ATMEGA16HVB_1

Features • High Performance, Low Power AVR® 8-bit Microcontroller • Advanced RISC Architecture – 124 Powerful Instructions - Most Single Clock Cycle Execution – 32 x 8 General Purpose Working Registers – Fully Static Operation – Up to 8 MIPS Throughput at 8 MHz High Endurance Non-volatile Memory Segments – 16K/32K Bytes of In-System Self-Programmable Flash (ATmega16HVB/32HVB) – 512/1K Bytes EEPROM – 1K/2K Bytes Internal SRAM – Write/Erase Cycles 10,000 Flash/100,000 EEPROM – Data retention: 20 years at 85°C/100 years at 25°C(1) – Optional Boot Code Section with Independent Lock Bits In-System Programming by On-chip Boot Program True Read-While-Write Operation – Programming Lock for Software Security Battery Management Features – Two, three or Four Cells in Series – High-current Protection (Charge and Discharge) – Over-current Protection (Charge and Discharge) – Short-circuit Protection (Discharge) – High Voltage Outputs to Drive N-Channel Charge/Discharge FETs – Optional Deep Under Voltage Recovery mode - allowing 0-volt charging without external Precharge FET – Optional High Voltage Open Drain ouput - allowing 0-volt charging with external Precharge FET – Integrated Cell Balancing FETs Peripheral Features – Two configurable 8- or 16-bit Timers with Separate Prescaler, Optional Input Capture (IC), Compare Mode and CTC – SPI - Serial Peripheral Interface – 12-bit Voltage ADC, Six External and One Internal ADC Input – High Resolution Coulomb Counter ADC for Current Measurements – TWI Serial Interface supporting SMBus implementation – Programmable Watchdog Timer Special Microcontroller Features – debugWIRE On-chip Debug System – In-System Programmable via SPI ports – Power-on Reset – On-chip Voltage Regulator with Short-circuit Monitoring Interface – External and Internal Interrupt Sources – Sleep Modes: Idle, ADC Noise Reduction, Power-save, and Power-off Additional Secure Authentication Features available only under NDA Packages – 44-pin TSSOP Operating Voltage: 4 - 25V Maximum Withstand Voltage (High-voltage pins): 35V Temperature Range: -40°C to 85°C Speed Grade: 1-8 MHz • • 8-bit Microcontroller with 16K/32K Bytes In-System Programmable Flash ATmega16HVB ATmega32HVB Preliminary • • • • • • • • Note: 1. See ”Data Retention” on page 8 for details. 8042A–AVR–07/09 ATmega16HVB/32HVB 1. Pin Configurations 1.1 TSSOP Figure 1-1. TSSOP - pinout ATmega16HVB/32HVB NI NNI VREFGND VREF GND VREG PA0(ADC0/SGND/PCINT0) PA1(ADC1/SGND/PCINT1) PA2(PCINT2/T0) PA3(PCINT3/T1) VCLMP10 VFET BATT VCC GND OD NC OC RESET/dw PB0(PCINT4/ICP00) PB1(PCINT5/CKOUT) PB2(PCINT6) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 PI PPI NV PV1 PV2 PV3 PV4 PVT VCC GND PC5 PC4(SCL) PC3(INT3/SDA) PC2(INT2) PC1(INT1) PC0(INT0/EXTPROT) PB7(MISO/PCINT11) NC PB6(MOSI/PCINT10) PB5(SCK/PCINT9) PB4(SS/PCINT8) PB3(PCINT7) 1.2 Pin Descriptions 1.2.1 VFET High voltage supply pin. This pin is used as supply for the internal voltage regulator, described in ”Voltage Regulator” on page 132. 1.2.2 VCLMP10 Internal 10V clamping of VFET voltage for external decoupling. 1.2.3 VCC Digital supply voltage. Normally connected to VREG. 2 8042A–AVR–07/09 ATmega16HVB/32HVB 1.2.4 VREG Output from the internal Voltage Regulator. Used for external decoupling to ensure stable regulator operation. For details, see ”Voltage Regulator” on page 132. 1.2.5 VREF Internal Voltage Reference for external decoupling. For details, see ”Voltage Reference and Temperature Sensor” on page 123. 1.2.6 VREFGND Ground for decoupling of Internal Voltage Reference. For details, see ”Voltage Reference and Temperature Sensor” on page 123. Do not connect to GND or SGND on PCB. 1.2.7 GND Ground 1.2.8 Port A (PA3..PA0) Port A serves as a low-voltage 4-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). As inputs, Port A pins that are externally pulled low will source current if the pull-up resistors are activated. The Port A pins are tri-stated when a reset condition becomes active, even if the clock is not running. Port A also serves the functions of various special features of the ATmega16HVB/32HVB as listed in ”Alternate Functions of Port A” on page 74. 1.2.9 Port B (PB7..PB0) Port B is a low-voltage 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). As inputs, Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-stated when a reset condition becomes active, even if the clock is not running. Port B also serves the functions of various special features of the ATmega16HVB/32HVB as listed in ”Alternate Functions of Port B” on page 75. 