The E20 in greater detail
Next, we’ll take a closer look at some of these capabilities by examining the schematic.
The serial interface and DTR reset enable
Serial0 is accessed on the E20 via a female DB-9 connector (J6). Only two of the traditional RS232 signals are supported, transmit (TX232) and receive (RX232). The Data Transmit Ready signal (DTR232X) is used primarily to reset TINI and access the bootstrap loader. Other signals on the DB-9 can be hardwired to one of two power supply nets (V+ or Vcc). These jumpers are open on the board as shipped. The DTR reset enable consists of a jumper that can be easily removed and replaced. With the jumper in place, TINI will reset if DTR232X is pulled low. This allows you to configure serial0 for use with JavaKit (jumper in place) or maybe a modem (jumper removed). The switch, S1, is not populated. It’s intended to act as a pushbutton reset.
Figure 6-19: Detail of the DTR Reset jumper
The external 1-Wire interface
The pull-up is of special note. XRX1 is a signal that can be used to override the iButton interface on TINI. By pulling it high here, we guarantee that serial1 will use the iButton interface circuitry on the TINI stick. Similarly, XRX0 ensures serial0 will use the RS232 interface on the TINI stick.
The Ethernet interface
The 10-base-T Ethernet interface uses an RJ-45 connector.
The CAN interface
CAN is supported via unpopulated board footprints that allow you to connect (via extension cable) to a DB-9 connector. There is also a spot for a CAN interface chip such as a PCA82C250.
Figure 6-25: CAN interface chip area
The regulated power supply
The regulated power supply circuitry on the E20 is shown in Figure 6-26.
Figure 6-26: Detail of the power supply circuitry
It accepts 8-24V AC/DC but can be bypassed by shorting jumpers J5 and J35. You would use this if you already have a 5V DC power supply you want to use or test with TINI. The connector is male, which mates to many inexpensive multi-voltage AC adapters.
The E20 has a variety of components related to using an external flash with TINI. There is space on the board for the FLASH, as well as selection circuitry. The selection circuitry, shown in Figure 6-28, is unpopulated, with the exception of J27, which is shorted on the board as shipped. The schematic says “default is J13 and J16 closed,” but that’s assuming we’ve installed the second flash. Since the E20 ships without the flash, the jumpers aren’t shorted either. With J27 shorted, the signal /RCE0 is equal to /CE0, which puts the stick’s on-board flash as the default flash.
Figure 6-28: Detail of flash selection circuitry
If we did install the flash on the E20, with J13 and J16 closed, our chip enables would become
Instead of having an onboard flash, and an image of it, we now have our external flash, then our internal flash. The external flash will be the one used by TINI. Closing J14 and J15 instead of J13 and J16 will cause the external flash and the TINI onboard flash to change places. The flash onboard the stick would now be the one used. In either case, if you intend to add the flash to the E20 and utilize jumpers J13, J14, J15, and J16, you will need to open J27. There is an additional feature on the socket board dealing with the flash: a flash override. This is J29 on the socket board. It is not populated.
This is a precautionary device for use in the unlikely event that you should want to upgrade the loader program in your flash. When updating the loader, the old loader first replicates itself in bank 1 of the flash, at 0×10000, then loads the new one at 0. If you short J29, the CPU is forced to execute the loader at 0×10000. So, if during the loader update, something goes wrong before the new loader is completely installed, you can always recover by shorting J29 and forcing the execution to begin with the old loader now residing at 0×10000.
167 – Support for Serial2 and Serial3