The MICROPROCESSOR PRINCIPLES AND APPLICATIONS Lab 7

The MICROPROCESSOR PRINCIPLES AND APPLICATIONS Lab 7 Timer, USART

Cheng-Chien Su 蘇正建 Home Automation, Networking, and Entertainment Lab

Dept. of Computer Science and Information Engineering National Cheng Kung University, TAIWAN

Outline

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Timer0 Introduction & Operation Timer1 Introduction & Operation C18 C Libraries of Timer USART Introduction & Operation C18 C Libraries of USART Lab Reference

Department of Computer Science and Information Engineering National Cheng Kung University, TAIWAN

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Timer0 Introduction

The Timer0 module incorporates the following features:

Software selectable operation as a timer or counter in both 8-bit or 16-bit modes Readable and writable registers Dedicated 8-bit, software programmable

prescaler

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Selectable clock source (internal or external) Edge select for external clock Interrupt-on-overflow


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Timer0 Introduction

Block Diagram

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The T0CON register controls all aspects of the module’s operation, including the prescale selection. It is both readable and writable. A simplified block diagram of the Timer0 module in 8-bit mode is shown in figure.


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Timer0 Introduction T0CON Register

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Timer0 Introduction

Prescaler

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An 8-bit counter is available as a prescaler for the Timer0 module. Its value is set by the PSA and T0PS2:T0PS0 bits (T0CON) which determine the prescaler assignment and prescale ratio. Clearing the PSA bit assigns the prescaler to the Timer0 module. When it is assigned, prescale values from 1:2 through 1:256 in power-of-2 increments are selectable.


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Timer0 Operation

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Timer0 can operate as either a timer or a counter; the mode is selected with the T0CS bit (T0CON). In Timer mode (T0CS = 0), the module increments on every clock by default unless a different prescaler value is selected. The user can work around this by writing an adjusted value to the TMR0 register. The Counter mode is selected by setting the T0CS bit (= 1). In this mode, Timer0 increments either on every rising or falling edge of pin RA4/T0CKI. The incrementing edge is determined by the Timer0 Source Edge Select bit, T0SE (T0CON); clearing this bit selects the rising edge.


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Timer0 Operation

Interrupt

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The TMR0 interrupt is generated when the TMR0 register overflows from FFh to 00h in 8-bit mode, or from FFFFh to 0000h in 16-bit mode. This overflow sets the TMR0IF flag bit. The interrupt can be masked by clearing the TMR0IE bit (INTCON). Before re-enabling the interrupt, the TMR0IF bit must be cleared in software by the Interrupt Service Routine. Since Timer0 is shut down in Sleep mode, the TMR0 interrupt cannot awaken the processor from Sleep.


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Timer1 Introduction

The Timer1 timer/counter module incorporates these features:

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Software selectable operation as a 16-bit timer or counter Readable and writable 8-bit registers (TMR1H and TMR1L) selectable clock source (internal or external) with device clock or Timer1 oscillator internal options Interrupt-on-overflow Reset on CCP Special Event Trigger Device clock status flag (T1RUN)


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Timer1 Introduction Block Diagram

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Timer1 Introduction T1CON Register

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C18 C Libraries of Timerx Overall

Timer Functions: the timer peripherals are supported with the following functions:

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CloseTimerx OpenTimerx ReadTimerx WriteTimerx


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C18 C Libraries of Timer

OpenTimer0: Configure and enable timer0. Include: timers.h Prototype: void OpenTimer0( unsigned char config ); Arguments: config

Enable Timer0 Interrupt:

OpenTimer0

Timer Width:

T0_PS_1_1 1:1 prescale T0_PS_1_2 1:2 prescale … T0_PS_1_256 1:256 prescale

Remarks: This function configures timer0 according to the options specified and then enables it. Code Example:

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T0_EDGE_FALL External clock on falling edge T0_EDGE_RISE External clock on rising edge

Prescale Value:

T0_SOURCE_EXT External clock source (I/O pin) T0_SOURCE_INT Internal clock source (TOSC)

External Clock Trigger (for T0_SOURCE_EXT):

T0_8BIT 8-bit mode T0_16BIT 16-bit mode

Clock Source:

TIMER_INT_ON Interrupt enabled TIMER_INT_OFF Interrupt disabled

OpenTimer0( TIMER_INT_OFF & T0_8BIT & T0_SOURCE_INT & T0_PS_1_32 );


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OpenTimer1: Configure and enable timer1. Include: timers.h Prototype: void OpenTimer1( unsigned char config ); Arguments: config