1.2.10 Port C (PC5) Port C (PC5) is a high voltage Open Drain output port. 1.2.11 Port C (PC4..PC0) Port C is a 5-bit high voltage Open Drain bi-directional I/O port. 1.2.12 OC/OD High voltage output to drive Charge/Discharge FET. For details, see ”FET Driver” on page 148. 1.2.13 PI/NI Filtered positive/negative input from external current sense resistor, used to by the Coulomb Counter ADC to measure charge/discharge currents flowing in the battery pack. For details, see ”Coulomb Counter - Dedicated Fuel Gauging Sigma-delta ADC” on page 108. 3 8042A–AVR–07/09 ATmega16HVB/32HVB 1.2.14 PPI/NNI Unfiltered positive/negative input from external current sense resistor, used by the battery protection circuit, for over-current and short-circuit detection. For details, see ”Battery Protection” on page 135. 1.2.15 NV/PV1/PV2/PV3/PV4 NV, PV1, PV2, PV3, and PV4 are the inputs for battery cells 1, 2, 3 and 4, used by the Voltage ADC to measure each cell voltage. For details, see ”Voltage ADC – 7-channel General Purpose 12-bit Sigma-Delta ADC” on page 117. 1.2.16 PVT Defines the source voltage level for the Charge FET driver. For details, see ”FET Driver” on page 148. 1.2.17 BATT Input for detecting when a charger is connected. Defines the source voltage level for the Discharge FET driver. For details, see ”FET Driver” on page 148. 1.2.18 RESET/dw Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. The minimum pulse length is given in Table 11 on page 38. Shorter pulses are not guaranteed to generate a reset. This pin is also used as debugWIRE communication pin. 4 8042A–AVR–07/09 ATmega16HVB/32HVB 2. Overview The ATmega16HVB/32HVB is a monitoring and protection circuit for 3 and 4-cell Li-ion applications with focus on highest safety including safe authentication, low cost and high utilization of the cell energy. The device contains secure authentication features as well as autonomous battery protection during charging and discharging. The External Protection Input can be used to implement other battery protection mechanisms using external components, e.g. protection against chargers with too high charge voltage can be easily implemented with a few low cost passive components. The feature set makes the ATmega16HVB/32HVB a key component in any system focusing on high security, battery protection, high system utilization and low cost. Figure 2-1. Block Diagram PB7..0 PC5..0 PB0 Oscillator Circuits / Clock Generation Oscillator Sampling Interface SPI VCC Watchdog Timer Flash SRAM 8/16-bit T/C1 Voltage ADC TWI 8/16-bit T/C0 FET Control OC OD PORTB (8) PORTC (6) Watchdog Oscillator Current Protection Cell Balancing PPI NNI PV4 PV3 PV2 PV1 NV RESET/dW Power Supervision POR & RESET Program Logic debugWIRE VPTAT CPU EEPROM Voltage Reference VREF VREFGND PI NI GND BATT Charger Detect Security Module DATA BUS Coulomb Counter ADC VFET VREG Voltage Regulator Voltage Regulator Monitor Interface PORTA (4) PA1..0 PA3..0 ATmega16HVB/32HVB provides the necessary redundancy on-chip to make sure that the battery is protected in critical failure modes. The chip is specifically designed to provide safety for the battery cells in case of pin shorting, loss of power (either caused by battery pack short or 5 8042A–AVR–07/09 ATmega16HVB/32HVB VCC short), illegal charger connection or software runaway. This makes ATmega16HVB/32HVB the ideal 1-chip solution for applications with focus on high safety. The ATmega16HVB/32HVB features an integrated voltage regulator that operates at a wide range of input voltages, 4 - 25 volts. This voltage is regulated to a constant supply voltage of nominally 3.3 volts for the integrated logic and analog functions. The regulator capabilities, combined with an extremely low power consumption in the power saving modes, greatly enhances the cell energy utilization compared to existing solutions. The chip utilizes Atmel's patented Deep Under-voltage Recovery (DUVR) mode that supports pre-charging of deeply discharged battery cells without using a separate Pre-charge FET. Optionally, Pre-charge FETs are supported for integration into many existing battery charging schemes. The battery protection monitors the charge and discharge current to detect illegal conditions and protect the battery from these when required. A 12-bit Voltage ADC allows software to monitor each cell voltage individually with high accuracy. The ADC also provides one internal input channel to measure on-chip temperature and two input channels intended for external thermistors. An 18-bit ADC optimized for Coulomb Counting accumulates charge and discharge currents and reports accumulated current with high resolution and accuracy. It can also be used to provide instantaneous current measurements with 13 