RFID freak

I’d like to let everyone who has received a baked Mikeycard  phase 1 spin 2 card to know that it there is a simple fix to get the board to power over USB. The pull-down resistors connected to D0, D1 and D2 lines of the LTC4088 LiPo charger IC are too large so that the lines are never go low. The fix is to replace R27, R28 and R29 with smaller value resistors. I’ve replaced the 10k originals with 1K value resistors and the board now powers up immediately when plugged into USB.

The explanation for the original behaviour  can be found on Table 1 on Page 11 of the datasheet. With the original resistors the D0, D1, and D2 lines are all high thus the LiPo IC is in 2.5mA (Susp High) mode which is insufficient power to boot the CPU, obviously. I measured ~0.8V on plugging in a USB cable to my board with the original pull-down resistors in place. The datasheet indicates on Page 4 indicated that V(IL), that is Input Low Voltage maximum for D0, D1 and D2 is 0.4V.

The pull-down resistors must pull-down D0, D1, and D2 to an acceptable low logic voltage on USB plug-in. Fix your three pull-downs and you’ll now have a board that boots when plugged into USB power only.

Eric