Designing an Intel 80386SX development board
Written by Pierre Surply
The LSE-PC aims to be a compact IBM-PC compatible development board based on an Intel
80386SXCPU and an Altera Cyclone IV
EP4CE22E22FPGA in order to emulate a custom chipset.
The main goal of this project is to create a simple, debuggable and customisable version of the well-known PC hardware architecture. Its purpose is mainly didactic for students or experienced developers who want to get started into x86 low-level programming.
The schematics were designed using gschem which is a part of the gEDA project. Although the provided component library is acceptable, most of the chips used on this board are outlandish and so need to be drawn before starting overall schematics. This rude work was achieved by using djboxsym tool which allows quick production of gschem symbols from a minimal description.
Central Processing Unit
The CPU used on this board is a
80386SXdesigned by Intel and released in 1986. It is basically a cut-down version of the original
386with a 16-bit physical data bus. Although memory access performance is hardly affected, it is still fully 32-bit internally and was designed to be used in a 16-bit environment which is simpler and cheaper to design that a full 32-bit compatible motherboard. The physical address bus is only 24-bit which limits address space to 16MB.
The model used here is an
NG80386SXLP20which is a low power version clocked at 20MHz and packaged in a 100-pin Plastic Quad Flat pack. Of course, this chip is today considered obsolete but is still the only 32-bit x86 CPU which is simple enough to be integrated in an amateur board.
Field-Programmable Gate Array
The main criterion for choosing an appropriate FPGA was about packaging. Knowing that this chip will be hand-soldered, selecting a Ball Grid Array based component was inconceivable. I'm also quite used to work with Altera's FPGA so one from the Cyclone IV series was a good compromise. The model chosen is an
EP4CE22E22C7Nreleased in 2009. With its 22320 logic elements, it is one of the largest FPGA available on EQFP. This package, only used by Altera, is an enhanced version of the standard plastic quad flat package which uses a step of 0.5 millimeter between each pins. This layout allows the FPGA to expose 144 pins where 62 can be used as I/O and 15 as clock inputs.
An other useful feature is the 3.3V PCI compliant mode of the IO banks. It provides compatibility with 5V devices by enabling a clamping diode which can supports 25mA. This explains the use of 120 Ohms resistors between CPU 5V signals and FPGA IO.
The CPU needs a 20MHz input clock to operate correctly. A unique oscillator is used to clock CPU and FPGA. The idea here is to assume that if the FPGA needs a higher clock speed, the use of an internal Phase Locked Loop will be considered to obtain the desired frequency from this 20MHz clock.
FPGA programming and debugging can be performed through JTAG. Altera provides a dedicated programmer called the USB Blaster which can be easily used with Quartus II. It provides a standard 10-pin connector and operates here at 2.5V.
As FPGA configuration is volatile, it is necessary to provide an external way to program it when the board is powered on. Here this is achieved by an external serial flash which contains the whole FPGA configuration. Altera sells
EPCQdevices which are dedicated to that purpose. However, most of the time those are expensive and it turns out that they are nothing more than SPI flash memories. That is why it has been decided to use an
M25P16, a 16Mbits flash memory from Micron which perfectly do the job.
In fact, several programming modes are available in this FPGA. In order to indicate what mode has to be used,
MSELpins must be pulled-up or pulled-down to encode the mode number. To select the Active Serial Programming mode, it is necessary to solder 120 Ohms resistors on
In addition to JTAG, it can be a good idea to provide USB connectivity to this design. However, implementing USB protocol stack in an FPGA can be really painful. The purpose of the
FT230Xchip is to provide a simple bridge between an USB and an UART interface which is simpler to implement in an FPGA. It is provided in a SSOP16 package and is really simple to wire thanks notably to the fully integrated clock generation which does no require an external crystal.
Static Random Access Memory
For the main RAM,
AS6C8016from Alliance Memory has been chosen. This is a 512K x 16-bit CMOS static RAM packaged in a 44-pin TSOP. It features tri-state output and data byte control (
UBsignals) as required by the
Although this chip was originally designed to be used as a battery backed-up non-volatile memory, its usage simplicity and its response time justify the low storage space. So 1MB ought to be enough for anybody. Also,
AS6C8016is powered by 5V but is still fully TTL compatible which means that it can be driven by the CPU as well as the 3.3V outputted by FPGA's IO. So control signals as
RAMWEare only driven by the FPGA which will perform address decoding.
