Difference between revisions of "ATmega documentation"
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Revision as of 15:10, 10 September 2019
This is an attempt to write documentation for the ATmega328P family of microcontrollers (and learn it myself as I go along). Send any corrections, suggestions, or other comments to email@example.com The ATmega328P Microcontroller - 1 Setting up the I/O pins - Sept. 9, 2019 version Note: In the following, the more negative voltage (Gnd) is 0 & the more positive voltage (Vcc) is 1. Addresses & data are given in hexadecimal. The ATmega328P is one of a large family of similar 8-bit microcontrollers and can operate on 1.8 to 5.5 V. The data sheet is available at ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf (in the following the section describing an item is often given afterwards in parentheses, i.e. SREG(7.3.1)); the instruction set manual for this family is available at ww1.microchip.com/downloads/en/devicedoc/atmel-0856-avr-instruction-set-manual.pdf (note that the 328P does not have all of the instructions listed) & also see en.wikipedia.org/wiki/Atmel_AVR_instruction_set. Other members differ in various ways; consult their data sheets for details. The ATmega328P has 32 8-bit general purpose registers (which can also be addressed as data memory locations 0000-001F), 224 8-bit I/O registers (which can also be addressed as data memory locations 0020-00FF; note that 20 must therefore be added to the I/O register number when addressing it as memory), 2kx8 data RAM(8.3) (data memory locations 0100-08FF), a separate 16kx16 program flash memory(8.2) (note that while it is an 8-bit chip the instructions are 16 bits), & a separate 1kx8 EEPROM(8.4). The ATmega328P is available as a 28-pin DIP (more exotic packages are also avilable) with 23 general purpose I/O pins which can also have specialized functions(14.3.1-14.3.3); in particular, pin 1 is an external reset(11.4) unless RSTDISBL(28.2) has been programmed to 0. Altho each pin can be individually controlled, they are grouped into the 8 bit port B (B0-B7), 7 bit port C (C0-C6), & 8 bit port D (D0-D7). 1=C6 B7=10 2=D0 B6=9 3=D1 B5=19 4=D2 B4=18 5=D3 B3=17 6=D4 B2=16 9=B6 B1=15 10=B7 B0=14 11=D5 C6=1 12=D6 C5=28 13=D7 C4=27 14=B0 C3=26 15=B1 C2=25 16=B2 C1=24 17=B3 C0=23 18=B4 D7=13 19=B5 D6=12 23=C0 D5=11 24=C1 D4=6 25=C2 D3=5 26=C3 D2=4 27=C4 D1=3 28=C5 D0=2 There are 3 I/O registers associated with each port: DDR(14.4.3, 14.4.6, 14.4.9) (data direction register), PORT(14.4.2, 14.4.5, 1.4.8), & PIN(14.4.4, 14.4.7, 14.4.10). The DDR determines whether the pins are inputs or outputs; if a bit in the DDR is 0 the corresponding pin is an input; if 1 it is an output. When the chip is reset (which happens automatically when power is turned on) all the DDR & PORT bits are cleared making all pins inputs; bit 6 if pin 1 is reset & unused bit 7 of DDRC, PORTC, & PINC are always 0. The PORT register contains the value that is output on the corresponding pins that are set as outputs. Note that if a bit corresponding to an input pin is set to 1 an internal "pullup" resistor is connected from the positve voltage to that pin unless PUD(14.4.1) has been set to 1. Reading the PIN register gives the value of the corresponding pins (regardless of whether they are an input or output). Note that unconnected inputs are not defined and may give erratic values when read. Unconnected inputs can also cause high power consumption so unused inputs(14.2.6) should be connected to something; this is easily done by turning on the pullup resistors. Also note that there is a 1 instruction delay(14.2.4) between writing data to the PORT and having it appear in the PIN register. Other than that, the value of PORT should be the same as PIN at those positions that are outputs except for pins that are heavily loaded (which is best avoided, at least at higher operating voltages). This can be used to do a limited self-test of the circuit. Note that while writing 0 to a PIN bit does nothing, writing a 1 will cause the corresponding bit in PORT to change state(14.2.2) (from 1 to 0 or from 0 to 1). Here are the I/O addresses for the various registers: 03 PINB 04 DDRB 05 PORTB 06 PINC 07 DDRC 08 PORTC 09 PIND 0A DDRD 0B PORTD Suppose that pin 1 is reset, 2-6 & 9 are to be inputs with the pullup resistors on for 2-3, 15-19 & 23-27 are to be outputs with 15-16 having an initial value of 0 & 17-18 having an