EE251: Thursday September 20

Transcription

1 EE251: Thursday September 20 Parallel I/O aka General Purpose I/O aka GPIO Common Devices: Switches, LEDs, Keypads Read Lab 4 carefully, and Chapter 14 in text Think about what you would like to review before mid-term. Mid-Term Exam is 2 weeks from today, October 4, in class. See web page for sample problems and topics we ve covered. Review in class on October 2. There will be one handwritten page allowed on this exam, one side of 8½ by 11 paper. You will be given appropriate reference material in the exam. Lab 3 is due next week Homework: #3 due next week. Lecture #10 1

2 A Particular Independent Project Create an interface to the x2 LCD Display Module, using HD44780 controller A GPIO device (see today s lecture) Most of code is available, but needs completion. Not very difficult, but should be done earlier rather than later in the semester TA/Instructor Consulting help available Lecture #10 2

3 Lecture #10 3

4 Lecture #10 4

5 Port System Ports, like harbors where ships are loaded and unloaded, are used to communicate with the external environment, and they each have 2 sides: Internally, each port is a register with a memory address that can be read or written. (See next slide) Externally, each port is a collection of up to 8 wires used to connect to external devices, to send data to those devices or to receive data from those devices (See slide-after-next) Lecture #10 5

6 Internal Port View Internally the processor communicates with Ports as memory addresses in special I/O memory area: Starts at address 0x The SAME instructions used with memory also control I/O! E.g. LDRB STRB (So, you can already do I/O, right?) Why the Byte load and store? See next slide. Memory-mapped I/O Architecture in industry. See Wikipedia entry. Lecture #10 6

7 External Parallel View The external side of these ports are wires (up to 8 per port) that connect to the outside world: Up to 8 pieces of data can be simultaneously: read from these wires or written to these wires This is why it s called Parallel I/O as well as GPIO It is a basic capability of ARM (and most processors); a powerful feature for doing fast I/O operations Some ports have fewer than 8 wires but exactly the same method of communication is used. Lecture #10 7

8 General-Purpose Input and Output (GPIO) Each GPIO port has A Data register (e.g. GPIO_PORTA_DATA) where each input or output value of a data pin is held by or fed to a bit in that register (up to 8 bits of data) A Direction register (e.g. GPIO_PORTA_DIR). The bits of that register correspond bit by bit with pins of the specified port. Setting a bit of the direction register to 1 makes it an output pin, while setting it to a 0 make it an input pin. (Yep, 1 = o and 0 = I. Go figure) And several more, but we ll get to the ones we need later. A Clock to each GPIO port is turned off by default to save power and must be turned on to use that port. The Run Clock Gate Control register for GPIO (RCGCGPIO) is used to turn on/off the clock to each GPIO Port independently. Lecture #10 8

9 TM4C Board vs. Our Text GPIO is done very differently on our board vs. that on the STM processor described in the text. The text information about GPIO in general ( ) is important. Read and understand this. The specific device information ( ), especially code examples, is less helpful, but the flow charts are good. We will give you code examples for the TM4C. For our GPIO see the TM4C123GH6PM Microcontroller DATA SHEET (TM4C DATA SHEET) referenced on our website, Chapter 10. And see the following slides. Lab 4 also has an excellent description of how GPIO works on our TM4C boards. Read and understand this well! Lecture #10 9

10 Tiva TM4C123GH6PM Microcontroller DATA SHEET (p. 340) on ECE251 web page TMI! Lecture #10 10

11 Tiva TM4C123GH6PM Microcontroller DATA SHEET (p. 340) on ECE251 web page Lecture #10 11

12 TM4C DATA SHEET(p. 340 continued) Register Address is Base + Offset = 0x400F.E Lecture #10 12

13 Ports/Pins and Their Locations on Our Board See TM4C Launchpad User s Guide: Ports A, B, C, D: Pins 7-0 Port E: Pins 5-0 Port F: Pins 4-0 How many total pins on our board? That s a lot! Lecture #10 13

14 Configuring Ports example Ports B,E I/O Register Addresses Used Lab 4 GPIO_PORTB_DATA EQU 0x400053FC ;PortB_DATA R/W all bits GPIO_PORTB_DIR EQU 0x GPIO_PORTB_AFSEL EQU 0x GPIO_PORTB_DEN EQU 0x C IOB EQU 0x07 ;for DIR setting bits 0-2 GPIO_PORTE_DATA EQU 0x400243FC ;PortE_DATA R/W all bits GPIO_PORTE_DIR EQU 0x GPIO_PORTE_AFSEL EQU 0x GPIO_PORTE_DEN EQU 0x C IOE EQU 0x00 ;for DIR setting SYSCTL_RCGCGPIO EQU 0x400FE608 ;clock control all ports ; This code can be copied from this PDF file. Lecture #10 14