bit resolution. Integrated Cell Balancing FETs allow cell balancing algorithms to be implemented in software. The MCU provides the following features: 16K/32K bytes of In-System Programmable Flash with Read-While-Write capabilities, 512/1K bytes EEPROM, 1K/2K bytes SRAM. 32 general purpose working registers, 12 general purpose I/O lines, 5 general purpose high voltage open drain I/O lines, one general purpose super high voltage open drain output, debugWIRE for On-chip debugging and SPI for In-system Programming, a SM-Bus compliant TWI module, two flexible Timer/Counters with Input Capture and compare modes. Internal and external interrupts, a 12-bit Sigma Delta ADC for voltage and temperature measurements, a high resolution Sigma Delta ADC for Coulomb Counting and instantaneous current measurements, integrated cell balancing FETs, Additional Secure Authentication Features, an autonomous Battery Protection module, a programmable Watchdog Timer with internal Oscillator, and software selectable power saving modes. The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers. The device is manufactured using Atmel’s high voltage high density non-volatile memory technology. The On-chip ISP Flash allows the program memory to be reprogrammed In-System, through an SPI serial interface, by a conventional non-volatile memory programmer or by an Onchip Boot program running on the AVR core. The Boot program can use any interface to download the application program in the Application Flash memory. Software in the Boot Flash section will continue to run while the Application Flash section is updated, providing true ReadWhile-Write operation. By combining an 8-bit RISC CPU with In-System Self-ProgrammableFlash and highly accurate analog front-end in a monolithic chip. The Atmel ATmega16HVB/32HVB is a powerful microcontroller that provides a highly flexible and cost effective solution. It is part of the AVR Battery Management family that provides secure 6 8042A–AVR–07/09 ATmega16HVB/32HVB authentication, highly accurate monitoring and autonomous protection for Lithium-ion battery cells. The ATmega16HVB/32HVB AVR is supported with a full suite of program and system development tools including: C Compilers, Macro Assemblers, Program Debugger/Simulators, and Onchip Debugger. 2.1 Comparison Between ATmega16HVB and ATmega32HVB The ATmega16HVB and ATmega32HVB differ only in memory size for Flash, EEPROM and internal SRAM. Table 2-1 summarizes the different configuration for the two devices. Table 2-1. Configuration summary Device ATmega16HVB ATmega32HVB Flash 16K 32K EEPROM 512 1K SRAM 1K 2K 7 8042A–AVR–07/09 ATmega16HVB/32HVB 3. Disclaimer All parameters contained in this datasheet are preliminary and based on characterization of ATmega16/32HVB. 4. Resources A comprehensive set of development tools, application notes and datasheets are available for download on http://www.atmel.com/avr.n1 Note: 1. 5. About Code Examples This documentation contains simple code examples that briefly show how to use various parts of the device. These code examples assume that the part specific header file is included before compilation. Be aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent. Please confirm with the C compiler documentation for more details. For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”. 6. Data Retention Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over 20 years at 85°C or 100 years at 25°C. 8 8042A–AVR–07/09 ATmega16HVB/32HVB 7. AVR CPU Core 7.1 Overview This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure correct program execution. The CPU must therefore be able to access memories, perform calculations, control peripherals, and handle interrupts. Figure 7-1. Block Diagram of the AVR Architecture Data Bus 8-bit Flash Program Memory Program Counter Status and Control Instruction Register 32 x 8 General Purpose Registrers Interrupt Unit Watchdog Timer Indirect Addressing Instruction Decoder Direct Addressing ALU I/O Module1 Control Lines I/O Module 2 Data SRAM I/O Module n EEPROM I/O Lines In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate memories and buses for program and data. Instructions in the program memory are executed with a single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the program memory. This concept enables instructions to be executed in every clock cycle. The program memory is In-System Reprogrammable Flash memory. The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typ- 9 8042A–AVR–07/09 ATmega16HVB/32HVB ical ALU operation, two operands are output from the Register File, the operation is executed, and the result is stored back in the Register File – in one clock cycle. Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing – enabling efficient address calculations. One of the these address pointers can also be used as an address pointer for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Z-register, described later in this section. The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated to reflect information about the result of the operation. Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address contains a 16- or 32-bit instruction. During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine (before subroutines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing modes supported in the AVR architecture. The memory spaces in the AVR architecture are all linear and regular memory maps. A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the priority. The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations following those of the Register File, 0x20 - 0x5F. In addition, the ATmega16HVB/32HVB has Extended I/O space from 0x60 - 0xFF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used. 7.2 ALU – Arithmetic Logic Unit The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and fractional format. See the “Instruction Set” section for a detailed description. 7.3 Status Register The Status Register contains information about the result of the most recently executed arithmetic instruction. This information can be used for altering program flow in order to perform conditional operations. Note that the Status Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases remove the need for using the dedicated compare instructions, resulting in faster and more compact code. The Status Register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt. This must be handled by software. 10 8042A–AVR–07/09 ATmega16HVB/32HVB 7.3.1 SREG – AVR Status Register Bit 0x3F (0x5F) Read/Write Initial Value 7 I R/W 0 6 T R/W 0 5 H R/W 0 4 S R/W 0 3 V R/W 0 2 N R/W 0 1 Z R/W 0 0 C R/W 0 SREG • Bit 7 – I: Global Interrupt Enable The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of the interrupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and CLI instructions, as described in the instruction set reference. • Bit 6 – T: Bit Copy Storage The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T can be copied into a bit in a register in the Register File by the BLD instruction. • Bit 5 – H: Half Carry Flag The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful in BCD arithmetic. See the “Instruction Set Description” for detailed information. • Bit 4 – S: Sign Bit, S = N ⊕ V The S-bit is always an exclusive or between the negative flag N and the Two’s Complement Overflow Flag V. See the “Instruction Set Description” for detailed information. • Bit 3 – V: Two’s Complement Overflow Flag The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the “Instruction Set Description” for detailed information. • Bit 2 – N: Negative Flag The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information. • Bit 1 – Z: Zero Flag The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information. • Bit 0 – C: Carry Flag The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information. 11 8042A–AVR–07/09 ATmega16HVB/32HVB 7.4 General Purpose Register File The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required performance and flexibility, the following input/output schemes are supported by the Register File: • One 8-bit output operand and one 8-bit result input • Two 8-bit output operands and one 8-bit result input • Two 8-bit output operands and one 16-bit result input • One 16-bit output operand and one 16-bit result input Figure 7-2 shows the structure of the 32 general purpose working registers in the CPU. Figure 7-2. AVR CPU General Purpose Working Registers 7 R0 R1 R2 … R13 General Purpose Working Registers R14 R15 R16 R17 … R26 R27 R28 R29 R30 R31 0x1A 0x1B 0x1C 0x1D 0x1E 0x1F X-register Low Byte X-register High Byte Y-register Low Byte Y-register High Byte Z-register Low Byte Z-register High Byte 0x0D 0x0E 0x0F 0x10 0x11 0 Addr. 0x00 0x01 0x02 Most of the instructions operating on the Register File have direct access to all registers, and most of them are single cycle instructions. As shown in Figure 7-2, each register is also assigned a data memory address, mapping them directly into the first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory organization provides great flexibility in access of the registers, as the X-, Y- and Z-pointer registers can be set to index any register in the file. 7.4.1 The X-register, Y-register, and Z-register The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are defined as described in Figure 7-3 on page 13. 