The power circuitry has to provide four sources of different voltages:
- 5V: CPU, SRAM
- 3.3V: FPGA In/Out
- 2.5V: FPGA Analog PLL
- 1.2V: FPGA internal logic, Digital PLL
Regulation is achieved by three fixed low drop positive voltage regulators which operate from the 5V supplied by the USB. Even though fixed regulators are often more expensive that adjustable regulators, they are easier to wire and reduce the number of passive components needed to perform adjustment. Only 250mA are provided for 2.5V because it is only used by FPGA Analog PLL and JTAG target voltage.
Routing and Manufacturing the Printed Circuit Board
Once the schematics completed, PCB has to be designed. This process has been assisted by pcb, an other part of gEDA project. As schematics and PCB designs are not performed using the same software (as KiCad or Eagle do), synchronization between those is ensured thanks to the gsch2pcb tool.
As some components on the board do not use standard packages, creating custom pcb footprint for those chip is necessary. Like symbols generation, footprints was generated using footgen.
The PCB routing here is a bit tricky due to the large number of signals needed to drive the CPU. A 4-layer PCB is unavoidable in order to achieve routing and to preserve signal integrity. As our manufacturer limits 4-layer board 5 x 10cm, this is the dimension adopted which is large enough for this design.
Each layer has a dedicated purpose:
- Top layer : it is mainly used for signals routing. Traces used for data signal are 0.20mm width which is the limit imposed by manufacturer. Unused spaces are recycled to ground planes. FPGA, CPU and voltage regulators are soldered on this layer.
- Ground layer : Used almost exclusively to get a common ground plane in the whole circuit. It has also been used to complete RAM routing.
- Power layer : Dedicated to conduct power rails through the board. Four areas corresponding to each voltage level can be clearly seen on this layer.
- Bottom layer : Like the top layer, this is mainly used for signals routing. Capacitors used to apply local filtering are soldered on this side as well as SRAM and 20MHz oscillator.
With a low end SMD soldering station, it takes approximately three hours to solder a whole board.
In addition to PCB, acrylic case was designed using FreeCAD and then manufactured.
Emulating a rudimentary chipset
Now that the board is correctly soldered, the last thing to do before being able to run code on the CPU is to configure the FPGA in order to emulate a basic chipset. The design is composed of two parts : the bus controller and the memory controller.
The bus controller has to handle
80386SXbus access protocol. In order to understand the exact purpose of it, it is necessary to detail signals involved in the process.
- The Data Bus (
D[15:0]) is composed of three-state bidirectional signals providing a general purpose data path between
386and other devices (such as memory).
- The Address Bus (
BLE#) is composed of three-state outputs providing physical memory addresses or I/O port addresses. The Byte Enable outputs (
BLE#) indicate which bytes of the 16-bit data bus are involved with the current transfer. If both of them are asserted, then 16 bits word is being transferred,
- A Bus Cycle is defined by
W/R#distinguishes between write and read cycles,
D/C#distinguishes between data and control cycles,
M/IO#distinguishes between memory and I/O cycles and
#LOCKindicates if the current operation is atomic or not.
- The Bus Access is controlled by
NA#. The Address Status (
ADS#) indicates that a valid bus cycle definition and address are being driven from the
386pins. Most of the bus controller logic must be based on the falling-edge of this signal.
READY#signal indicates a transfer acknowledge driven by the bus controller to the
NA#signal is used to request address pipelining which is not relevant in this case.
As an example, here is a waveform of bus signals during these operations :
- Write data1 to address1
- Read data2 from address2
- Write data3 to address3
- Read data4 from address4
Each bus access operates in two steps. The first one, indicated by
ADS#is used to drive Bus Cycle Definition signals and an address. The second one take place during the next rising edge of the main clock. Depending on the
W/R#pin state, the data bus is driven with the value the CPU wants to write. During all these sequences
ADS#is still asserted.
