ATmega328P Ports Mapping

This is in continuation of last blog. I will validate the ATmega328P concepts with practical approach.

How ATmega328P Peripherals are mapped in Memory?

mem5

Which AVR core is used in ATmega328P ?

ATmega328P name implies that it is from megaAVR family of AVR core series. This precise information is required to understand instructions supported in this device.

Why there is a need to learn Assembly when we have C++ based Library ?

I will not recommend any hobbyist to do into in-depth assembly because that will nullify whole concept of Arduino. However there is no harm in spending some time to understand the importance of Assembly. Here are some basic points

High level language like C depends on Assembly for creating basic stack. You may find some information on video C Learning Tips with x86 platform
Assembly is helpful for designing optimized time critical functions
Assembly is helpful in understanding MCU architecture better because C is architecture independent language
Additional skill set for Engineering students interested in embedded domain

Is Learning assembly a tough job ?

Not at all, It’s not a rocket science. For RISC based architecture, You need mechanism:

To read(load) from a memory or memory mapped I/O
To write(store) to a memory or memory mapped I/O
To change program flow(branch) with or without a condition
To perform arithmetical and logical operations.

Examples for decoding AVR Assembly Code :

I wrote a simple C code to display basic GPIO ports and analyzed the disassembly of the same. The method to fetch assembly from executable file was discussed in blog.

Reading GPIO using Indirect Addressing :

dump_GPIO_registers() //From sample C code

You will notice that the disassembly file has a mixture of 16 and 32-bit instructions.

16-bit Instruction decoding

41a: 8b a1 ldd r24, Y+35 ; 0x23

LD (LDD) – Load Indirect from Data Space to Register using Index Y
- LDD Rd, Y+q 0 ≤ d ≤ 31, 0 ≤ q ≤ 63 //From Datasheet
- d=24=0x18 =0001_1000 //From instruction line given above
- q=35=0x23 = 0010_0011 //6 bits to address 64 I/O addresses
Opcode Explanation
- Note the sequence of d and q bits in the opcode below with color coding
- opcode format : 10q0 qq0d dddd 1qqq //From Data Sheet
- 1010_0001_1000_1011 =0xA1_8B //Match with disassembly code
- //after changing byte order
- Here is the logic explained in datasheet

32-bit Instruction decoding

Lets analyze a 32-bit instruction/opcode in the same code listing

41c: 0e 94 b3 01 call 0x366 ; 0x366 <_ZN5Print7printlnEhi.constprop.7>
- CALL k 0 ≤ k < 64K PC ← k Devices with 16-bit PC
- opcode format 1001 010k kkkk 111k kkkk kkkk kkkk kkkk
- K = 0x366=0000_0011_0110_0110 (shift right 1) ->0000_0001_1011_0011
  - Note: shift right is done above because opcode will have word addr
- 1001_0100_0000_1110_0000_0001_1011_0011 = 0x940E_01B3
  - 14 bit PC for 16K range

16 bit I/O specific instruction

OUT – Store Register to I/O Location
4c2: 0f be out 0x3f, r0 ; 63
OUT A,Rr 0 ≤ r ≤ 31, 0 ≤ A ≤ 63
1011 1AAr rrrr AAAA // Note: 6-bit I/O address can cover 64 I/O addresses
- A=0x3F=11_1111
- r=0x00 = 0_0000
- 1011 1110_0000_1111 = 0xBE_0F

Can we analyze assembly of PORTB reading ?

- - As per Register Summary PortB is at 0x23
  - IN – Load an I/O Location to Register
  - IN Rd,A 0 ≤ d ≤ 31, 0 ≤ A ≤ 63
  - 1011 0AAd dddd AAAA
  - 83 b1 in r24, 0x03
    - d=24 = 0x18=0001_1000
    - A=0x03=0x0000_0011 //Subtract 0x20 in I/O specific command
    - 1011 0001_1000_0011 =0xB1_83
The extended I/O memory from address 64 to 255 can only be reached by data addressing, not I/O addressing.

Exercise to be completed :

One can understand this concept better by repeating the exercise > Hope you can decode the following long jump instruction discussed in last blog

7fee: ee 27 eor r30, r30 //Exclusive OR i.e clear r307ff0: ff 27 eor r31, r31 //Exclusive OR i.e clear r307ff2: 09 94 ijmp //Indirect Jump to address set in Z register(R31:R30)

References : All documents are available here. I am referring to most of these :

To understand the data addressing better , We can look into section The Program and Data Addressing Modes of AVR Instruction set reference manual

Here are few other interesting topics @Embedkari

How to Learn C practically ?

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