SCSI2SD Schematic Notes: Difference between revisions
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== Termination == | == Termination == | ||
* The [http://www.ti.com/product/ucc5617?qgpn=ucc5617 ucc5617] will be powered by | * The [http://www.ti.com/product/ucc5617?qgpn=ucc5617 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 [http://www.ti.com/product/ucc5617?qgpn=ucc5617 ucc5617] to ground if the user wants to disable termination. The pin will be pulled-up to | * A DIP Switch will be used to connect the DISCNCT pin of the [http://www.ti.com/product/ucc5617?qgpn=ucc5617 ucc5617] to ground if the user wants to disable termination. The pin will be pulled-up to +5V via a 10k resistor. | ||
== Switches == | == Switches == | ||
Revision as of 04:19, 29 February 2012
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
Power Requirements
| 3.3V | 5V | |
|---|---|---|
| LPC1751 | 600mA
Rated at 100mA per supply pin. |
0 |
| SD Card | 200mA | 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)