The next bus cycle is performed when the
386detects a falling edge on the
READY#signal. So the bus controller can be easily modeled as the following Finite-State Machine :
It is simple to implement this behavior in Verilog :
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always @(posedge clk) begin if (!_ads) begin capture_bus(); // Capture values driven on // A[23:1], D[15:0], /BLE, /BHE, WR, DC and MIO _ready <= 1; state <= `ST_T1; end else if (state == `ST_T1) begin _ready <= 0; state <= `ST_T2; end end
As data bus is bidirectional, it is sometimes necessary to set it in high impedance in order to let another device driving the bus. It is also needed to respect bytes requested by the CPU via
assign d[15:8] = wr || _bhe || !ramcs ? 8'hzz : dout[15:8]; assign d[7:0] = wr || _ble || !ramcs ? 8'hzz : dout[7:0];
Once the bus protocol is properly respected, the address requested by the CPU must be decoded in order to figure out which device must be selected. This is here the purpose of the memory controller unit.
Altera Cyclone IV devices features embedded memory structures. It consists of M9K memory blocks that can be configured to provide various memory functions, such as RAM, shift registers or ROM. The idea here is to use it to create a small memory which is initialized with a basic piece of code dedicated to CPU initialization. An other useful feature of this memory is to be easily readable and editable through JTAG using the In-System Content Editor provided by Quartus II.
Basically, the main address space is composed of two memories : an external (i.e. the SRAM) and an internal (i.e. the M9K blocks).
The first megabyte of addressable memory is organized as the layout of the traditional IBM-PC. It means that only the first 640K of external memory are mapped from
0x0A0000and BIOS shadow ROM (implemented here with internal memory) is mapped from
0x100000. Shadow ROM was originally a 64KB memory which contains a copy of the BIOS ROM mapped on the last 64KB of the address space. As the CPU starts fetching instructions at
0xFFFFF0after a reset, the mechanism consists of mapping a ROM at this address, copying ROM content on the shadow ROM and then jumping on a subroutine located on the first megabyte.
Here, the internal RAM is only 32KB due to the FPGA limitations and is located at
0x0F8000which allows simulation of the original machinery. Moreover, the whole SRAM is mapped from 1MB which means that first 640KB of external RAM are mapped twice.
Memory controller unit can be simplified as :
The actual address space layout is achieved by applying a logic expression to the chip select signal of each memory. Notice that
WE#signal of SRAM is not active on the same level that
386signal. So this signal is inverted by the FPGA.
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assign eramwe = !wr; assign eramcs = !(cs && ((addr[23:16] < 8'h0A) || (addr[23:20] == 4'h1))); assign iramcs = cs && ((addr[23:15] == 9'h1FF) || (addr[23:15] == 9'h01F));
Skeleton of a basic firmware
As an example, this section will present a basic firmware which can be run on the LSE-PC.
Firstly, it is considered here that the entire firmware will be located on the internal memory which is automatically initialized when the design is loaded into the FPGA.
On reset, the
80386CPU is running in real mode and will start to execute the instructions located at the end of the address space:
0xFFFFF0. So the purpose of these instructions are to jump to the first megabyte by reloading Code Segment. However, the last 16 bytes can be used to set a minimal environment to allow 16-bit application execution. The following code is an example of 5 instructions that can be assembled to 16 bytes of opcodes. It basically sets Data, Stack and Code Segment Selector, sets the stack pointer and then jumps to the beginning of the internal ram mapped at
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org 0xFFF0 ;; CS:0xF000, IP:0xFFF0 reset: mov ax, 0xF000 mov ds, ax mov ss, ax mov sp, 0xFFF0 jmp 0xF000:0x8000
Now that the execution flow has exited the reset state, it is now possible to set the CPU to protected mode. This can be achieved by loading a simple Global Descriptor Table which defines memory segments that will be used in protected mode. Notice that the jump to
reload_segsis used to flush instruction the prefetch queue after enabling protected mode in order to validate segment reloading. This code can be improved by the setting of an Interrupt Descriptor Table in addition of a Global Descriptor Table.
