Modifying the AT91SAM7S256-EK to support three UARTs

The AT91SAM7S ARM MCUs provide two full featured USARTs (I'll just call them UARTs for the rest of this page) and a DEBUG serial port which has similar features without synchronous capability and is comparable to a UART in most other MCUs. I wanted to be able to use all three serial ports on the AT91SAM7S256-EK, but they only bring out two of the three to connectors. The two they bring out to connectors are UART0 and the DEBUG serial port. The Debug port is the one that is used by SAM-BA to load code into the Flash of an unprogrammed part. I really wanted all three ports so I decided to add it to the Debug port connector. Here is how I did it. Please read all instructions carefully before starting the work. This will require blind drilling a via which can be difficult. Please evaluate your skills before starting.

Looking at the schematic I saw that there are two RS-232 converter chips to work with the two ports. Each chip has two drivers and two receivers. The chip supporting UART0 uses one pair for serial data and another pair for the handshake lines. The DEBUG serial port has an unused driver and receiver since there are no handshake signals. It seemed a simple matter to wire the unused pair to the UART1 serial data pins and bring the RS-232 side to unused pins on the DEBUG serial port connector.

Here is the component side of the board after the modifications. The connections to the RS-232 transceiver chip are on the top side of the board and the connections to the AT91SAM7S256 and the DEBUG serial port connector are on the bottom.

Figure 1 - Component Side Full Board

The top side wires are shown in more detail here. Two go to the DEBUG serial port connector and two go to the expander connector which connects to the AT91SAM7S256 MCU.

Figure 2 - Component Side Wires

The unused sections of the transceiver chip have their inputs grounded, so the grounds have to be cut. One is at pin 9 on IC2 and can be cut with a knife, but the other is pin 12 which connects to a via that must be drilled out. Worse the via is under the chip so it has to be done without drilling through the chip. This is not too hard to do if you have a small drill press, but otherwise will take some care to do by hand. Note that the schematic counts the pins on IC1 and IC2 as 18 pin chips. I assume they did this to be able to use some chip that has a compatible pinout, but in an 18 pin package. So since these chips have 16 pins, count the first pin on the chip as pin 2.

Here is what the back side of the board looks like with the wires and the drilled via. The drilled via is roughly centered left-right and is about a quarter way down from the top.

Figure 3 - Back Side Wires and Drilled Via

Here is a closeup of the drilled via. I used an 0.032" drill after finding out that an 0.018" drill was not large enough. Both drills were very hard to center over the via since this requires a steady hand and a good eye. I also had a drill press with a dial gauge mounted so I could accurately measure the depth. The PCB is 0.062" with another 0.010" or so clearance below the IC on the other side. If you don't have a drill press you can carefully measure 0.070" up from the tip of your drill and wrap a piece of masking tape. Then drill down until the tape is touching the board. As you can see below, the hole is a bit hard to get centered, I had to drill it three times before it was disconnected fully. The only trace in the area is the IC pad on the top side. So you can use a larger drill, say 0.050", but be careful not to drill the chip or the chip lead on the other side of the board. You can check to make sure you have disconnected the trace from the ground plane by checking with an ohm meter.

Figure 4 - Drilled Via Closeup

You have to cut the trace between ground and pin 9. That can be done on the top side with a sharp knife or a razor blade. I used the drill press since it was handy. You do not need to drill through the board for this one, so I just drilled enough to cut the trace. This trace also grounds another part, so be careful to cut it just next to the pad for pin 9. Otherwise you will cut the ground trace to a capacitor which may make the RS-232 driver stop working. The trace is largely covered by the white silkscreen but you can see where it leaves the via and turns toward the capacitor. Do not cut that trace, only cut it where it turns toward pin 9 of IC2.

Figure 5 - Ground to Pin 9 Cut

I think that is it for the board modifications. Now you need the info on a cable to bring out both the Debug UART and the standard UART1 from the DB-9 connector. I put the extra signals for UART1 on different pins than are used for the handshaking on the connector for UART0. This way I can use the same cable on either connector if I want. Here is a picture of the cable.

Figure 6 - Cable

The cable uses female connectors on all three ends. The AT91SAM7S256-EK uses male connectors as does a standard PC. The cable is wired as a null modem so it can be used without a null modem cable or with just a standard male-female extension cable if needed. The schematics for the cable and for the modified board are shown here.

Figure 7 - Modified AT91SAM7S256-EK

Figure 8 - Cable

I think this is pretty clear and complete. If I have left anything out or anything is not clear, please let me know and I will try to improve it. The photos are fairly high resolution at about 200 kB or more each. If this is inconvenient for users on dialup, I can provide this page with images that have smaller file sizes.

gnuarm dot 2006 at arius dot com