15 Configuring Ports example Ports B,E Getting Started AREA.text, READONLY, CODE, ALIGN=2 THUMB EXPORT Start ; Turn on Clock to Ports B (bit 1) and E (bit 4) Start LDR R1, =SYSCTL_RCGCGPIO ;Clock Register LDR R0, [R1] ORR R0, R0, #2_ ; Bits for Ports E,B STR R0, [R1] NOP ; Execute 3 NOP instructions NOP ; to stabilize clocks before NOP ; using them Lecture #10 15

16 Configuring Ports example Ports B,E Doing Configuration on B (E similar) LDR R1, =GPIO_PORTB_DIR ; Direction Register Addr LDR R0, [R1] ; Direction Register Data BIC R0, #0xFF ; Clear lower 8 bits ORR R0, #IOB ; Set 2,1,0 to output STR R0, [R1] ; Store back LDR R1, =GPIO_PORTB_AFSEL ; Alternate Fn Reg Addr LDR R0, [R1] BIC R0, #0xFF ; Clear all-use GPIO fn STR R0, [R1] LDR R1, =GPIO_PORTB_DEN ; Digital Output Enable LDR R0, [R1] ; Register. Tristate at reset ORR R0, #0xFF ; so enable all STR R0, [R1] ; bits with drive Lecture #10 16

17 Digital Input and Output Devices Lecture #10 17

18 Switches as Input Devices V DD R To pin of port on processor When switch is in the open position, a logic 1 voltage (~V DD ) is provided to the port pin it is connected to. When switch is closed, the input to the pin is grounded and provides a logic 0 to this pin. The pull-up resistor R defines pin voltage when switch is open and limits the current flow when the switch is pushed. What determines size of this resistor? Answer: Make it as large as possible (to reduce power consumption), while still being able to provide enough drive current to port. It isn t critical 20 kω works fine. How would you reverse signal from this switch? (Open= 0 and Closed = 1 ) Lecture #10 18

19 Switches and HiZ Inputs A digital input can have three states: High, Low, and High- Impedance (also called floating, tri-stated, HiZ) If external input is HiZ, the input is read as a valid HIGH. If external input is HiZ, the input is read as a valid LOW. Lecture #10 19

20 Real Switches They bounce! They typically make/break contact several times when depressed and released. Multiple solutions can be used, (E.g. a filter) but cheapest and easiest for us is a simple delay in software. Don t read switch for about milliseconds after detecting that it has changed. If multiple switches are required, it s convenient to use DIP (Dual Inline Package), usually 8 per package, along with built-in pull-up resistors. These DIPs are slide switches, rather than pushbutton and are not appropriate for all applications. Anyone want an easier independent project? See me. Lecture #10 20

21 Output--Light Emitting Diodes (LEDs) anode cathode Most any color you want, including white. They really are diodes, and emit light in forward bias mode, i.e. anode at higher voltage than cathode. Care must be taken to bias LED correctly: about 1.5 volts, typical about 15 ma., typical (Note: TM4C123GH6PM pins deliver only 8 ma. max) Therefore, you must choose a series resistor carefully to give the right voltage and current level for the brightness you desire. You might also need a buffer to drive your LEDs Lecture #10 21

22 Output--LEDs continued Sizing the series resistor: If supply voltage V CC (aka V DD ) is +5 v. and brightly lit LED drops 1.5 v., then 3.5 v. is dropped across resistor (Kirchoff voltage law). If current I is 15 ma, then R=V/I=(5-1.5)/.015 = 233Ω (Ohms law). Choose a convenient value larger than this. (Why larger?) What if suppy voltage had been +3.6 v.? DIP LED bars are available to display 8 outputs simultaneously. Lecture #10 22

23 Output--Seven Segment Displays Common Anode Configuration Display consists of 7 individual bar LEDs or LCDs in shape above Can display all ten digits, as well as hex letters and several other symbols (c.f. ECE102 labs.) A simple SW routine can turn hex nibbles into the appropriate display. They are available as either common cathode or common anode (shown above). Lecture #10 23

24 Lab Keypad and TIVA Ports B & E Lecture #10 24

25 A Keypad Scan Routine Start Output 0 to all rows Read Column bits No Is any column low? Yes Call DELAY subroutine Key Number = 0 Row Pointer = 0 Column Pointer = 0 Output 0 to Row Pointer Read Column Pointer Is Column Pointer low? Yes Save the Key Number No Increment Column Pointer and Key Number Return No Was last column checked? Yes Increment Row Pointer No Was last row checked? Yes Lecture #10 25

26 Next Lecture GPIO Continued More detailed programming examples More Info on TM4C Input and Output Lab 4 Brief Discussion What do V DD, V CC and V SS stand for? What is the history of these names? Are they standard names? Look this up somewhere if you don t already know. Likely a quiz sometime next week Lecture #10 26