SCSI2SD Schematic Notes

Details for the circuit design of SCSI2SD.

Changes between V1 and V2

The First_reflow_soldering_attempt was a failure. As a result, the schematic is being improved to simplify construction.

Change of microcontroller footprint

The V1 incorrectly made use of a 10x10mm QFN68 0.5mm pitch footprint for the CY8C53 microcontroller, instead of the smaller 8x8mm QFN68. 8x8mm is seriously tiny, with a pitch of only 0.4mm. The TSSOP packages (0.635mm pitch) on the V1 boards had many solder bridges etc, and moving to an even finer pitch device would be well beyond my soldering ability.

The solution is to make use of the 100-pin TQFP versions of the microcontroller instead. The devices use a 0.5mm pitch, 25% larger than the QFN68 devices, and the same as the (incorrect) footprint on the V1 board.

Change of PHY

The V1 schematic made use of a typical bus arrangement to interface to the SCSI wires. SCSI signals were split into read and write paths, with the microcontroller using a signal to switch the bus between read or write mode. This arrangement uses minimal microcontroller pins (18 SCSI pins + 1 R/W signal), but additional interface IC's.

The V2 schematic will switch back to a much simpler arrangement whereby each SCSI signal is connected to the microcontroller twice - once for read, and another for write (via an inverting buffer). This arrangement is possible due to the additional I/Os available with the 100-pin QFP version of the microcontroller, and saves the cost/board space/soldering of 6 additional ICs.

Use of a solder stencil

The 0.5mm pitch of the microcontroller will require the use of a stencil to limit the amount of solder paste applied. I applied way too much paste to the 0.635mm pitch devices on the V1 boards, and this caused many solder bridges.

The 0.5mm pitch will require a laser-cut stencil.

1. Laser-cut 3mil mylar (plastic) stencils available from pololu for $25. Non-framed.
2. Laser-cut 3.5mil kapton (plastic) stencils available from ohararp for $25. Non-framed.

Board specifications

1. 0805 sized components will be used where applicable. These represent a good tradeoff between hand-solderability and PCB board space.
2. Pin pitch will be 0.5mm or (preferably) larger.

Power Supply

Power Requirements

	3.3V
CY8C53	200mA rough estimate
SD Card	200mA Peak value from [1]
Termination	466mA 18 * (2.85V / 110ohms)
Total	866mA

5V supply from a hard drive molex connector should provide more than sufficient current. The 3.3v supply will be regulated from the 5v supply to share input capacitance and therefore reduce complexity.

Switching Regulator Requirements

• Require at least 83% duty cycle, to allow operation down to Vin = 4v.
• Require >= 90% efficiency to reduce heat.
• > 866mA output.
• Fsw >= 1MHz for small output filtering capacitance.
• Easy hand-soldering.

MAX1951 Design

• MAX1951
• Supports Vout == Vin.
  • Regulator won't dropout if the 5V rail temporarily drops down to 3.3V.
• Over 90% efficiency with 5V input.
• 2A output
  • Max load current without a heatsink is 1.36A
• 1MHz fixed switching
• Digital soft-start
• Reasonably priced for single units $4.41
• Designed for use with small ceramic capacitors.
  • Only 10uF is required for both the input & output filters.
  • Additional bulk capacitance will be provided on input to deal with the 5V IC's.

We will take 400mA as the expected load; in truth, it is likely to be much less. Derating to provide a safety margin will be done as a last step in chosing components - this derating should allow a higher load without any problems.

Output Inductor

The MAX1951 recommends a 2μH inductor, making selection very simple.

• 2A minimum current rating, 3A minimum saturation.
• 20mΩ maximum DC resistance.

Given LIR = 30% (Inductor ripple %, recommended range 20%-40%):

Peak inductor current = <math>\left ( 1 + \frac{LIR}{2} \right ) \times I_{OUT} = \left ( 1 + \frac{0.3}{2} \right ) \times 0.4 = 460mA</math>

Given the above values, the chosen inductor is TDK SPM6530T-2R2M ($1.41 for single units).

Inductance	Tolerance	DC Resistance	Current
2.2μH	20%	19mΩ	8.4A

Filter Capacitors

The MAX1951 recommends a 10μF input and output ceramic capacitor. There is no point calculating ripple current/voltages etc, because manufacturers don't bother supplying a ripple voltage spec on their datasheets; the low ESR and ESL of ceramic capacitors makes it somewhat irrelevant.

The value of ceramic capacitors decreases significantly (80% or greater) as they approach their rated voltage. For this reason, the filter caps will be over-rated to 25V, and the bypass caps to 10V.

The chosen capacitors are:

Use	Value	Voltage	Type	Package	Device
Input and Output filtering	10μF	25V	X5R or X7R	0805	Murata GRM21BR61E106KA73L
Input Bulk	47μF	25V	X5R or X7R	1206	TDK C3216X5R1E476M
Bypass	100nF	25V	X5R or X7R	0805	TDK C2012X7R1E104K
Compensation	220pF	50V See Table 2 in datasheet.	C0G/NP0	0805	TDK C2012C0G1H221J

Ferrite Bead

A ferrite bead will be used on the incoming +5V line to reduce EMI being conducted back to the host.

• Ideally, the bead should reject all frequencies from the switching frequency (1MHz) to the CPU Frequency (100MHz). Realistically, very few beads attenuate from 1MHz.
• 2A current rating.

To be safe, we'll chose a ferrite bead with a very minimal DC resistance.

DC Resistance	Peak current	Case style	Impedence	Part
5mΩ	6A	1206	48Ω @ 100MHz	Taiyo Yuden FBMJ3216HS480NT

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)