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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 gippgig@gmail.com
 
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The ATmega328P Microcontroller - 1 Setting up the I/O pins - Sept. 19, 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 are outputs, 9-13 are unused, 14-19 are inputs with the pullup resistors on for 14-15, 23 is an output set to 0, 24 is unused, & 25-28 are outputs with 25-27 set to 1. Also suppose that setting pin 23 to 1 (except under special conditions) could damage the circuit; to reduce the chance of this happening this output is placed next to Gnd (pin 22), a resistor (~5k ohms suggested) is connected from the pin to Gnd, & the other adjacent pin (24) is unused & is connected thru a "pulldown" resistor (1k suggested) to Gnd instead of using the internal pullup. 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 1 unused
 
B5 19 0 0 in
 
B4 18 0 0 in
 
B3 17 0 0 in
 
B2 16 0 0 in
 
B1 15 0 1 in, pullup on
 
B0 14 0 1 in, pullup on  DDRB=00 PORTB=C3
 
C6  1 0 0 reset
 
C5 28 1 0 out
 
C4 27 1 1 out, =1
 
C3 26 1 1 out, =1
 
C2 25 1 1 out, =1
 
C1 24 0 0 unused, resistor to Gnd
 
C0 23 1 0 out, =0, must not be 1, resistor to Gnd  DDRC=3D PORTC=1C
 
D7 13 0 1 unused
 
D6 12 0 1 unused
 
D5 11 0 1 unused
 
D4  6 1 0 out
 
D3  5 1 0 out
 
D2  4 1 0 out
 
D1  3 1 0 out
 
D0  2 1 0 out  DDRD=1F PORTD=E0
 
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 E13C LDI R19,1C Load value to set pin 23=0 & pins 25-27=1 into arbitrarily chosen temporary register
 
1001 B938 OUT 08,R19 Store into PORTC (note that this will immediately turn on the pullup resistors for pins 25-27 which shouldn't matter since they are about to be set to 1)
 
1002 E34D LDI R20,3D Load value to make pins 23 & 25-28 outputs
 
1003 B947 OUT 07,R20 Store into DDRC
 
1004 EE50 LDI R21,E0 Load value to turn on pullups for unused pins 11-13
 
1005 B95B OUT 0B,R21 Store into PORTD
 
1006 E16F LDI R22,1F Load value to make pins 2-6 outputs
 
1007 B96A OUT 0A,R22 Store into DDRD
 
1008 EC13 LDI R17,C3 Load value to turn on pullups for pins 14-15 & unused pins 9-10
 
1009 B915 OUT 05,R17 Store into PORTB
 
100A rest of program No need to load DDRB since it was reset to 00 when power was turned on
 
  
If possible, do not connect or disconnect whatever would be damaged if pin 23 was 1 & connect it to Gnd instead if applicable. Once the rest of the circuit is built, turn on a voltmeter, set it on DC volts, & select the lowest range that includes the voltage chosen to power the circuit. Before connecting power to the circuit turn on the power source, select the correct voltage, & measure the output. If the voltage is wrong make sure the power source is plugged into an outlet that has power or test the batteries & check for corroded battery contacts. If the voltage is correct verify which connection is positive & which is negative. Turn off power, connect power to the circuit MAKING SURE THE POSITIVE VOLTAGE IS CONNECTED TO VCC, turn on power, & measure the voltage at pin 7 (Vcc). If it is negative shut off power (& hope the circuit hasn't blow out) because the power is somehow connected backwards, if it is 0 check for a loose connection or short circuit, if it is low check for weak batteries or a short.
 
If Vcc is correct check the voltage at pin 23. If it is not 0 find & fix the problem and repeat the check. If it is 0 turn off power, connect the rest of the circuit, & turn on power. Check pin 23 again; if it is not 0 shut off power, find & fix the problem, & repeat the check. If it is 0 repeat the check of Vcc. If Vcc is not correct fix the problem, repeat the check of pin 23, & then test Vcc again. If Vcc is correct see if the circuit works.
 
