Texas Instruments MSC1210 Reset Line RST, Address Latch Enable ALE, Program Store Enable Psen

MSC1210 Pin-Out

1.2.3Reset Line (RST)

Pin 13 is the master reset line for the microcontroller. When this pin is brought high for two instruction cycles, the microcontroller is effectively reset. SFRs, including the I/O ports, are restored to their default conditions and the program counter is reset to 0000H. Keep in mind that Internal RAM is not affected by a reset. The microcontroller begins executing code at 0000H when pin 13 re- turns to a low state.

The reset line is often connected to a reset button/switch that you can press to reset the circuit. It is also common to connect the reset line to a watchdog IC or a supervisor IC (such as MAX707). Traditional resistor-capacitor net- works attached to the reset line also work well because the RST input is a Schmitt trigger input.

1.2.4Address Latch Enable (ALE)

The ALE at pin 45 is an output-only pin that is controlled entirely by the micro- controller and allows the microcontroller to multiplex the low-byte of a memory address and the 8-bit data itself on port 0. This is because, while the high byte of the memory address is sent on port 2, port 0 is used both to send the low byte of the memory address and the data itself. This is accomplished by plac- ing the low byte of the address on port 0, exerting an ALE high-to-low transition to latch the low byte of the address into a latch IC (such as the 74HC573), and then placing the 8 data bits on port 0. In this way, the MSC1210 is able to output a 16-bit address and an 8-bit data word with 16 I/O lines instead of 24.

The ALE line is used in this fashion both to access external RAM with MOVX @DPTR, as well as to accessi instructions in external code memory. When the pro- gram is executed from external code memory, ALE pulses at a rate that is ¼ that of the oscillator frequency. Thus, if the oscillator operates at 11.0592MHz, ALE pulses at a rate of 2 764 800 times per second. When the MOVX instruction is exe- cuted, one PSEN pulse is missed in lieu of a pulse on WR or RD.

This pin is also used when programming the part, along with PSEN, as an input during reset to indicate whether programming will occur in serial or parallel mode. If this line is held high when in programming mode, programming will occur in serial mode.

1.2.5Program Store Enable (PSEN)

The program store enable (PSEN) line at pin 44 is exerted low automatically by the microcontroller whenever it accesses external code memory. This line should be attached to the output enable (OE) pin of the device that contains your code memory. The PSEN signal is applied for both internal and external memory access.

This pin is also used when programming the part, along with ALE, as an input to indicate whether programming will occur in serial or parallel mode. If this line is held high when in programming mode, programming will occur in parallel mode.

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