Board/Rev2/pico-ice.kicad_sch - 2022/09/23

[josuah]

Review for the board after the first production batch.

[josuah]

Add a schottky for protection when two voltage sources are present (avoid powering the host USB port)

From @z80 on the chat:

So after my "review" of the pico-ice schematic, I think my only small concern is there could be a schottky diode around about where PTC1 is. That would make it more useable in projects that are not powered by USB but are sometimes connected to a USB host.
For instance, 3v3 of the board connecting to an USB device using 5V logics?
Let's say I connected a 4V-ish lithium battery to VBUS for a battery powered project. Then I connected it to a PC via a USB cable. The PC would charge the battery.
So you can prevent that by putting a diode between VBUS and the battery.
But that doesn't stop the issue in the other direction where VBUS is connected to a 5.5V external power supply. Now current could flow from the 5.5V supply into the PC's power supply.
The designer using pico-ice is free to add the first diode but has no way to retrofit the second into pico-ice.

[josuah]

Change the pinout to avoid GPIO15

From @z80 on the chat:

:
More feedback. GP15 is troublesome on RP2040B0 & RP2040B1. See errata RP2040-E5 in the RP2040 datasheet. TLDR; sometimes the Pico SDK will seize control of GP15 without warning to workaround a USB hardware bug. On the current schematic, Pico GP15 falls in the PICO_PMOD range, which is better than the ICE range imho. Ideally, If possible, I think it would be best if Pico GP15 fell in the PICO_LED_x range, where a temporary takeover would be imperceptible to the human eye. That said, I think it is hugely beneficial that the PICO_PMOD range is on consecutive GPIOs. Because of the enormous value of that from the point-of-view of PIO state machine pin mappings, one should think very carefully about breaking that if necessary to reassign GP15.

Proposed pinout:

0-7 Pico <-> FPGA Bus - SPI0, I2C0,1, UART0,1, PWM0,1,2,3

8 ICE_SO   (SPI1 RX)
9 ICE_SSN  (SPI1 CS)
10 ICE_SCK (SPI1 SCK)
11 ICE_SI  (SPI1 TX)

12 SRAM_SSN
13-15 PICO_LED_x (includes GP15)

16-23  Pico PMOD - SPI0, I2C0,1, UART0,1, PWM0,1,2,3

24 ICE_CLK (CLOCK GPOUT2)
25 PICO_CLKOUT (CLOCK GPOUT 3)
26 ADC0 / ICE_DONE (ADC, PWM5)
27 ADC1 / ICE_RSTn (ADC, PWM5)
28 ADC2 (ADC, PWM6)
29 ADC3 (ADC, PWM6) 

Which brings these changes:

• the ICE_CLK are moved at the other side of the LEDs,
• the LEDs move near the pin GPIO15,
• the ICE_CLK fills the hole of the 3 LEDs block(4),
• the PICO-FPGA and FPGA-FLASH are swapped
• the PICO-PMOD lands after the LED block (hence avoiding the GPIO15)

[josuah]

Resistor array

From z80 on the chat:

Replace R8, R9, R12, R13 & R14 with a single resistor array, like https://www.mouser.com/datasheet/2/315/AOC0000C14-1108062.pdf
Looks like rerouting wouldn't be too much trouble.
That would free up space to make the SRAM's pads a bit bigger. With 1.27mm pitch, that should be pretty easy to solder by hand.