Parallel Input/Output. Microcomputer Architecture and Interfacing Colorado School of Mines Professor William Hoff

Transcription

1 Parallel Input/Output 1

2 Parallel Input/Output Ports A HCS12 device may have from 48 to 144 pins arranged in 3 to 12 I/O Ports An I/O pin can be configured for input or output An I/O pin usually serves multiple functions. When it is not used as a peripheral function, it can be used as a general-purpose I/O pin. From MC9S12C Family Reference Manual (document ReferenceManualS12C PUV2rev4) 2

3 Addressing I/O Ports When inputting or outputting to/from an I/O port, you read from or write to the port data register Each I/O port register is assigned to an address in the HCS12 memory space. For example, Port T data register is assigned to address $240: movb #$35,$240 ; output a $35 to Port T A name is also assigned to each register so that we can access a register by referring to its name: PTT equ $240 ; this is in the include file movb #$35,PTT ; output a $35 to Port T Names and addresses are defined in the mc9s12c32.inc file that you include your program using the include statement 3

4 I/O Ports in C Port names are also pre-defined for C programs, so you can just do PTT = 0x35; // output 35 (hex) to port T Definitions are in the include file mc9s12c32.h Note In C, port access is done with pointers, since you can t just do 0x240 = 0x35; // this is invalid See notes on Advanced C for how this is done 4

5 Bi-Directional Ports A data direction register (DDR) for each port, controls the direction of each port pin If the DDR bit is a 0, the pin is input Reading the data bit (e.g., D0) from the port gets the current value of the input pin 0 1 You can write a data bit, but it won t come out to the pin This circuit implements one bit of Port T 5

6 Bi-Directional Ports A data direction register (DDR) for each port, controls the direction of each port pin If the DDR bit is a 1, the pin is output Writing the data bit (D0) stores the bit into the latch You can still read out the value of the bit in the latch 1 0 6

7 General purpose digital IO Ports in the MC9S12C32 Port name # pins Pin names Notes * Pins 0 and 1 are hardwired to other things on the SSMI board (don t use them) T 8 PT0..PT7 Also used for timer functions PAD0 8 6 * PAD00..PAD07 Also used for A/D conversion M 6 PM0..PM5 Also used for communications interfaces (CANbus, SPI) E 2 PE0,PE1 Also used for interrupt inputs S 2 PS0,PS1 Also used for serial communications (so we can t use these unless we don t need communications to the PC) All port pins can be configured separately for digital input or output by writing a 0 or 1 to the associated data direction register Example: DDRT = 0xf0; // configure bits 0:3 of Port T for //.. input, bits 4:7 for output 7

8 (Side note on using Port PAD for digital I/O) Port PAD is normally used for A/D conversion However, you can also use it for digital I/O, by reading and writing to port PTAD To set up: In register ATDDIEN, set to 1 s the bits corresponding to the pins you want to use as digital I/O Then, as with the other ports, set the data direction register (DDRAD, in this case) to 0 for input and 1 for output Avoid PAD0 and PAD1 (they are hardwired to other components on the SSMI board) This is described in detail in the reference manual (MC9S12C128V1.pdf) on pages and

9 Example Continuously read switches S0..S3, and light LEDs L0..L3 if corresponding input signal is high PT7 PT6 PT5 PT4 PT3 PT2 PT1 PT0 L3 L2 L1 L0 S3 S2 S1 S0 main() { char c; DDRT = 0xf0; while (1) { c = PTT; // configure bits 0:3 for input, bits 4:7 for output // get data from Port T // We want to get bits 0:3 up into places 4:7. c = c * 16; // Multiply by 16 to shift left 4 places. } } PTT = c; // Output to Port T bits 4:7 (doesn t affect bits // 0:3 since they are configured as input) 9

10 Example: Seven Segment Display We can use 7 output pins (e.g., PT0:PT6) to drive the LEDs of a 7- segment display PT6 PT5 PT4 PT3 PT2 PT1 PT0 470 Ω each a a b f c b g d e e c f d g common cathode Digit abcdefg a b c d e f g 0 0x7E x x6D x x x5B x5F x x7F x7B What would be the code to display the hex digit A? 10

11 Example: Pushbutton switch You must use a pull-up or pull-down resistor, so that when the switch is open, you are reading a valid voltage logic level (as opposed to floating ). Pull-up Pull-down VDD VDD R PT0 PT0 R When the switch is pushed, is pin PT0 is high or low? When the switch is pushed, is pin PT0 is high or low? 11

12 Example: DIP switches A set of pull-up resistors are needed to pull the voltage to high on one side VCC of the DIP SW DIP-8 10 KΩ PA0 PA1 PA2 PA3 PA4 PA5 PA6 PA7 HCS12 DIP = dual-in-line package Figure 4.19 Connecting a set of eight DIP switches to Port A of the HCS 12 To read data from switches DDRA = 0x00; // configure port A for input n = PTA; // read port A Note: if you make a mistake in this program, you can physically damage the MCU! How? 12

13 Testing a single bit Sometimes you just want to test a single bit of an input Port (e.g., Port T) and don t care about the rest of the bits We will cover this more thoroughly in the next lecture, but you will need this for Lab 3 Example: To test if PT7 is inputting a 1 (and ignore all the other bits of Port T): if (PTT & 0x80!= 0) { // PT7 is a 1 : } else { // PT7 is a 0 : } This does a bit-wise AND of PTT and the mask 0x80. The result is zero in every bit position except bit 7 (which is the just the original value of PT7) 13

14 Example: Passive motion sensor Uses a pyroelectric sensor which can detect levels of infrared radiation Everything (including people) emits some low level infrared radiation The hotter something is, the more radiation is emitted The sensor is actually split in two halves If a person walks by, the relative amount of radiation seen by the two halves changes, and the output triggers TR257-1 From: 30 Years of Passive Infrared Motion Detectors - a Technology Review, by Hans J. Keller, KUBE Electronics Ltd, 14

15 Example: Digital motion sensor (continued) A Fresnel lens which splits up the field of view into a set of discrete directions, or zones The sensor has three pins: (1) +5V power, (2) a digital output signal, and (3) ground When the sensor detects a moving heat source, its output pin is pulled low for a short period of time (~2-3 seconds) Vcc MCU The output pin is open collector, so when it is not being pulled low, it is floating Therefore, you must use a pullup resistor so that when the output is not being pulled low, it is pulled up to a valid logic high 15

16 Example: Break beam sensor A break beam sensor can be constructed using a light source (e.g., an infrared LED) and a photodetector (e.g., a PIN diode) The light normally shines on the detector, which senses it If something passes between the source and detector, it breaks the beam and the detector can sense that the light is no longer there LED Problem: the detector is also sensitive to ambient light One solution: Modulate (turn on and off) the light source at a specific frequency Use a bandpass circuit on the detector that only passes a signal with that frequency 16

17 Example: Break beam sensor (continued) TSOP 4838 IR Receiver Module Sensitive to IR light at 950 nm, modulated at 38 khz Can be used to transmit data and do remote control applications (e.g., turn on a TV) from datasheet for TSOP 4838, 17

18 Summary / Questions Most digital I/O pins can be either input or output. How do you configure them for input or output? How many general purpose digital IO pins does our MC9S12C32 chip have? When driving an LED from a digital output pin, why should you use a resistor in series with the LED? 18