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org 0x8000 startup: lgdt [gdtr] ;; Load Glocal Descriptor Table mov eax, cr0 ;; Enable protected mode or eax, 1 mov cr0, eax jmp reload_segs ;; Flush prefetch queue reload_segs: mov ax, 0x10 ;; Reload segment selectors mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ss, ax ;; ljmp 0x08:0xF8400 dw 0xEA66 ;; Reload CS and jump to application code dd 0xF8400 dw 0x08 align 16 gdt: ... gdtr: Limit dw gdtr - gdt - 1 Base dd 0xF0000 + gdt
A 32-bit application can then be located at 0xF8400. The internal RAM is segmented according to the following layout :
As the In-Sytem Memory Content Editor accepts a special binary format called MIF (Memory Initialization File), a dedicated OCaml script has been created to facilitate linking of several raw binary object files.
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bin2mif -o fw.mif -b 0xF8000 0 \ # Memory base address -i pm.bin 0xF8000 0 \ # Jump to protected mode code -i app.bin 0xFC000 0 \ # Application code -i reset.bin 0xFFFF0 0 # Reset routine code
Providing debug facilities
Even though Altera's FPGA provide an efficient internal signal analyser thanks to SignalTap, it is a real pain to make software debugging when the size of applications running on the
386become significant. Adding a flexible on-chip debug facility based on the UART communication to this design is one of the main challenge of this project.
The supervisor is designed using Altera's QSys tool which assists the creation of systems based on the NIOS II soft-processor. This system is composed of a private on-chip memory which contains NIOS instructions and data, and of an UART which is connected to
The protocol between the host and the supervisor is pretty simple and it considers that the CPU is at any time in one of these states :
STOP: CPU is stopped.
RESETsignal is asserted.
RUN: CPU is running.
IOWR: CPU is trying to perform an access to IO ports. Distinction between read and write operation is done. Those states are used to allow device emulation.
BRK32: CPU is ready to accept debug operations. Distinction between real and protected mode is done.
It is accurate to implement the protocol logic through NIOS software instead of having it hardwired in Verilog. However, directly handling
386signals on the NIOS is inefficient due to execution speed of this system. The idea here is to export the
386signal handling job to an other module dedicated to it : the On-Chip Debug Unit.
The OCD Unit can take the control of
386buses at anytime by asserting the
ocd.ensignal, which disable the original bus controller described before. The communication between those two units is ensured by a dual-port shared memory accessible through Avalon bus and two PIO registers. The first one,
OCD_CTL, is used to reset the OCD Unit from supervisor. The second,
OCD_STATUSindicates if the unit is running or not. The shared memory contains a routine that must be applied on
On-Chip Debug Unit
This unit is basically a processor specially designed to handle
386signals. It fetches its instructions from the 256 x 16-bit Avalon memory filled by the supervisor and operates on a 16 x 16-bit data space also located on shared memory.
While supervisor can access OCD program and data unrestrictedly, the OCD Unit can only operates on its data space which corresponds to offset
0x100from supervisor point of view. In the dedicated assembler, data memory is addressed using
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module ocd ( // OCD Control input rst, // Connected to OCD_CTL input clk, // 40MHz clock (synchronous with 20MHz CPU clock) output reg en, // Asserted if OCD Unit is attached to the 386 output reg stop, // Connected to OCD_STATUS // 80386 signals ... // RAM signals (Avalon) ... );
Implementing this kind of processor is quite simple and a basic one will be based on the following state machine :
As Avalon memory signals are always latched, reading on it takes two clock cycles : the first cycle is used to latch the address value and the second one latches the result on the data bus. Taking that into account, execution of a single instruction which reads and writes on data memory cannot take less than five clock cycles.
- FETCH : Get instruction from program memory.
- LOAD : Latch source address into data memory.
- EXEC : Load source value from data memory and execute the instruction.
- STORE : Store result and compute next address of the next instruction.
- LATCH : Latch instruction address into program memory.
Instruction set is composed of several categories. The first one is used to control the OCD :
DETACH: Connect/Disconnect the OCD unit to 386 signals.
The second category includes instructions related to
386signals processing :
LDD d: Load data bus value into
LDAH d: Load address bus value into
LDWR d: Load
LDDC d: Load
LDMIO d: Load
STD s: Set data bus value to
RESET: Start/Reset the CPU.
