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Contributions from Michael Baar:

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CC1020

o   Turns off only after transmission has ended and not in between

o   Using events for DMA takes too long. It should use a callback when
    DMA finishes to turn off the transmit mode as early as possible.

Core/UART

o   Added profiling and energest initialization

o   Changed low-power mode from LPM1 to LPM3
This commit is contained in:
nvt-se 2007-12-13 12:51:38 +00:00
parent b1d87bf0d3
commit 665c58fd2a
4 changed files with 267 additions and 163 deletions
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142
platform/msb430/contiki-msb430-main.c
View file

@ -47,21 +47,21 @@
#include "sys/procinit.h"
#include "sys/autostart.h"
#include "sys/rtimer.h"
#include "dev/adc.h"
#include "dev/dma.h"
#include "dev/sht11.h"
#include "dev/slip.h"
#include "net/mac/nullmac.h"
#include "net/mac/xmac.h"
#include "dev/slip.h"
extern volatile bool uart_edge;
SENSORS(NULL);
#if WITH_UIP
static struct uip_fw_netif slipif =
{UIP_FW_NETIF(172,16,0,1, 255,255,255,0, slip_send)};
{UIP_FW_NETIF(192,168,1,2, 255,255,255,255, slip_send)};
#else
int
putchar(int c)
@ -75,7 +75,6 @@ static void
set_rime_addr(void)
{
rimeaddr_t addr;
addr.u16[0] = node_id;
rimeaddr_set_node_addr(&addr);
}
@ -83,12 +82,29 @@ set_rime_addr(void)
static void
msb_ports_init(void)
{
P1DIR = 0x00; P1SEL = 0x00; P1OUT = 0x00;
P2DIR = 0x1A; P2SEL = 0x00; P2OUT = 0x18;
P3DIR = 0x21; P3SEL = 0x00; P3OUT = 0x09;
P4DIR = 0x00; P4SEL = 0x00; P4OUT = 0x07;
P5DIR = 0xFD; P5SEL = 0x0E; P5OUT = 0xF9;
P6DIR = 0xC8; P6SEL = 0x07; P6OUT = 0x00;
P1SEL = 0x00;
P1OUT = 0x00;
P1DIR = 0x00;
P2SEL = 0x00;
P2OUT = 0x18;
P2DIR = 0x1A;
P3SEL = 0x00;
P3OUT = 0x09;
P3DIR = 0x21;
P4SEL = 0x00;
P4OUT = 0x00;
P4DIR = 0x00;
P5SEL = 0x0E;
P5OUT = 0xF9;
P5DIR = 0xFD;
P6SEL = 0x07;
P6OUT = 0x00;
P6DIR = 0xC8;
}
int
@ -99,12 +115,10 @@ main(void)
/* Platform-specific initialization. */
msb_ports_init();
adc_init();
dma_init();
clock_init();
leds_init();
leds_on(LEDS_ALL);
sht11_init();
// low level
irq_init();
@ -123,55 +137,101 @@ main(void)
slip_arch_init(BAUD2UBR(115200));
#endif
rtimer_init();
ctimer_init();
energest_init();
node_id_restore();
/* System services */
process_start(&etimer_process, NULL);
#if 0
process_start(&sensors_process, NULL);
#endif
//process_start(&sensors_process, NULL);
/* Radio driver */
cc1020_init(cc1020_config_19200);
//cc1020_init(cc1020_config_19200);
/* Network configuration */
// network configuration
node_id_restore();
#if WITH_UIP
uip_init();
uip_sethostaddr(&slipif.ipaddr);
uip_setnetmask(&slipif.netmask);
/* Point-to-point, no default router. */
uip_fw_default(&slipif);
uip_fw_default(&slipif); /* Point2point, no default router. */
tcpip_set_forwarding(0);
process_start(&tcpip_process, NULL);
process_start(&uip_fw_process, NULL);
process_start(&slip_process, NULL);
#endif /* WITH_UIP */
nullmac_init(&cc1020_driver);
rime_init(&nullmac_driver);
set_rime_addr();
#if WITH_UIP
process_start(&tcpip_process, NULL);
process_start(&uip_fw_process, NULL); /* Start IP output */
process_start(&slip_process, NULL);
#endif /* WITH_UIP */
#if PROFILE_CONF_ON
profile_init();
#endif /* PROFILE_CONF_ON */
printf(CONTIKI_VERSION_STRING " started. Node id %d.\n", node_id);
printf("Autostarting processes\n");
autostart_start((struct process **) autostart_processes);
leds_off(LEDS_ALL);
lpm_on();
for (;;) {
while (process_run() > 0);
if (process_nevents() == 0) {
printf(CONTIKI_VERSION_STRING " started. Node id %u.", node_id);
autostart_start((struct process **) autostart_processes);
energest_init();
/*
* This is the scheduler loop.
*/
ENERGEST_ON(ENERGEST_TYPE_CPU);
while (1) {
int r;
#if PROFILE_CONF_ON
profile_episode_start();
#endif /* PROFILE_CONF_ON */
do {
/* Reset watchdog. */
r = process_run();
} while(r > 0);
#if PROFILE_CONF_ON
profile_episode_end();
#endif /* PROFILE_CONF_ON */
/*
* Idle processing.
*/
int s = splhigh(); /* Disable interrupts. */
if (process_nevents() != 0) {
splx(s); /* Re-enable interrupts. */
} else {
static unsigned long irq_energest = 0;
/* Re-enable interrupts and go to sleep atomically. */
ENERGEST_OFF(ENERGEST_TYPE_CPU);
ENERGEST_ON(ENERGEST_TYPE_LPM);
LPM_SLEEP();
/*
* We only want to measure the processing done in IRQs when we
* are asleep, so we discard the processing time done when we
* were awake.
*/
energest_type_set(ENERGEST_TYPE_IRQ, irq_energest);
if (uart_edge) {
_BIS_SR(LPM1_bits + GIE);
} else {
_BIS_SR(LPM1_bits + GIE);
}
/*
* We get the current processing time for interrupts that was
* done during the LPM and store it for next time around.
*/
dint();
irq_energest = energest_type_time(ENERGEST_TYPE_IRQ);
eint();
ENERGEST_OFF(ENERGEST_TYPE_LPM);
ENERGEST_ON(ENERGEST_TYPE_CPU);
#if PROFILE_CONF_ON
profile_clear_timestamps();
#endif /* PROFILE_CONF_ON */
}
}