Enable Timer1 Interrupt:

OpenTimer1

Timer Width:

prescale prescale prescale prescale

T1_OSC1EN_ON Enable Timer1 oscillator T1_OSC1EN_OFF Disable Timer1 oscillator T1_SYNC_EXT_ON Sync external clock input T1_SYNC_EXT_OFF Don’t sync external clock input

Remarks: This function configures timer1 according to the options specified and then enables it. Code Example:

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1:1 1:2 1:4 1:8

Synchronize Clock Input:

T1_PS_1_1 T1_PS_1_2 T1_PS_1_4 T1_PS_1_8

Oscillator Use:

T1_SOURCE_EXT External clock source (I/O pin) T1_SOURCE_INT Internal clock source (TOSC)

Prescaler:

T1_8BIT_RW 8-bit mode T1_16BIT_RW 16-bit mode

Clock Source:

TIMER_INT_ON Interrupt enabled TIMER_INT_OFF Interrupt disabled

OpenTimer1( TIMER_INT_ON & T1_8BIT_RW & T1_SOURCE_EXT & T1_PS_1_1 & T1_OSC1EN_OFF & T1_SYNC_EXT_OFF );


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WriteTimer0

WriteTimer0: Write a value into the specified timer. Include: timers.h Prototype: void WriteTimer0( unsigned int timer ); Arguments: timer

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The value that will be loaded into the specified timer.

Remarks: These functions write a value to the respective timer register(s):TMR0L,TMR0H Code Example: WriteTimer0( 10000 );


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ReadTimer0

ReadTimer0: Read the value of the specified timer. Include: timers.h Prototype: unsigned int ReadTimer0( void ); Remarks: These functions read the value of the respective timer register(s): TMR0L,TMR0H Return Value: The current value of the timer. Note: When using a timer in 8-bit mode that may be configured in 16-bit mode (e.g., timer0), the upper byte is not ensured to be zero. The user may wish to cast the result to a char for correct results. Code Example: unsigned int result; result = (unsigned char) ReadTimer0();

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CloseTimer0

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CloseTimer0: Disable the specified timer. Include: timers.h Prototype: void CloseTimer0( void ); Remarks: This function disables the interrupt and the specified timer.


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C18 C Libraries of Timer Example

#include #include #include void main( void ){ int result; char str[7]; // configure timer0 OpenTimer0( TIMER_INT_OFF & T0_SOURCE_INT & T0_PS_1_32 ); TRISD = 0; while( 1 ) { while( ! PORTBbits.RB3 ); result = ReadTimer0(); if( result > 0xc000 ) break; WriteTimer0( 0 );

// wait for RB3 high // read timer // exit loop if value // is out of range // restart timer

PORTD = (unsinged char) result; } CloseTimer0();

// close modules

}

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USART Introduction

The Enhanced Universal Synchronous Asynchronous Receiver Transmitter (USART) module is one of the two serial I/O modules. The USART can be configured

The Enhanced USART module implements additional features:

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A full-duplex asynchronous system, such as personal computers, etc. A half-duplex, synchronous system, such as serial EEPROMs, etc.

automatic baud rate detection and calibration automatic wake-up on Sync Break reception 12-bit Break character transmit.


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USART Introduction

The USART can be configured in the following modes:

Auto-wake-up on character reception Auto-baud calibration 12-bit Break character transmission

(cont’)

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Asynchronous (full duplex) with:

Synchronous – Master (half duplex) with selectable clock polarity Synchronous – Slave (half duplex) with selectable clock polarity


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USART Introduction

(cont’)

The pins of the Enhanced USART are multiplexed with PORTC. In order to configure RC6/TX/CK and RC7/RX/DT as an USART:

The operation of the Enhanced USART module is controlled through three registers:

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bit SPEN (RCSTA) must be set (= 1) bit TRISC must be set (= 1) bit TRISC must be set (= 1)

Transmit Status and Control (TXSTA) Receive Status and Control (RCSTA) Baud Rate Control (BAUDCON)


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Registers of USART TXSTA Register

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Registers of USART RCSTA Register

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Registers of USART BAUDCON Register

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USART Operation Baud Rate Generator (BRG)

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The BRG is a dedicated, 8-bit or 16-bit generator that supports both the Asynchronous and Synchronous modes of the USART. By default, the BRG operates in 8-bit mode; setting the BRG16 bit (BAUDCON) selects 16-bit mode. The SPBRGH:SPBRG register pair controls the period of a free-running timer. In Asynchronous mode, bits, BRGH (TXSTA) and BRG16 (BAUDCON), also control the baud rate. In Synchronous mode, BRGH is ignored.