12 8042A–AVR–07/09 ATmega16HVB/32HVB Figure 7-3. The X-, Y-, and Z-registers 15 X-register 7 R27 (0x1B) XH 0 7 R26 (0x1A) XL 0 0 15 Y-register 7 R29 (0x1D) 15 Z-register 7 R31 (0x1F) YH 0 7 R28 (0x1C) ZH 0 7 R30 (0x1E) YL 0 0 ZL 0 0 In the different addressing modes these address registers have functions as fixed displacement, automatic increment, and automatic decrement (see the instruction set reference for details). 7.5 Stack Pointer The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that the Stack is implemented as growing from higher memory locations to lower memory locations. This implies that a Stack PUSH command decreases the Stack Pointer. The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to point above 0x100. The Stack Pointer is decremented by one when data is pushed onto the Stack with the PUSH instruction, and it is decremented by two when the return address is pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is popped from the Stack with the POP instruction, and it is incremented by two when data is popped from the Stack with return from subroutine RET or return from interrupt RETI. The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is implementation dependent. Note that the data space in some implementations of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register will not be present. 7.5.1 SPH and SPL – Stack Pointer High and Stack Pointer Low Bit 0x3E (0x5E) 0x3D (0x5D) 15 SP15 SP7 7 14 SP14 SP6 6 R/W R/W 0 0 13 SP13 SP5 5 R/W R/W 0 0 12 SP12 SP4 4 R/W R/W 0 0 11 SP11 SP3 3 R/W R/W 0 0 10 SP10 SP2 2 R/W R/W 0 0 9 SP9 SP1 1 R/W R/W 0 0 8 SP8 SP0 0 R/W R/W 0 0 SPH SPL Read/Write R/W R/W Initial Value 0 0 13 8042A–AVR–07/09 ATmega16HVB/32HVB 7.6 Instruction Execution Timing This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is used. Figure 7-4 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit. Figure 7-4. The Parallel Instruction Fetches and Instruction Executions T1 T2 T3 T4 clkCPU 1st Instruction Fetch 1st Instruction Execute 2nd Instruction Fetch 2nd Instruction Execute 3rd Instruction Fetch 3rd Instruction Execute 4th Instruction Fetch Figure 7-5 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using two register operands is executed, and the result is stored back to the destination register. Figure 7-5. Single Cycle ALU Operation T1 T2 T3 T4 clkCPU Total Execution Time Register Operands Fetch ALU Operation Execute Result Write Back 7.7 Reset and Interrupt Handling The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each have a separate program vector in the program memory space. All interrupts are assigned individual enable bits which must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to enable the interrupt. The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors. The complete list of vectors is shown in ”Interrupts” on page 52. The list also determines the priority levels of the different interrupts. The lower the address the higher is the priority level. RESET has the highest priority. 14 8042A–AVR–07/09 ATmega16HVB/32HVB When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is executed. There are basically two types of interrupts. The first type is triggered by an event that sets the interrupt flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, and hardware clears the corresponding interrupt flag. Interrupt flags can also be cleared by writing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is cleared, the interrupt flag will be set and remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt Enable bit is cleared, the corresponding interrupt flag(s) will be set and remembered until the Global Interrupt Enable bit is set, and will then be executed by order of priority. The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not necessarily have interrupt flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will not be triggered. When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served. Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when returning from an interrupt routine. This must be handled by software. When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example shows how this can be used to avoid interrupts during the timed EEPROM write sequence. Assembly Code Example in r16, SREG cli sbi EECR, EEMPE sbi EECR, EEPE out SREG, r16 ; restore SREG value (I-bit) ; store SREG value ; start EEPROM write ; disable interrupts during timed sequence C Code Example char cSREG; cSREG = SREG; /* store SREG value */ /* disable interrupts during timed sequence */ _CLI(); EECR |= (1