 
The ATmega328P Microcontroller - 2 Adding a self-test - Sept. 19, 2019 version
 
The self-test works by checking whether the pins have the expected values. Note that the value of PIN should equal the value of PORT for output pins unless a pin is heavily loaded (which should generally be avoided, especially at higher voltage); this is also the case for unused pins with the pullup enabled & unused pins with the pullup off if pulled down to Gnd (i.e., pin 24). Unless it would cause something bad to happen to whatever the microcontroller is controlling, 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 are outputs, 9-12 are unused, 13 is an output, 14-19 are inputs with the pullup resistors on for 14-15, 23 is an output set to 0, 24 is unused, & 25-28 are outputs with 25-27 set to 1. Pin 6 is heavily loaded, pin 13 is the self-test output (0=fail, 1=pass), pin 15 should initially be 1 & pin 17 0, pin 18 should be 1 after the self-test sets pin 5 to 1, pin 19 should be the same as pin 16, pin 23 must not be 1 & has a resistor connected to Gnd, & pin 24 also has a resistor to Gnd. Note that the self-test momentarily sets pin 13 to 0 so this must not trigger an error response. Here is the port bit map; 3 values are given for PORTC (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 PORTD (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 13 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 unused
 
B6  9 0 1 unused
 
B5 19 0 0 in, initially same as B2
 
B4 18 0 0 in, =1 after self-test sets pin 5=1
 
B3 17 0 0 in, initially 0
 
B2 16 0 0 in, initially same as B5
 
B1 15 0 1 in, pullup on, initially 1
 
B0 14 0 1 in, pullup on  DDRB=00 PORTB=C3
 
C6  1 0  0 0 0 reset
 
C5 28 1  0 1 0 out
 
C4 27 1  1 0 1 out, =1
 
C3 26 1  0 1 1 out, =1
 
C2 25 1  1 0 1 out, =1
 
C1 24 0  0 0 0 unused, resistor to Gnd
 
C0 23 1  0 0 0 out, =0, must not be 1, resistor to Gnd  DDRC=3D PORTC=14,28,1C
 
D7 13 1  0 1 0 out, self-test
 
D6 12 0  1 1 1 unused
 
D5 11 0  1 1 1 unused
 
D4  6 1  1 0 0 out, heavy load
 
D3  5 1  0 1 1 out
 
D2  4 1  1 0 0 out
 
D1  3 1  0 1 1 out
 
D0  2 1  1 0 0 out  DDRD=9F PORTD=75,EA,6A
 
The following code fragment does the self-test and sets up the pins.
 
0000 940C JMP
 
0001 1000 1000
 
...
 
1000 EC13 LDI R17,C3 Load value to turn on pullups for 14-15 & unused 9-10
 
1001 B915 OUT 05,R17 Store into PORTB
 
1002 E134 LDI R19,14 Load value to do 1st C test
 
1003 B938 OUT 08,R19 Store into PORTC
 
1004 E34D LDI R20,3D Load value to make 23 & 25-28 outputs
 
1005 B947 OUT 07,R20 Store into DDRC
 
1006 E755 LDI R21,75 Load value to do 1st D test
 
1007 B95B OUT 0B,R21 Store into PORTD
 
1008 E96F LDI R22,9F Load value to make 2-6 & 13 outputs
 
1009 B96A OUT 0A,R22 Store into DDRD
 
100A B176 IN R23,06 Load temporary register from PINC
 
100B 2773 EOR R23,R19 Clear bits where PINC=PORTC
 
100C F4E1 BRNE 1C PINC not same as PORTC, self-test failed, pin 13 already 0, go to 1029 (note that the branch distance is relative to the following instruction so this jumps ahead 1D)
 
