SD Loader

Introduction

Legacy computer systems typically load and save programs over serial ports. This is often done by attaching a PC and running a terminal program to send/receive programs to the legacy systems. Integrating modern storage, like SD cards, can be a serious challenge into legacy designs.

Replacing the external PC offers the opportunity to more easily transport the system. A compact solution that emulates the PC for transfers would be useful.

This design transfers files to/from an SD card via a serial port. It runs a menu driven program on small OLED to control transfers and supports SD cards up to 32GB (FAT32 formatted). The serial interface has RTS/CTS hardware handshake support. It can run with 5V or 3.3V target systems or at RS-232 levels. Custom cards could easily be added to emulate cassette interfaces or run.

Hackaday page

Features

• Supports SD cards up to 32GB
  • FAT32 formatted
• Serial interface
  • Full hardware Handshake support

Cards

• Constructed out of existing Land Boards cards
• RP2040 board versions
  • SAMD board does not have enough SRAM to use CircuitPython, needs Arduino IDE
  • RP2040 board has a lot more SRAM and is easier to program for directory lists
  • QTPy49-01 QT Py or XIAO RP2040 Adapter card
• MyMenu
  • MCP23008 I2C interface
  • OLED
  • 5 Pushbuttons
  • 3 LEDs
• Optional DCE or DTE RS-232 Level Translator card
• Optional Level Shifter mounted on GRID49 card
  • Used for legacy 5V system targets
• Optional Mini360 Adapter Board
  • Wide-range input Power card (7-23V)

QTPy49-01 Card

• Easier to cable external cards
• SD Card socket
• Flexible power (7-12V)

MyMenu

• I2C interface to OLED, I2C Port Expander with 3 LEDs, 5 switches

MCP23008 Data Sheet

GPIO Port Map
GP0 - LED D3
GP1 - LED D2
GP2 - LED D1
GP3 - Select button 0 = pressed
GP4 - Right button
GP5 - Down button
GP6 - Up button
GP7 - Left button

DTE - RS-232 to TTL Level Shifter (Optional)

• The DTE (PC) has the male DB-9 connector

5V Level Shifter (Optional)

• Use for "legacy" 5V Serial target systems
  • Not used for 3.3V systems
• Convert 3.3V QT Py Tx/Rx/RTS/CTS to "legacy" 5V
• Isolated 3.3V/5V power
• Common Ground
• Built on GRID49 card
• Adafruit 4-channel I2C-safe Bi-directional Logic Level Converter
• SH-LEV-01 - New

QTPy49-01 Card Cabling

• QTPy49-01 Card has function specific headers

QTPy49-01 Card to MyMenu Cabling

QTPy49	SIGNAL	MyMenu	COLOR
J5-1	GND	P2-1	BLK
J5-2	+3.3V	P2-2	RED
J5-3	SDA	P2-3	WHT
J5-4	SCL	P2-4	BRN

QTPy49 Card to Host Serial

QTPy49	SIGNAL	COLOR
J8-3 (D1)	RTS* (in)	GRY
J7-1 (D2)	CTS* (out)	ORA
J4-4 (RX)	RX	BRN
J4-3 (TX)	TX	WHT
J4-2 (VCC)	+3.3V	Xlator (only)
J4-1 (GND)	GND	BLK

QT Py Breakout Card (Alternate)

• Do not install 5mm Terminal Block

• Power via USB C cable

QT Py Breakout Card (on GRID49 card) Cabling

• GRID49 card
• QT Py uses (2) 1x7 male headers for signals
  • Cut off 1 set of holes of (2) female housings to make 1x7 female headers
• SD Card uses (1) 1x8 header
• MyMenu uses (1) 1x5 header
• QT Py uses 2x8 header for 3.3V and Ground
  • Use 2x3 female housings
• 40mm cables to Host Target