initial value of 1, & 10-14 & 28 are unused. Here is the port bit map (the port bit is given first followed by the corresponding pin, the value of the DDR for that position, the value (if the value doesn't matter it is generally set to 0 in these examples) of the PORT for that position, & a description; after the last bit of a port the hexadecimal value of DDR & PORT is shown): B7 10 0 1 unused (set PORT to 1 for unused pins to turn on pullup resistor) B6 9 0 0 in B5 19 1 0 out B4 18 1 1 out, =1 B3 17 1 1 out, =1 B2 16 1 0 out, =0 B1 15 1 0 out, =0 B0 14 0 1 unused DDRB=3E PORTB=99 C6 1 0 0 reset C5 28 0 1 unused C4 27 1 0 out C3 26 1 0 out C2 25 1 0 out C1 24 1 0 out C0 23 1 0 out DDRC=1F PORTC=20 D7 13 0 1 unused D6 12 0 1 unused D5 11 0 1 unused D4 6 0 0 in D3 5 0 0 in D2 4 0 0 in D1 3 0 1 in, pullup on D0 2 0 1 in, pullup on DDRD=00 PORTD=E3 The following code fragment sets up the pins (in all code examples the program memory address is given first (4 hexadecimal digits) followed by the opcode (4 digits), the instruction, & an explanation). When power is turned on the processor resets and starts at address 0000(11.1). However, other things can cause execution to start nearby (for example, INT0 can cause execution to start at 0002(12.4)) so the first instruction is often a jump to get out of the way. 0000 940C JMP 0001 1000 1000 ... 1000 E919 LDI R17,99 Load value to turn on pullup resistors for unused pins 14 & 10, set pins 15-16 to 0, & pins 17-18 to 1 into temporary register 1001 B915 OUT 05,R17 Store into PORTB; note that this will immediately turn on the pullup resistors on pins 17-18 (which should not cause a problem since those pins are about to be set to 1 (positive)) 1002 E32E LDI R18,3E Load value to make pins 15-19 outputs 1003 B924 OUT 04,R18 Store into DDRB 1004 E230 LDI R19,20 Load value to turn on pullup for unused pin 28 1005 B938 OUT 08,R19 Store into PORTC 1006 E14F LDI R20,1F Load value to make pins 23-27 outputs 1007 B947 OUT 07,R20 Store into DDRC 1008 EE53 LDI R21,E3 Load value to turn on pullups for pins 2-3 & unused pins 11-13 1009 B95B OUT 0B,R21 Store into PORTD 100A rest of program No need to load DDRD since it was reset to 00 when power was turned on The ATmega328P Microcontroller - 2 Adding a self-test - Sept. 9, 2019 version The self-test uses the fact that PORT & PIN should normally have the same value for output pins to test for excessive loads. The test should be repeated with each output pin set to both 0 & 1 to detect excessively low resistance to either Vcc or Gnd and adjacent pins should have opposite values to detect shorts between pins. Using the previous example, suppose that pin 1 is reset, 2-6 & 9 are to be inputs with the pullups on for 2-3, 15-19 & 23-28 are to be outputs with 15-16 initially being 0 & 17-18 being 1 and that pin 16 cannot be set to 1 (except under special conditions) and pin 23 is heavily loaded, with pins 10-14 unused. Pin 28 is to be a self-test output (0=fail, 1=pass). Note that the self-test momentarily sets pin 28 to 0 so this must not trigger an error response. Here is the port bit map; 3 values are given for PORTB (the first 2 being the values used for self-testing & the 3rd being the final value that sets the outputs to the required initial values) & 3 values are given for PORTC (the first 2 again being the values used for self-testing, the 2nd is also the final value if the self-test passes (there is no need to change it since pin 28 is already 1 & none of the other outputs need to be set to a specific value), & the 1st or 3rd is the final value if the self-test fails). B7 10 0 1 1 1 unused B6 9 0 0 0 0 in B5 19 1 0 1 0 out B4 18 1 1 0 1 out, =1 B3 17 1 0 1 1 out, =1 B2 16 1 0 0 0 out, =0, cannot be set to 1 B1 15 1 0 1 0 out, =0 B0 14 0 1 1 1 unused DDRB=3E PORTB=91,AB,99 C6 1 0 0 0 0 reset C5 28 1 0 1 0 out, self-test C4 27 1 1 0 0 out C3 26 1 0 1 1 out C2 25 1 1 0 0 out C1 24 1 0 1 1 out C0 23 1 1 0 0 out, heavy load DDRC=3F PORTC=15,2A,0A D7 13 0 1 unused D6 12 0 1 unused D5 11 0 1 unused D4 6 0 0 in D3 5 0 0 in D2 4 0 0 in D1 3 0 1 in, pullup on D0 2 0 1 in, pullup on DDRD=00 PORTD=E3 The following code fragment does the self-test and sets up the pins. 0000 940C JMP 0001 1000 1000 ... 