SCSI2SD Schematic Notes
Details for the circuit design of SCSI2SD.
SMT Type
- 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 should provide more than sufficient current.
A LMS1585A linear LDO regulator will be used to convert the 5v supply to the required 3.3v.
- 5A max current is more than enough
- Easy TO-220 mounting
- 1.3V dropout @ 3A allows for 5V supply to drop to 4.6V
- Significantly cheaper than a switching regulator
- Simpler than a switching regulator.
- At an expected peak current of 800mA, the regulator will dissipate: (5-3.3)*0.8 = 1.36W
- Thermal Resistance Junction-to-Case: 2.3C/W
- Assuming 40C ambient temperature, a 10C/W (or lower) heatsink will keep temperatures under 57C. [1]. Choosing a better heatsink will reduce temperatures further. eg. Multicomp MC33265 w/ 3.9C/W will keep temps under 49C for $1.71
TODO: Do a better job of calculating LPC heat. Do we need a heatsink for it ?
- Idd * Vdd = 42mA * 3.3 = 0.14W
- + current used by I/O pins
- + SPI peripheral current
- or Flash programming current ?
- Datasheet gives 1.5W as max package dissipation
Maximum power requirements:
| 3.3V | 5V | |
|---|---|---|
| LPC1751 | 600mA
Rated at 100mA per supply pin. |
0 |
| SD Card | 200mA | 0 |
| UCC5617
Optionally powered by SCSI TERMPWR |
0 | 440mA |
| 74HCT05 | 0 | ?
Likely to be insignificant |
| Total | 800mA | > 440mA |
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