100D B179 IN R23,09 Load from PIND
 
100E 2775 EOR R23,R21 Clear bits where PIND=PORTD
 
100F 7E7F ANDI R23,EF Clear heavily loaded D4
 
1010 F401 BRNE 18 PIND outputs not same as PORTD besides D4, fail, pin 13 already 0, go to 1029
 
1011 B173 IN R23,03 Load from PINB
 
1012 7E7E ANDI R23,EE Clear unpredictable bits B0 & B4
 
1013 3C72 CPI R23,C2 Test for pass with B2 & B5=0
 
1014 F011 BREQ 02 Pass, go to 1017
 
1015 3E76 CPI R23,E6 Test for pass with B2 & B5=1
 
1016 F491 BRNE 12 PORTB wrong, fail, pin 13 already 0, go to 1029
 
1017 E238 LDI R19,28 Load value to reverse C outputs except C0
 
1018 B938 OUT 08,R19 Store into PORTC
 
1019 EE5A LDI R21,EA Load value to reverse D outputs
 
101A B95B OUT 0B,R21 Store into PORTD
 
101B B176 IN R23,03 Load from PINC (note that PIND couldn't be tested yet because of 1 instruction delay)
 
101C 2773 EOR R23,R19 Clear bits where PINC=PORTC
 
101D F451 BRNE 0A PINC outputs not same as PORTC, fail, go to 1028
 
101E B179 IN R23,09 Load from PIND
 
101F 2775 EOR R23,R21 Clear bits where PIND=PORTD
 
1020 7E7F ANDI R23,EF Clear heavily loaded bit D4
 
1021 F431 BRNE 06 PIND not same as PORTD, fail, go to 1028
 
1022 B173 IN R23,03 Load from PINB
 
1023 7F7E ANDI R23,FE Clear unpredictable bit B0
 
1024 3D72 CPI R23,D2 Test for pass with B2 & B5=0
 
1025 F019 BREQ 03 Self-test passes, pin 13=1, go to 1029
 
1026 3F76 CPI R23,F6 Test for pass with B2 & B5=1
 
1027 F009 BREQ 01 Self-test passes, pin 13=1, go to 1029
 
1028 985F CBI 0B,7 Set pin 13 to 0
 
1029 E13C LDI R19,1C Load value to set 23=0 & 25-27=1
 
102A B938 OUT 08,R19 Store into PORTC
 
102B... 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. However, the LED may be too dim to see if the voltage is under 2V (test by connecting it directly from Vcc to Gnd (no resistor needed if under 2V)).
 
To do more than just set pin 13 if the self-test fails, add additional code after 1028 & change the value of BREQ at 1025 & 1027 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:
 
1025 F021 BREQ 04 Pass, go to 102A (increased by 1 since 1 instruction added)
 
1026 3E76 CPI R23,E6 Test for pass with B2 & B5=1
 
1027 F011 BREQ 02 Pass, go to 102A (increased by 1)
 
1028 985F CBI 0B,7 Set pin 13 to 0
 
1029 CFFF RJMP FFF Go to 1029 (halt by going into infinite loop)
 
102A E13C LDI R19,1C Load C value
 
102B B938 OUT 08,R19 Store into PORTC
 
102C... rest of program
 
(In this particular case only, it makes sense to also change the BRNE at 100C,1010,& 1016 to F7F9 BRNE 7F to halt the program at those points.)
 
 
After all the tests described previously pass, if the self-test does not pass 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 2.7k 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 pins. Suppose that the supply is 3V & the voltages on pins 2-6 are 3,0,0,0,2, on 9-19 are 3,3,3,3,0,0,3,3,0,0,3, & 23-28 are 0,0,3,0,3,0. The odd value at pin 6 is not unexpected because of the heavy load and 23-28 match the 1st test value for port C indicating that the first part of the self-test (at 100C) passed (but the test failed before PORTC was reloaded at 1018). Pins 2-6 & 11-13 match the 1st value for port B except for pin 4 (B2), which should be 3V but was 0. This indicates that pin 4 is shorted to Gnd, check the wiring to that pin. On the other hand, if 2-6 were 3,1,1,0,2 that would indicate that pins 3 & 4 are shorted together. Fix the problem & try again.
 
If the self-test passes see if the circuit works.
 
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Latest revision as of 23:49, 28 September 2019