Intel mcs-48 manual 1 T1 Input, High Current Outputs, Expanded I/O

SINGLE COMPONENT SYSTEM

2.9.1T1 Input

The 8021 T1 input line can be used as an input for the following functions:

•Event Counter (external input)

•Test input for branch instructions

•Zero voltage crossing detection

The operation of T1 as an input to the Event Counter is described in the Timer/Event Counter section. When used as a test input, the JT1 and JNT1 instructions test for 1 and o levels, respectively.

The T1 pin can also be used to detect the zero crossing of slowly moving AC signals (60 Hz)'The self-biasing circuit shown in Figure 4 permits the Test 1 input to detect when the input voltage crosses zero within ±5%; the voltage is then coupled through a 1.01-lf capacitor. Maximum input voltage is 3V peak-to-peak. The zero cross detection is especially useful in SCR control of 60 Hz power and in developing time-of~dayand other timing routines. As a ROM mask option there is a pullup resistor that is useful for switch contact input or standard TTL.

2.9.2High Current Outputs

Very hig~ current drive is desirable for

minimizing external parts required to do high power control. P10 and P11 have been designated high drive outputs capable of sinking 7mA at Vss + 2.5 volts. (For clarity, this is 7mA to Vss with a 2.5 volt drop across the buffer.) These pins may, of course, be paralleled for 14mA drive if the output logic states are always the same.

2.9.3Expanded I/O

The 8021 can be used with the 8243 I/O expander chip, which provides additional I/O capability with a limited number of overhead pins. This chip has 4 directly addressable 4-bit ports. It connects to the PROG pin, which provides a clock, and pins P20-P23, which provide address and data. These ports can be written with a MOVD P,A; ANLD P,A; and ORlD P,A forPorts4-7. A high to low transition on PROG signifies that address and control are available on P20-P23. The previous data on P20-P23 before an output expander instruction is lost. Therefore, when using an output expander P20-P23 are not useful for general input/output. Reading is via the MOVD A,P. This circuit configuration is shown in Figure 5. The timing diagram is shown in Figure 6.

(0)ZERO CROSS OETECT EXTERNAL

The 8021 can also use standard low cost TTL to expand the number of I/O lines. An example is shown in the Applications section.

Ibl OPTIONAL PULLUP RESISTOR

EXTERNAL

SWITCH ~

FIGURE 4. TEST 1 PIN FIGURE 5. I/O EXPANDER INTERFACE

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