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Commodore PET Projects - petSD+

petSD Revisions / Enhancements

petSD+ Prototype

(minus the LCD)

 

During initial discussions with Nils, he suggested some changes that could be made to the design that would enhance petSD functionality, but would not have the same cost impact as petSD-duo (an enhanced version of petSD).

My original plan was just to try and build a copy of the original petSD, but, going further than just suggesting changes to the original design, Nils has now taken those changes and produced  an updated petSD ,that we're calling petSD+.

 

 
  petSD petSD+
Microcontroller

Atmel  ATmega 1284P

CPU 8-bit AVR
Flash 128 Kbytes
SRAM 16 Kbytes
EEPROM 4 Kbytes
Clock 18.432 MHz 8 or 18.432 MHz
Media SD Card
Interfaces

PET

IEEE-488

Expansion

I2C-Bus

Programming

6-pin ISP 6-pin ISP

Ethernet

ENC28J60 2

(none) 2

USB to Serial

FT232RL 3 (none) 3
RTC DS1307 4 PCF8583 4
Buttons
1. MCU Reset
2. Previous Image
3. Next Image
 
1. MCU Reset
2. Previous Image
3. Next Image
4. Select Image
Indicators

LEDs

4 2

Red

1
  2
   
  +5 VDC power
  On = unsaved buffers
  Flash = Error
 
  On = unsaved buffers
  Flash = Error

Green

1
  2
  +3.3 VDC power
  Access / Busy
 
  Access / Busy
Display (none) ASCII LCD 5
Board Size 1
Horizontal 116 mm 99 mm
Vertical 62 mm 62 mm

 

Board Size

When I looked at the costs of having a new PCB produced, I found that the size and shape of the original petSD PCB was just outside the footprint of the most cost effective size that the PCB manufacturer that I use offers. The price differential was not really significant, particularly if someone only needed a single PCB, and would not in itself been enough of a justification to redesign the PCB. However, Nils had other suggestions for changes that could be made that does justify some minor PCB component and layout changes (below). Another advantage of modifying the board layout is having the opportunity to place mounting holes to make fitting the board into a case easier.

 

Ethernet Controller

The original petSD design makes provision for the installation of a Microchip ENC28J60 10MB/s Ethernet controller, but although the hardware has been tested, there is no software available that can take advantage of the port. Since the Ethernet port would probably never be used in practice, the space allocated to it on the PCB is pretty much wasted, and dropping the connector and supporting components is probably enough to reduce the board footprint below the 100mm price-point.   

 

USB to Serial Interface

The original petSD included an FTDI FT232RL USB to serial UART interface chip to provide a USB interface via a virtual serial port. However, the USB functionality in petSD was severely limited and given the limited uses for the USB/serial interface, for petSD+, it was decided that, at the expense of the USB port, some of the MCU I/O pins could be better utilised to support the addition of an LCD text display (see below).

Should anyone have a need for the serial interface, this will be available in petSD-duo, albeit, that this may be some way off.

 

Real Time Clock

The original petSD had provision for a battery backed Real Time Clock (RTC), using a Dallas Semiconductor (Maxim) DS1307, that could be used to set time stamps for files. In the light of experience, Nils describes the DS1307 as "ridiculously inaccurate", so, the RTC on the new version will be implemented using an NXP Semiconductors PCF8583. However, although the PCF8583 is capable of much greater accuracy than the DS1307, this requires a highly accurate frequency meter to fine-tune the oscillator frequency and achieve an accuracy of +/- 5 minutes per year. Without a high accuracy frequency meter, "trial and error" adjustment of the RTC will be possible, so a reasonable level of accuracy should be possible.

 

LCD Text Display

petSD supported files residing in directories of the FAT tree as well as inside disk images like D64, D80, D82 and others, but there was no indication on the device of which disk image has been selected. In petSD+, the LCD display can display the image currently made available as an emulated floppy disk and a rotary switch will allow the selected image to be rotated as desired.

 

 

 

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