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USART Operation

Example

Lookup table Baud Rate Formula

Baud Rate Generator (BRG)

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USART Operation

Asynchronous Mode

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The Asynchronous mode of operation is selected by clearing the SYNC bit (TXSTA). In this mode, the USART uses standard NonReturn-to-Zero (NRZ) format (one Start bit, eight or nine data bits and one Stop bit). The USART transmits and receives the LSB first. The USART’s transmitter and receiver are functionally independent but use the same data format and baud rate. Parity is not supported by the hardware but can be implemented in software and stored as the 9th data bit.


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USART Operation Asynchronous Transmitter

USART Asynchronous Transmitter

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The heart of the transmitter is the Transmit (Serial) Shift Register (TSR). The Shift register obtains its data from the Read/Write Transmit Buffer register, TXREG. The TXREG register is loaded with data in software. The TSR register is not loaded until the Stop bit has been transmitted from the previous load. As soon as the Stop bit is transmitted, the TSR is loaded with new data from the TXREG register (if available).


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This interrupt can be enabled or disabled by setting or clearing the interrupt enable bit, TXIE (PIE1). TRMT is a read-only bit which is set when the TSR register is empty.


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To set up an Asynchronous Transmission: 1. Initialize the SPBRGH:SPBRG registers for the appropriate baud rate. Set or clear the BRGH and BRG16 bits, as required, to achieve the desired baud rate. 2. Enable the asynchronous serial port, SYNC=0 and SPEN=1. 3. If interrupts are desired, TXIE=1. 4. If 9-bit transmission is desired, TX9=1. 5. Enable the transmission by setting bit, TXEN, which will also set bit, TXIF. 6. If 9-bit transmission is selected, the ninth bit should be loaded in bit, TX9D. 7. Load data to the TXREG register (starts transmission). 8. If using interrupts, ensure that the GIE and PEIE bits in the INTCON register (INTCON) are set.


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USART Asynchronous Receiver

USART Operation Asynchronous Receiver

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The data is received on the RX pin and drives the data recovery block. The data recovery block is actually a high-speed shifter operating at x16 times the baud rate, whereas the main receive serial shifter operates at the bit rate or at FOSC. This mode would typically be used in RS-232 systems.


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USART Operation Asynchronous Receiver

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To set up an Asynchronous Reception: 1. Initialize the SPBRGH:SPBRG registers for the appropriate baud rate. Set or clear the BRGH and BRG16 bits, as required, to achieve the desired baud rate. 2. Enable the asynchronous serial port, SYNC=0 and SPEN=1. 3. If interrupts are desired, RCIE=1. 4. If 9-bit reception is desired, RX9=1. 5. Enable the reception by CREN=1. 6. Flag bit, RCIF, will be set when reception is complete and an interrupt will be generated if enable bit, RCIE, was set. 7. Read the RCSTA register to get the 9th bit (if enabled) and determine if any error occurred during reception. 8. Read the 8-bit received data by reading the RCREG register. 9. If any error occurred, clear the error by clearing enable bit, CREN. 10. If using interrupts, ensure that the GIE and PEIE bits in the INTCON register (INTCON) are set.


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C18 Libraries of USART

USART Functions: The following routines are provided for devices with a single USART peripheral:

Overall

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OpenUSART: Configure the specified USART module. Include: usart.h Prototype: void OpenUSART( unsigned char config, unsigned int spbrg); Arguments: config

Interrupt on Transmission:

OpenUSART

Interrupt on Receipt:

USART_SINGLE_RX Single reception USART_CONT_RX Continuous reception

Baud rate:

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USART_SYNC_SLAVE Synchronous Slave mode USART_SYNC_MASTER Synchronous Master mode

Reception mode:

USART_EIGHT_BIT 8-bit transmit/receive USART_NINE_BIT 9-bit transmit/receive

Slave/Master Select*:

USART_ASYNCH_MODE Asynchronous Mode USART_SYNCH_MODE Synchronous Mode

Transmission Width:

USART_RX_INT_ON Receive interrupt ON USART_RX_INT_OFF Receive interrupt OFF

USART Mode:

USART_TX_INT_ON Transmit interrupt ON USART_TX_INT_OFF Transmit interrupt OFF

USART_BRGH_HIGH High baud rate USART_BRGH_LOW Low baud rate


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Arguments: spbrg


Asynchronous mode, high speed: – FOSC / (16 * (spbrg + 1)) Asynchronous mode, low speed: – FOSC / (64 * (spbrg + 1)) Synchronous mode: – FOSC / (4 * (spbrg + 1))

OpenUSART (cont’)

Where FOSC is the oscillator frequency.

