES – Special Function Registers Embedded Systems 48 Consider the following two points about the SFR addresses. A special function register can have an address between 80H to FFH. These addresses are above 80H, as the addresses from 00 to 7FH are the addresses of RAM memory inside the 8051. Not all the address space of 80 to FF are used by the SFR. Unused locations, 80H to FFH, are reserved and must not be used by the 8051 programmer. In the following example, the SFR registers’ names are replaced with their addresses. CY AC F0 RS1 RS0 OV – P We can select the corresponding Register Bank bit using RS0 and RS1 bits. RS1 RS2 Register Bank Address 0 0 0 00H-07H 0 1 1 08H-0FH 1 0 2 10H-17H 1 1 3 18H-1FH The Program Status Word (PSW) contains status bits to reflect the current state of the CPU. The 8051 variants provide one special function register, PSW, with this status information. The 8251 provides two additional status flags, Z and N, which are available in a second special function register called PSW1. CY PSW.7 Carry Flag AC PSW.6 Auxiliary Carry Flag F0 PSW.5 Flag 0 available to user for general purpose. RS1 PSW.4 Register Bank selector bit 1 RS0 PSW.3 Register Bank selector bit 0 OV PSW.2 Overflow Flag - PSW.1 User definable FLAG P PSW.0 Parity FLAG. Set/ cleared by hardware during instruction cycle to indicate even/odd number of 1 bit in accumulator. Embedded Systems 49 A timer is a specialized type of clock which is used to measure time intervals. A timer that counts from zero upwards for measuring time elapsed is often called a stopwatch. It is a device that counts down from a specified time interval and used to generate a time delay, for example, an hourglass is a timer. A counter is a device that stores (and sometimes displays) the number of times a particular event or process occurred, with respect to a clock signal. It is used to count the events happening outside the microcontroller. In electronics, counters can be implemented quite easily using register-type circuits such as a flip-flop. Download 1.78 Mb. Do'stlaringiz bilan baham: 
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