QT Py Breakout Card Cabling

QT Py	SIGNAL	CARD	PIN	COLOR
01 (AD0)	INT(AD0)	MyMenu	P2-5	GRY
02 (AD1)	RTS*	TARGET	TBD	GRY
03 (AD2)	CTS*	TARGET	TBD	ORA
04 (AD3)	SS*	SD_CARD	J1-5	GRY
05 (SDA)	SDA	MyMenu	P2-3	WHT
06 (SCL)	SCL	MyMenu	P2-4	BRN
07 (TX)	TX	TARGET	TBD	WHT
08 (RX)	RX	TARGET	TBD	BRN
09 (SCK)	SCK	SD_CARD	J1-3	BLU
10 (MI)	MISO	SD_CARD	J1-2	BRN
11 (MO)	MOSI	SD_CARD	J1-4	WHT
12 (3V)	+3.3V	GRID49	Q7	WIRE
13 (GND)	GND	GRID49	R7	WIRE
14 (5V)	5V	N/C
PWR-01	+3.3V	SD_CARD	J1-8	RED
PWR-02	+3.3V	TARGET	TBD	RED
PWR-03	+3.3V	MyMenu	P2-2	RED
PWR-14	GND	MyMenu	P2-1	BLK
PWR-15	GND	SD_CARD	J1-1	BLK
PWR-16	GND	TARGET	TBD	BLK

Arduino Software

• i2cscan of attached cards shows OLED at 0x3c and MCP23008 at 0x20

I2C addresses found: ['0x20', '0x3c']

Arduino Software Development Environment

• Developed using Arduino IDE V1.8.xx
• Arduino IDE Setup for QT Py
• Requires Arduino IDE version 1.8 or higher
• Add to Preferences Additional Boards Manager URLs

https://adafruit.github.io/arduino-board-index/package_adafruit_index.json
 

• Add Adafruit SAMD Boards in Boards Manager
• Adapting Sketches to M0 & M4
  • Specifics

pinMode(pin, INPUT_PULLUP)

#if defined(ARDUINO_SAMD_ZERO) && defined(SERIAL_PORT_USBVIRTUAL)
  // Required for Serial on Zero based boards
  #define Serial SERIAL_PORT_USBVIRTUAL
#endif

const char str[] = "My very long string";

• UF2 Bootloader Details

Arduino Human Interface Design (HID)

• Menu driven
  • MyMenu card driver
• OLED display
  • ug2 u8x8 OLED display driver software

Arduino SD Card

• Library
• Example files

Arduino SD Card Library

• SD Card library
  • The SD library allows for reading from and writing to SD cards, e.g. on the Arduino Ethernet Shield
  • Based on sdfatlib
  • The library supports FAT16 and FAT32 file systems on standard SD cards and SDHC cards
  • It uses short 8.3 names for files
  • The file names passed to the SD library functions can include paths separated by forward-slashes, /, e.g. “directory/filename.txt”
  • Because the working directory is always the root of the SD card, a name refers to the same file whether or not it includes a leading slash (e.g. “/file.txt” is equivalent to “file.txt”)
  • As of version 1.0, the library supports opening multiple files.
  • The communication between the microcontroller and the SD card uses SPI, which takes place on digital pins 11, 12, and 13 (on most Arduino boards) or 50, 51, and 52 (Arduino Mega)
  • Additionally, another pin must be used to select the SD card
  • This can be the hardware SS pin - pin 10 (on most Arduino boards) or pin 53 (on the Mega) - or another pin specified in the call to SD.begin()

Arduino SD Card Example Code

• SD Card example Code
• Read and Write File example
• Read Card Information
• Read a file from a SD card using the SD library and send it over the serial port
• Create and destroy a file on a SD card
• List the files available in the directory of the SD card

Arduino SD Card Connections

• Uses default board SPI pins
• Uses D3 as chip select

Serial

• Serial Reference
• Serial.begin(speed)
• Serial.available()
  • Get the number of bytes (characters) available for reading from the serial port.
  • This is data that’s already arrived and stored in the serial receive buffer (which holds 64 bytes).
  • Turn off/on receive handshake based on Serial.available() count and storage
• Serial.write()
  • Writes binary data to the serial port
  • This data is sent as a byte or series of bytes
  • To send the characters representing the digits of a number use the print() function instead.
  • Syntax

Serial.write(val)
Serial.write(str)
Serial.write(buf, len)