Of course, some instructions only operate on registers :
LDI d, imm16: Load a 16-bit immediate into
MOV d, s: Move
sregister value into
CLR d: Clear
Third category is about flow control. As the data memory only exposes one port to the OCD Unit, implementing a compare instruction which loads two registers is not possible in a single cycle. So a compare register as been added to the core. All comparisons will be related to that register.
LDCMP s: Load
sregister value into the compare register.
CMP s: Compare
sregister value with compare register value and store the result into the compare register.
BNE addr: Branch to the specified address according to compare register value.
As example, those instructions performs a jump to
labelif R1 is equal to R2 :
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LDCMP R1 ;; cmpr <- R1 CMP R2 ;; cmpr <- cmpr == R2 BEQ label ;; pc <- label if cmpr != 0
Some instructions can stay more than one cycle in the EXEC state order to wait for an acknowledge from the CPU during some bus operation :
HOLDsignal and wait for
INTsignal and wait for
EXIT: Stop OCD routine execution. Never leaves EXEC state and assert
This wait state mechanism is also used to implement instructions used to wait for a particular event on the bus. All those instructions deassert
READY#signal and attach the
386when the expected condition is triggered.
WAITADS: Wait for
ADS#signal to be asserted
WAITIO: Wait for
WAITLOCK: Wait for
LOCK#to be asserted
The block diagram of this unit can be represented as :
Here is routines used to reset and start the CPU from OCD Unit. Notice that the start routine let the original bus controller operates on the
386until an IO access is performed. The supervisor has just to be interrupted when the OCD is exited from the start routine to handle the IO request. Devices can then be emulated by the supervisor or by the host.
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.func ocd_prgm_reset RESET ;; RESET <- 1 EXIT .func ocd_prgm_start START ;; RESET <- 0 DETACH ;; Let bus controller to handle CPU signals WAITIO ;; Wait for IO access to attach OCD Unit LDAL R1 ;; Get IO port address LDWR R2 ;; Get IO operation type EXIT
Example : Obtaining CPU registers
Now that the OCD Unit internals have been presented, the purpose now is to use it to get CPU registers.
Before applying debug operations on the CPU, it is necessary to stop execution and set it up in a known state. The simplest method to interrupt a
386without having to mind about the interrupt flag is to send a Non Maskable Interrupt. Unlike
NMImechanism does not provide any acknowledge from the CPU. So the way only to know if the CPU actually took into account the NMI is to wait
LOCK#signal assertion. Indeed, the
386locks the whole bus when it accesses an IDT or IVT entry. The
WAITLOCKinstruction has been designed for that specific purpose.
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.func ocd_prgm_break NMI ;; Set NMI signal WAITLOCK ;; Wait for ADS# and LOCK# signals then attach OCD unit
On the next step, the behaviour of the CPU is different according to its mode. If the
386is still in real mode, it will fetch the code segment and the offset of the NMI handler located on the Interrupt Vector Table. As IVT always starts at
0x0000000, the address
0x0000008will be outputted after triggering the NMI.
In the other hand, if protected mode is enabled, the CPU will fetch an Interrupt Descriptor corresponding of the NMI interrupt. This structure is located on the Interrupt Descriptor Table which can be found anywhere on the address space.
As the processor mode is unknown at that moment, it can be deduced from the first requested address after NMI :
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;; Get CPU Mode LDAL R2 ;; Load requested address LDAH R3 LDCMP R2 LDI R1, 0x0008 CMP R1 BNE break_protected_mode ;; Branch to protected mode handler if ;; A[15:0] != 0x0008 LDCMP R3 BEQ break_real_mode ;; Branch to real mode handler if ;; A[23:16] is equal to the NMI entry ;; offset on the IVT
Only protected mode will be considered for the rest of the example.
As IDT set by the application cannot be trusted, using the OCD Unit to drive a valid interrupt gate is conceivable :
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;; Fake IDT entry LDI R1, 0b1000111000000000 ;; Flags STD R1 WAITADS LDI R1, 0x000D ;; Offset[31:16] STD R1 WAITADS LDI R1, 0x0000 ;; Offset[15:0] STD R1 WAITADS LDI R1, 0x0008 ;; Segment Selector STD R1 WAITADS
A code segment reload is always performed before jumping to the interrupt handler. So a read to a GDT entry will be requested by the CPU.
