SCSI2SD Schematic Notes

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Revision as of 04:19, 29 February 2012 by Michael (talk | contribs) (→‎Termination)
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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

Power Requirements

3.3V 5V
LPC1751 600mA

Rated at 100mA per supply pin.
Analog supply pin unused.

0
SD Card 200mA

[1]

0
UCC5617 0 440mA
74HCT05 0 ?

Likely to be insignificant

Total 800mA > 440mA

5V supply from a hard drive molex connector should provide more than sufficient current. A regulator will be required to convert the 5v supply to 3.3V.

Preferred Option: Switching Regulator

  • NCP3170
  • Over 90% efficiency with 5V input.
  • CHEAP $1.73
  • * Max load current without a heatsink is 2.75A

Backup Option: Linear Regulator

A LMS1585A linear LDO regulator can 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
  • Max load current without a heatsink is insufficient at 0.7A

In-circuit programming

The LPC17xx micro will be programmed via JTAG using Open OCD.

The standard ARM 0.1" 20-pin JTAG header will be used (see http://www.keil.com/support/man/docs/ulink2/ulink2_hw_connectors.htm for connector and necessary pull-up/pull-down details).

Serial programming of the LPC1751 is performed via the UART0 TX and RX pins. To enter programming mode, P2.10 must be low on RESET. The active-low P2.10 and RESET lines will be pulled up to +3.3V via a 10kΩ resistor to ensure the micro isn't reset.

Termination

  • The ucc5617 will be powered by +5v, not TERMPWR. This enables testing the device without connecting to a live SCSI bus. The PHY essentially connects the outputs back to the inputs, but we still need the terminator powered to provide pullups.
  • A DIP Switch will be used to connect the DISCNCT pin of the ucc5617 to ground if the user wants to disable termination. The pin will be pulled-up to +5V via a 10k resistor.

Switches

  • Parity and SCSI ID will be set via a set of DIP switches to ground.
  • The micro GPIO port pull-ups will be enabled (this is the default anyway).
  • Parity requires 1 bit, SCSI ID requires 3 bits, SCSI Terminator DISCNT requires 1 bit. (5-way DIP switch required)