Level 5: Byte Buster (UART)

01The situation

An incoming craft is calling for docking. Nobody answers, because the station's serial line is dead. Reopen it before the signal fades.

02Why this matters

UART is the first thing an engineer connects on a brand-new board. If you can talk over UART, you can debug anything.

Debug consoleEvery "printf" from a microcontroller travels over UART.
GPS & BluetoothMost plug-in modules talk to the main chip over UART.
Serial MonitorThe Arduino Serial Monitor? That's UART.

03The idea in one picture

Illustration: a start bit, 8 data bits (highest bit first, as in the game) and a stop bit cross the TX wire.

Think of it as: two people reading aloud to each other at a pace they agreed in advance, with no metronome between them. Each can talk (TX) and listen (RX) at the same time, but a listener who isn't paying attention misses the words.

Go deeper

UART is asynchronous: no clock wire is shared, so both ends agree on the speed (the baud rate) beforehand. Transmit and receive are separate wires, so data can flow both ways at once. Two enables are needed: SPEN switches the serial port on, then TXEN wakes the transmitter and CREN opens the receiver.

04Your mission

OSCCONStart the clock. Any speed works.
TXSTASwitch the serial port on and wake the transmitter.
RCSTASwitch the serial port on and open the receiver.

Hint 1

Both registers share one master switch: SPEN. Without it the port stays off, whatever else you set.

Hint 2

SPEN is bit 0 in both registers. TXEN (bit 1 of TXSTA) wakes the transmitter; CREN (bit 1 of RCSTA) opens the receiver.

Show the solution

1Step 1 — OSCCON (0x20)Set it to 0x05 (0000 0101). Switch the internal oscillator on at 4 MHz (any speed wins).
2Step 2 — TXSTA (0x2B)Set it to 0x03 (0000 0011). Enable the serial port and the transmitter.
3Step 3 — RCSTA (0x2C)Set it to 0x03 (0000 0011). Enable the serial port and the receiver.
4Program & ExecutePress P to compile your configuration into machine code in ROM, then press E to run it. When every register matches you get Target Reached.

The same thing as a program (MOVLW loads a value into W, MOVWF copies W into a register):

MOVLW 0x05
MOVWF 0x20   ; OSCCONMOVLW 0x03
MOVWF 0x2B   ; TXSTAMOVLW 0x03
MOVWF 0x2C   ; RCSTA

Bit-by-bit breakdown

OSCCON0x20= 0x05

Switch the internal oscillator on at 4 MHz (any speed wins). 0x05 = 0000 0101 = SCS0 = 1, IRCF = 10

Bit	7	6	5	4	3	2	1	0
Value	0	0	0	0	0	1	0	1
Field	—					IRCF		SCS0

IRCF (bits 2:1) = 10 → 4 MHz
SCS0 (bit 0) = 1 → Internal oscillator on
Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

TXSTA0x2B= 0x03

Enable the serial port and the transmitter. 0x03 = 0000 0011 = SPEN = 1, TXEN = 1

Bit	7	6	5	4	3	2	1	0
Value	0	0	0	0	0	0	1	1
Field	—			TX9	BRGH	SYNC	TXEN	SPEN

TXEN (bit 1) = 1 → Transmitter enabled
SPEN (bit 0) = 1 → Serial port enabled
Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

RCSTA0x2C= 0x03

Enable the serial port and the receiver. 0x03 = 0000 0011 = SPEN = 1, CREN = 1

Bit	7	6	5	4	3	2	1	0
Value	0	0	0	0	0	0	1	1
Field	—			RX9	BRGH	SYNC	CREN	SPEN

CREN (bit 1) = 1 → Receiver enabled
SPEN (bit 0) = 1 → Serial port enabled
Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

Try it: build the values

Flip the bits until each register matches the mission. It's practice only; nothing is saved.

0 of 3 registers match.

OSCCON 0x20

✗ Not yet – 2 bits differ

7–

6–

5–

4–

3–

2IRCF1

1IRCF0

0SCS0

Hex 0x00 Binary 0000 0000 Target 0x05 (0000 0101)

IRCF (bits 2:1) = 00 → 1 MHz
SCS0 (bit 0) = 0 → Internal oscillator not enabled

TXSTA 0x2B

✗ Not yet – 2 bits differ

7–

6–

5–

4TX9

3BRGH

2SYNC

1TXEN

0SPEN

Hex 0x00 Binary 0000 0000 Target 0x03 (0000 0011)

TX9 (bit 4) = 0 → 9-bit transmit bit: not part of the win value (leave 0)
BRGH (bit 3) = 0 → BRGH bit: not part of the win value (leave 0)
SYNC (bit 2) = 0 → SYNC bit: not part of the win value (leave 0)
TXEN (bit 1) = 0 → Transmitter disabled
SPEN (bit 0) = 0 → Serial port disabled

RCSTA 0x2C

✗ Not yet – 2 bits differ

7–

6–

5–

4RX9

3BRGH

2SYNC

1CREN

0SPEN

Hex 0x00 Binary 0000 0000 Target 0x03 (0000 0011)

RX9 (bit 4) = 0 → 9-bit receive bit: not part of the win value (leave 0)
BRGH (bit 3) = 0 → BRGH bit: not part of the win value (leave 0)
SYNC (bit 2) = 0 → SYNC bit: not part of the win value (leave 0)
CREN (bit 1) = 0 → Receiver disabled
SPEN (bit 0) = 0 → Serial port disabled

05How you know it worked

1The transmitter wakesTXREG loads a byte and copies it into the shift register.
Screenshot coming soon
2Bits ride the wireOne bit leaves per clock beat and travels to the pin.
Screenshot coming soon
3The receiver rebuilds itAfter the 8th bit the byte lands in RXREG; the fault panel shows TX and RX.TARGET REACHED
Screenshot coming soon

Simulation note

The game feeds the transmitter itself (a repeating 0x00 → 0xFF sequence); you only switch the port on.

Stuck?

Nothing leaves the TX roomSPEN is missing in TXSTA. Set bit 0.
Bits leave, but the RX room stays emptyThe receiver is off. RCSTA needs CREN (bit 1).
Port is on, but nothing is sentTXEN is off: 0x01 enables the port but not the transmitter.
Everything moves, but no Target ReachedAn extra bit is set (e.g. BRGH). Use exactly 0x03.

06Debrief

You just did day one of every embedded job.

Bringing up a new board starts exactly here: clock on, serial port on, say hello.

// the same idea in CTXSTA = 0x03;  // serial port + transmitter onRCSTA = 0x03;  // serial port + receiver on

On a real chip the same switches can sit at different bit positions; its datasheet has the map. The idea is identical.

Next mission · 06Slave Adventure (SPI)