CircuitPython Software

• CircuitPython has several advantages over Arduino IDE
  • Build-test-built cycle time much faster
  • Much easier to do list processing for directory structure
  • Speed not critical in this application
• Not enough SRAM with SAMD version of the card
  • RP2040 version of the card has plenty of SRAM
  • XAIO RP2040 is $5.40 and ships quickly from US Warehouse

CircuitPython Software Development Environment

• CircuitPython repo
• SDLoader GitHub repo
• Use Thonny IDE
  • Left click bottom right corner
  • Select CircuitPython (generic)
  • Left click bottom right corner
  • Select Configure Interpreter
  • Select USB Serial Device (COM<x)
• Pins list

import board
dir(board)
['__class__', '__name__', 'A0', 'A1', 'A2', 'A3', 'D0', 'D1', 'D10', 'D2', 'D3', 'D4', 'D5', 'D6', 'D7', 'D8', 'D9', 'I2C', 'LED', 'LED_BLUE', 'LED_GREEN', 'LED_RED', 'MISO', 'MOSI', 'NEOPIXEL', 'NEOPIXEL_POWER', 'RX', 'SCK', 'SCL', 'SDA', 'SPI', 'TX', 'UART', 'board_id']

>>> print(board.board_id)
seeeduino_xiao_rp2040

Install CircuitPython on Seeed XIAO RP2040

• Long press the "BOOT" button. (The "B" is written on the board shown to the right)
• Connect the XIAO PR2040 to your computer while you still pressing the button.
• The computer then will appear a disk driver(RP1-RP2)
• Download the XIAO-RP2040-CircuitPython.uf2 file for XIAO RP2040
• Drag the .uf2 file to the disk driver("RP1-RP2")
• Check the disk drive if the name has changed to "CIRCUITPY" (takes a few seconds)
• Free memory

>>> import gc
>>> gc.mem_free()
208128
>>> import time
>>> gc.mem_free()
207952

CircuitPython Human Interface Design (HID)

• MyMenu card
  • 0.91" OLED
  • 3 LEDs
  • 5 pushbuttons
• i2cscan of attached cards shows OLED at 0x3c and MCP23008 at 0x21
• Added i2cscan code to find the MCP23008 on the MyMenu card
• MyMenu demo/test code
  • Read pushbuttons
  • Write OLED
  • Write LEDs
  • Exit by pressing Select button

CircuitPython SD Card

• SD card module reference
• storage module reference
• Using sdcardio
• Listing files on SD card
• sdcardio class constructor

sdcardio.SDCard(bus: busio.SPI, cs: microcontroller.Pin, baudrate: int = 8000000)

Example code

• Initialize SD card and read directory
• Result

>>> %Run -c $EDITOR_CONTENT
Files on filesystem:
====================
System Volume Information/               Size:       0 by
   WPSettings.dat                        Size:      12 by
   IndexerVolumeGuid                     Size:      76 by
SuperStarTrek.bas                        Size:    20.0 KB
scramble.bas                             Size:    19.2 KB
CivilWar.bas                             Size:    13.5 KB
>>> 

• Storing path, file name as set pairs, gives

dirFileNames [('/sd', 'System Volume Information'), ('/sd/System Volume Information', 'WPSettings.dat'), ('/sd/System Volume Information', 'IndexerVolumeGuid'), ('/sd', 'SuperStarTrek.bas'), ('/sd', 'scramble.bas'), ('/sd', 'CivilWar.bas')]
('/sd', 'System Volume Information')
('/sd/System Volume Information', 'WPSettings.dat')
('/sd/System Volume Information', 'IndexerVolumeGuid')
('/sd', 'SuperStarTrek.bas')
('/sd', 'scramble.bas')
('/sd', 'CivilWar.bas')

CircuitPython Serial

• busio UART reference
• Supports RTS/CTS
  • MicroPython UART flow control
• Constructor

lassbusio.UART(tx: microcontroller.Pin, rx: microcontroller.Pin, *, baudrate: int = 9600, bits: int = 8, parity: Optional[Parity] = None, stop: int = 1, timeout: float = 1, receiver_buffer_size: int = 64)

• RTS, CTS pins defines on QT Py RP2040
  • RTS = pin D2
  • CTS = pin D3

`

Serial Protocol References

• XMODEM Protocol
• S record format