In the same way, it is painless with this mechanism to drive a valid code segment :
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;; Fake GDT entry LDI R1, 0b1001101000000000 ;; Flags | Base[23:16] STD R1 WAITADS LDI R1, 0x00CF ;; Base[31:24] | G | D/B | Limit[19:16] STD R1 WAITADS LDI R1, 0xFFFF ;; Limit[15:00] STD R1 WAITADS LDI R1, 0x0000 ;; Base[15:0] STD R1 WAITADS READY ;; GDT Access bit WAITADS
CSregisters have been modified, they are pushed on the stack. However the bus controller is disconnected from CPU signals : this means that no actual write on the memory are performed during this operation. Instead, it is straightforward to load those values into OCD registers :
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;; Context saving LDD R2 ;; EFLAGS[15:0] READY WAITADS LDD R3 ;; EFLAGS[31:16] READY WAITADS LDD R4 ;; CS READY WAITADS LDD R5 ;; EIP[15:0] READY WAITADS LDD R6 ;; EIP[31:16] READY
Afterwards, the CPU will try to fetch instructions from the interrupt handler. So
HOLDsignal is asserted at the end of the
breakroutine. This leaves the supervisor time to load the next routine to the OCD program memory.
At this point,
386is on a known and valid state which allows us to inject any instructions sequences. In order to obtain CPU registers, the
pushainstruction can be injected :
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.func ocd_prgm_get_regs LDI R1, 0x9060 LDI R2, 0x9090 WAITADS ;; Fill instruction prefetch queue STD R1 ;; Drive PUSHA; NOP WAITADS STD R2 ;; Drive NOP; NOP WAITADS STD R2 ;; Drive NOP; NOP WAITADS STD R2 ;; Drive NOP; NOP WAITADS STD R2 ;; Drive NOP; NOP WAITADS ;; PUSHA LDD R0 READY WAITADS ... LDD R15 READY HOLD ;; Hold CPU in order to avoid instruction fetch during ;; loading of the next OCD routine EXIT
ESPvalue. In the same way, a
movinstruction can be used to restore
ESPand set any register value.
When debugging phase is over, a continue routine is executed which basically inject an
iretand drive original values of
For now, the debug unit is provided with a CLI interface allowing simple CPU interactions. When more debug features will be available, the goal is to embed a gdb stub into the host application.
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[lsepc-monitor] start [lsepc-monitor] status CPU Status: RUN [lsepc-monitor] break [lsepc-monitor] status CPU Status: Break (Protected Mode) [lsepc-monitor] getregs EFLAGS: 00000046 EIP: 000fd024 ESP: ffe4000c EBP: 00000123 EAX: 1100bbaa EBX: 5544000f ECX: 9988ffee EDX: ddcc7766 ESI: 456789ab EDI: cdef9090 CS: 0008 [lsepc-monitor] continue [lsepc-monitor] status CPU Status: RUN
Developing and testing on the LSE-PC is still mainly based on the JTAG interface. When connected to a JTAG interface, the FPGA design exposes the following entry points :
- RAM/ROM editor : used to perform on-chip operation on the internal memory
- NIOS II interface : used to program and debug the NIOS II contained on the supervisor
- Serial Flash Loader : used to program the SPI flash which contains FPGA configuration
- SignalTap : used to perform signal analysis.
This board is still a proof a concept. However, its composition was an excellent exercise to understand how the original
80386CPU works under the hood.
Although some work need to be done to get a profitable on-chip debugger, the hardware part and the simple chipset embedded are reliable enough to allow execution of simple applications.
- 80386SX datasheet
- Cyclone IV Handbook
- M25P16 Serial Flash datasheet
- USB Blaster User Guide
- FTDI FT230X datasheet
- AS6C8016 Static RAM datasheet
- LD1117A voltage regulator datasheet
- MCP1700 voltage regulator datasheet
- MCP1826 voltage regulator datasheet
- Avalon Interface Specifications