Remarks: This function configures the USART module according to the specified configuration options. Code Example:

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This is the value that is written to the baud rate generator register which determines the baud rate at which the USART operates. The formulas for baud rate are:

OpenUSART1( USART_TX_INT_OFF & USART_RX_INT_OFF & USART_ASYNCH_MODE & USART_EIGHT_BIT & USART_CONT_RX & USART_BRGH_HIGH, 25);


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baudUSART: Set the baud rate configuration bits for enhanced USART operation. Include: usart.h Prototype: void baudUSART( unsigned char baudconfig ); Arguments: baudconfig

Clock Idle State:

baudUSART

Baud Rate Generation:

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BAUD_WAKEUP_ON RX pin monitored BAUD_WAKEUP_OFF RX pin not monitored

Baud Rate Measurement:

BAUD_16_BIT_RATE 16-bit baud generation rate BAUD_8_BIT_RATE 8-bit baud generation rate

RX Pin Monitoring:

BAUD_IDLE_CLK_HIGH Clock idle state is a high level BAUD_IDLE_CLK_LOW Clock idle state is a low level

BAUD_AUTO_ON Auto baud rate measurement enabled BAUD_AUTO_OFF Auto baud rate measurement disabled

Remarks: These functions are only available for processors with enhanced USART capability. Code Example: baudUSART (BAUD_IDLE_CLK_HIGH & BAUD_16_BIT_RATE & BAUD_WAKEUP_ON & BAUD_AUTO_ON);


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BusyUSART

BusyUSART: Is the USART transmitting? Include: usart.h Prototype: char BusyUSART( void ); Remarks:

Return Value:

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Returns a value indicating if the USART transmitter is currently busy. This function should be used prior to commencing a new transmission. BusyUSART should be used on parts with a single USART peripheral. 0 if the USART transmitter is idle 1 if the USART transmitter is in use

Code Example: while (BusyUSART());


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WritUSART putcUSART

WritUSART / putcUSART: Write a byte (one character) to the USART transmit buffer, including the 9th bit if enabled. Include: usart.h Prototype:

Arguments: data

The value to be written to the USART.

Remarks:

void WritUSART( char data ); void putcUSART( char data );

This function writes a byte to the USART transmit buffer. If 9-bit mode is enabled, the 9th bit is written from the field TX_NINE, found in a variable of type USART.

Code Example: unsigned int outval; USART_Status.TX_NINE = (outval & 0x0100) >> 8; Write1USART( (char) outval );

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putsUSART putrsUSART

putsUSART and putrsUSART: Writes a string of characters to the USART including the null character. Include: usart.h Prototype:

Arguments: data

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Pointer to a null-terminated string of data.

Remarks:

void putsUSART( char *data ); void putrsUSART( const rom char *data );

This function only works in 8-bit transmit/receive mode. This function writes a string of data to the USART including the null character. Strings located in data memory should be used with the “puts” versions of these functions. Strings located in program memory, including string literals, should be used with the “putrs” versions of these functions.

Code Example: putrsUSART( “Hello World!” );


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DataRdyUSART

DataRdyUSART: Is data available in the read buffer? Include: usart.h Prototype: char DataRdyUSART( void ); Remarks: This function returns the status of the RCIF flag bit in the PIR register. Return Value:

Code Example:

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1 if data is available 0 if data is not available while (!DataRdyUSART());


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ReadUSART / getcUSART : Read a byte (one character) out of the USART receive buffer, including the 9th bit if enabled. Include: usart.h Prototype:

Remarks:

ReadUSART getcUSART

This function reads a byte out of the USART receive buffer. The Status bits and the 9th data bits are saved in a union with the following declaration: union USART { unsigned char val; struct { unsigned RX_NINE:1; unsigned TX_NINE:1; unsigned FRAME_ERROR:1; unsigned OVERRUN_ERROR:1; unsigned fill:4; }; };

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char ReadUSART( void ); char getcUSART(void );

The 9th bit is read-only if 9-bit mode is enabled. The Status bits are always read.

Return Value: This function returns the next character in the USART receive buffer. Code Example: int result; result = ReadUSART(); result |= (unsigned int) USART_Status.RX_NINE