SCSI2SD Schematic Notes

Details for the circuit design of SCSI2SD.

SMT Type

1. 0805 sized components will be used where applicable. These represent a good tradeoff between hand-solderability and PCB board space.

Crystal Oscillator

• LCP1751 requires a 25MHz crystal, which results in a 100MHz clock with x4 PLL
• The crystal requires 2 caps for stability. The required value is:

2 * (CL - CS)

Where CL is the crystal's load capacitance, as specified by the crystal manufacturer, and CS is the PCB's stray capacitance (around 5pF for a reasonable PCB).

TXC - 9C-25.000MEEJ-T Load capacitance 18pF. Therefore, use 2x 22pF standard ceramic capacitors.

Power Supply

5V supply from hard drive molex connector will provide more than sufficient current.

3.3V current requirements:

1. LPC1751: Rated at 100mA per supply pin, 4 I/O power pins, 2 core power pins, 1 analog supply pin = 700mA.
2. SD Card: 100mA

5V power requirements:

1. SCSI terminator: 440mA (optionally powered by SCSI TERMPWR)
2. 74HCT05: ?? Not much

Linear LDO: Simple, but may get hot Switching: Less heat.

In-circuit programming

UART0 pins, + active low program enable power supply while programming ? Custom header ? Custom bootloader as well ? Reuse for serial debug output ?

Switches

TERMPWR, terminator, parity, SCSI ID