1000 E911 LDI R17,91 Load value to turn on pullups for unused pins 10 & 14 and do 1st B test (B2 can't be 1) 1001 B915 OUT 05,R17 Store into PORTB 1002 E32E LDI R18,3E Load value to make pins 15-19 outputs 1003 B924 OUT 04,R18 Store into DDRB 1004 E135 LDI R19,15 Load value to do 1st C test 1005 B938 OUT 08,R19 Store into PORTC 1006 E34F LDI R20,3F Load value to make pins 23-28 outputs 1007 B947 OUT 07,R20 Store into DDRC 1008 EE53 LDI R21,E3 Load value to turn on pullups for pins 2-3 & unused pins 11-13 1009 B95B OUT 0B,R21 Store into PORTD 100A B176 IN R23,06 Load temporary register from PINC 100B 2773 EOR R23,R19 Clear bits where PINC=PORTC 100C 737E ANDI R23,3E Clear heavily loaded C0 (& the bits that aren't outputs, but they're already 0 anyway) 100D F489 BRNE 11 PINC outputs besides C0 not same as PORTC, self-test failed, pin 28 already 0, go to 101F (note that the branch distance is relative to the following instruction so this jumps ahead 12) 100E B173 IN R23,03 Load from PINB 100F 2771 EOR R23,R17 Clear bits where PINB=PORTB 1010 2372 AND R23,R18 Clear bits that aren't outputs (ANDI R23,3E would work equally well) 1011 F469 BRNE 0D PINB outputs not same as PORTB, fail, pin 28 already 0, go to 101F 1012 EA1B LDI R17,AB Load value that reverses B output pins except pin 16 1013 B915 OUT 05,R17 Store into PORTB 1014 E23A LDI R19,2A Load value that reverses C output pins 1015 B938 OUT 08,R19 Store into PORTC 1016 B173 IN R23,03 Load from PINB (note that PINC can't be tested yet because of 1 instruction delay) 1017 2771 EOR R23,R17 Clear bits where PINB=PORTB 1018 2372 AND R23,R18 Clear bits that aren't outputs 1019 F421 BRNE 04 PINB outputs not same as PORTB, fail, go to 101E 101A B176 IN R23,06 Load from PINC 101B 2773 EOR R23,R19 Clear bits where PINC=PORTC 101C 737E ANDI R23,3E Clear bit C0 101D F009 BREQ 01 PINC=PORTC, pass, no need to test port D since none of the pins are outputs, pin 28 already 1, go to 101F 101E 9845 CBI 08,5 Set pin 28 to 0 101F E919 LDI R17,99 Load value to turn on pullups for unused pins 10 & 14, set 15-16 to 0, & 17-18 to 1 1020 B915 OUT 05,R17 Store into PORTB 1021... rest of program To turn on an LED if the self-test passes (recommended because it indicates that the initialization routine executed) connect the positive LED lead to pin 28 & the negative lead thru a resistor (the value depends on the voltage used) to Gnd. To do more than just set pin 28 if the self-test fails, add additional code after 101E & change the value of BREQ at 101D to jump past the end of the added code. For example, this alternate ending will halt the program at the point the self-test failed to make it easier to find (as described below) the problem: 101D F011 BREQ 02 Pass, go to 1020 (increased by 1 since 1 instruction added) 101E 9845 CBI 08,5 Set pin 28 to 0 101F CFFF RJMP FFF Go to 101F (halt by going into infinite loop) 1020 E919 LDI R17,99 Load B value 1021 B915 OUT 05,R17 Store into PORTB 1022... rest of program If the self-test does not pass, check for other problems. Measure the voltage (unless otherwise stated, always measure voltages relative to Gnd (pins 8 & 22)) on pin 7 (Vcc); if it is negative power is connected backwards, if it is 0 measure the outputs of the power source directly. If the power source voltage is 0 make sure it is turned on and test the batteries & check for corroded battery contacts or make sure it is plugged in to an outlet that has power. If the power source voltage is correct there is a wiring error or the microcontroller board is faulty. If the pin 7 voltage is correct measure the voltage at pin 28; if it is not 0 test the resistor & LED, make sure the LED isn't backwards, & check for a wiring error. If pin 28 is 0 connect a 1k resistor from pin 28 to Vcc; if it now reads >1V the pin apparently was not initialized to an output, check the program for errors (beware typos) & make sure it was loaded into the microcontroller correctly. If it still reads 0 the self-test really did fail. Check the voltages on all of the output pins. Suppose that the supply is 3V & the voltages are 0,3,0,3,0,1 at pins 28-23 & 3,3,0,0,0 at 19-15. The odd value at 23 is not unexpected because of the heavy load and 28-24 match the 1st test value for port C indicating that the first part of the self-test (at 100D) passed (but the test failed before PORTC was reloaded at 1015). Pins 19-15 match the 1st value for port B except for pin 19, which should be 0 but was 3V. This indicates that pin 19 is shorted to Vcc, check the wiring to that pin. On the other hand, if 19-15 were 2,2,0,0,0 that would indicate that pins 19 & 18 are shorted together.