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# Clock Port

The Sentinel 65X clock port is a small, 28-pin female pin header on the mainboard, which exposes the system's debug serial line, I2C bus, reset and main interrupt lines, `PHI2` clock signal, and a 64-address memory-mapped I/O region, mapped to memory beginning at `0x00DFC0`. This region of uncommitted expansion is reserved for the use of user-installed clock port accessories.

The clock port's pin header has two rows, and a pin and row pitch of 2.54mm.

## Pinout

The clock port's pinout, as seen from above, is shown in the following table:

| Pin   | Label |     | Label  | Pin   |
| :---: | :----:| --- | :----: | :---: |
| 1     |  GND  |     | +3.3V  | 2     |
| 3     |  TXD  |     |  SCL   | 4     |
| 5     |  RXD  |     |  SDA   | 6     |
| 7     |  /CTS |     |  /RES  | 8     |
| 9     |  /RTS |     |  /IRQ  | 10    |
| 11    |  PHI2 |     |  /CS1  | 12    |
| 13    |  /RD  |     |  /WR   | 14    |
| 15    |  A5   |     |  A4    | 16    |
| 17    |  A3   |     |  A2    | 18    |
| 19    |  A1   |     |  A0    | 20    |
| 21    |  D7   |     |  D6    | 22    |
| 23    |  D5   |     |  D4    | 24    |
| 25    |  D3   |     |  D2    | 26    |
| 27    |  D1   |     |  D0    | 28    |