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SERIAL: Use Timer1 instead of Timer2

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Use Timer1 as baudrate generator.
Timer1 can be programmed to have the same accuracy and deviation as
Timer2, up to 115200 at F_SYS = 125 Mhz. See comment
setup_serial_timer1() comment for valid baudrates and settings.
Timer1 is used in 8-bit auto-reload mode.

Timer2 16-bit auto-reload can now be used for other tasks like SYS_TICK.
This commit is contained in:
René van Dorst
2026-01-04 14:15:30 +01:00
parent 8b061d5fb9
commit bbd4656810
1 changed files with 38 additions and 14 deletions
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rtlplayground.c
+38 -14
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@@ -28,6 +28,7 @@ void crc16(__xdata uint8_t *v) __naked;
// Upload Firmware to 1M
#define FIRMWARE_UPLOAD_START 0x100000
// See setup_serial_timer1() for valid baudrate settings!
#define SERIAL_BAUD_RATE 115200
/* All RTL839x switches have an external 25MHz Oscillator,
@@ -1662,25 +1663,48 @@ void init_smi(void)
}
/* Set up serial port 0 using Timer 2 with an external trigger
* as baud generator.
* The external clock generator uses a crystal at 25MHz.
/* Set up serial port 0 using Timer 1 as baudrate generator.
* For x Bd these settings are needed, see table below.
* NOTE: Settings only valid for F_SYS = 125 MHz!
* | Wanted | | TMR | F_SYS | | Actual | |
* | baudrate | SMOD0 | DIV | DIV | TH1 | baudrate | Error |
* | -------- | ----- | --- | ----- | ---- | -------- | ------ |
* | 1200 | 0 | 12 | 255 | 0x01 | 1276.6 | 6.00% |
* | 2400 | 0 | 12 | 136 | 0x78 | 2393.5 | −0.27% |
* | 4800 | 0 | 4 | 203 | 0x35 | 4810.7 | 0.22% |
* | 9600 | 1 | 4 | 203 | 0x35 | 9621.3 | 0.22% |
* | 14400 | 1 | 4 | 136 | 0x78 | 14361.2 | −0.27% |
* | 19200 | 1 | 4 | 102 | 0x9a | 19148.3 | −0.27% |
* | 38400 | 1 | 4 | 51 | 0xcd | 38296.6 | −0.27% |
* | 57600 | 1 | 4 | 34 | 0xde | 57444.9 | −0.27% |
* | 115200 | 1 | 4 | 17 | 0xef | 114889.7 | −0.27% |
*/
void setup_serial(void)
#if CLOCK_HZ != 125000000
#warning "SERIAL 0 baudrate setting may only valid for F_CPU = 125 MHz!"
#endif
void setup_serial_timer1(void)
{
IE = 0;
// Timer 1: Mode 2: automatic reload
TMOD &= 0x0F;
TMOD |= 0xA0; // Timer1: GATE, Mode2: Timer, 8-bit with auto-reload
CKCON |= 0x10; // Timer1 clock divider: F_SYS / 4: T2M = 1, Timer 1 uses clk/4
T2CON = 0x34; // Enable RCLK/TCLK (serial transmit/receive clock for T2), TR2 (Timer 2 RUN), disable CP/RL2 (bit 0)
SCON = 0x50; // Mode = 1: ASYNC 8N1 with T2 as baud-rate generator, REN_0 Receive enable
PCON |= 0x80; // SMOD0 = 1; Double the Baud Rate, don't divide Timer 1 Overflag signal.
// The RCAP2 registers contain the high/low byte that is loaded into
// timer2 when T2 overflows to 0x10000
RCAP2H = (0x10000 - ((CLOCK_HZ / SERIAL_BAUD_RATE + 16) / 32)) >> 8;
RCAP2L = (0x10000 - ((CLOCK_HZ / SERIAL_BAUD_RATE + 16) / 32)) % 0xff;
SCON = 0x50; // Mode = 1: ASYNC 8N1 with Timer 2 as baud-rate generator, REN_0 Receive enable
PCON |= 0x80; // Double the Baud Rate
/* The TH1 register contain the reload value, timer1 when T1 overflows to 0x100.
* NOTE: compiler computs the wrong value. 0xF0 is calculated but 0xEF is the right value for 115200.
* Also https://www.keil.com/products/c51/baudrate.asp confirms this.
* Added 32 before div by 64 to make sure rounding is correct so that the results are right.
*
* TH1 = 0x100 - (2^SMOD0 * F_SYS) / ( TMR1_DIV / BAUDRATE * 32)
*/
TH1 = (0x100 - (((CLOCK_HZ / SERIAL_BAUD_RATE) + 32) / (4 * 16))) & 0xff;
SCON = 0x50;
TCON |= 0x40; // Start timer 1
ET1 = 0; // Timer1 Interrupt is NOT wanted!
TI = 1;
RI = 0;
@@ -1726,7 +1750,7 @@ void bootloader(void)
// HW setup, serial, timer, external IRQs
setup_clock();
setup_timer2();
setup_serial();
setup_serial_timer1();
setup_external_irqs();
EA = 1; // Enable global interrupt