CH559 Programming (Part 4): Using UART

After toggling some I/O pins, the first thing you should get up and running on a new platform is UART. CH559 chip provides two full-duplex asynchronous serial ports: UART0 and UART1. UART0 is a standard MCS51 serial port, and UART1 is an enhanced asynchronous serial port with a lot of features that we talk about in future tutorials.

We need the datasheet for register definitions. WCH only provides Datasheet in Chinese. I have translated the datasheet, you can find it here. Complete source code is also available on this GitHub Repo.

UART0 Initialization

First, we need to select the UART mode by setting SM0 and SM1 bit in the SCON register. We are going to use Mode 1 for UART0.

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// SM0 & SM1: UART0 mode
// 00 - mode 0, shift Register, baud rate fixed at: Fsys/12
// 01 - mode 1, 8-bit UART,     baud rate = variable by timer1 or timer2 overflow rate
// 10 - mode 2, 9-bit UART,     baud rate fixed at: Fsys/128@SMOD=0, Fsys/32@SMOD=1
// 11 - mode 3, 9-bit UART,     baud rate = variable by timer1 or timer2 overflow rate
SM0 = 0;
SM1 = 1;

Next, we need to select a clock source from either timer1 or timer2. We will choose timer1 as our clock source for UART0 by setting RCLK and TCLK bits in the T2CON register.

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// RCLK select UART0 receiving clock: 0=timer1 overflow pulse, 1=timer2 overflow pulse
// TCLK select UART0 transmittal clock: 0=timer1 overflow pulse, 1=timer2 overflow pulse
RCLK = 0;
TCLK = 0;

We configure timer1 with 12MHz internal clock in mode 2 for baud rate 9600 by setting bT1_M1 and bT1_M0 bits in the TMOD register.

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// bT1_M1 & bT1_M0: timer1 mode
// 00: mode 0, 13-bit timer or counter by cascaded TH1 and lower 5 bits of TL1, the upper 3 bits of TL1 are ignored
// 01: mode 1, 16-bit timer or counter by cascaded TH1 and TL1
// 10: mode 2, TL1 operates as 8-bit timer or counter, and TH1 provide initial value for TL1 auto-reload
// 11: mode 3, stop timer1

TMOD = MASK_T1_MOD & (bT1_M1 | ~bT1_M0);

CH559 provide different speed mode for UART0 baud rate generation for power saving purpose. We are going to use fast Mode by setting a SMOD bit in the PCON register. We also need to set bTMR_CLK and bT1_CLK in the T2MOD register to get a 12MHz clock.

CH559 provides several ways to generate the baud rate. The baud rate generation formula for different UART0, speeds are listed in the table below.

We will choose the 2nd option in the list above and set TH1 register for 9600 baud rate.

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PCON |= SMOD;
T2MOD = T2MOD | bTMR_CLK | bT1_CLK;
TH1 = 256 - FREQ_SYS/16/UART0_BAUD;

Now, we will start the timer1 by setting the TR1 bit. Set the transmit interrupt flag TI for UART0. and enable the UART0 by setting REN bit in the SCON register.

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TR1 = 1;
TI = 1;
REN = 1;

Sending Data to UART0

We can send data via UART0 by writing on the SBUF register.

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// Send one byte data to UART0
void uart0_write(uint8_t data) {
    while (!TI);
    TI = 0;
    SBUF = data & 0xFF;
}

Receiving Data from UART0

We can receive data from UART0 by reading the SBUF register.

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// Receive one byte data from UART0
uint8_t uart0_read() {
    while(!RI);
    RI = 0;
    return SBUF;
}

Using printf with UART0

Redirecting stdout is easy with SDCC

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/*
 * Redirect stdout to UART0
 */
int putchar(int c) {
    uart0_write(c);
    return 0;
}

/*
 * Redirect stdin to UART0
 */
int getchar() {
    return uart0_read();
}

Now we can use printf() for debugging.