23
platform/msb430/dev/cc1020-internal.h
View file

@ -214,11 +214,21 @@ const uint8_t cc1020_config_115200[41] = {
/// cc1020 state
enum cc1020_state {
CC1020_OFF,
CC1020_RX,
CC1020_TX
CC1020_OFF = 0,
CC1020_RX = 0x01,
CC1020_TX = 0x02,
CC1020_RX_SEARCHING = 0x10, // searching for preamble + sync word
CC1020_RX_RECEIVING = 0x20, // receiving bytes
CC1020_RX_PROCESSING = 0x40, // processing data in buffer
CC1020_OP_STATE = 0x73,
CC1020_TURN_OFF = 0x80,
};
#define CC1020_SET_OPSTATE(opstate) cc1020_state = ((cc1020_state & ~CC1020_OP_STATE) | (opstate))
/******************************************************************************
* @name Packet specification
* @{
@ -241,10 +251,3 @@ struct cc1020_header {
#define HDRSIZE (sizeof (struct cc1020_header))
///@}
/// cc1020 receiver state
enum cc1020_rxstate {
CC1020_RX_SEARCHING, // searching for preamble + sync word
CC1020_RX_RECEIVING, // receiving bytes
CC1020_RX_PROCESSING // processing data in buffer
};

113
platform/msb430/dev/cc1020.c
View file

@ -52,6 +52,7 @@ Berlin, 2006
#include <string.h>
#include <signal.h>
#include "contiki.h"
#include "contiki-msb430.h"
#include "cc1020-internal.h"
#include "cc1020.h"
@ -59,6 +60,7 @@ Berlin, 2006
#include "net/rime/rimestats.h"
#include "dev/irq.h"
#include "dev/dma.h"
#include "energest.h"
static int cc1020_calibrate(void);
static int cc1020_setupTX(int);
@ -76,16 +78,16 @@ static enum cc1020_state cc1020_state = CC1020_OFF;
static volatile uint8_t cc1020_rxbuf[HDRSIZE + CC1020_BUFFERSIZE];
static uint8_t cc1020_txbuf[PREAMBLESIZE + HDRSIZE + CC1020_BUFFERSIZE +
TAILSIZE];
static volatile enum cc1020_rxstate cc1020_rxstate = CC1020_RX_SEARCHING;
//static volatile enum cc1020_rxstate cc1020_rxstate = CC1020_RX_SEARCHING;
/// number of bytes in receive and transmit buffers respectively.
// number of bytes in receive and transmit buffers respectively.
static uint16_t cc1020_rxlen;
static uint16_t cc1020_txlen;
/// received signal strength indicator reading for last received packet
// received signal strength indicator reading for last received packet
static volatile uint8_t rssi;
/// callback when a packet has been received
// callback when a packet has been received
static uint8_t cc1020_pa_power = PA_POWER;
static void (*receiver_callback)(const struct radio_driver *);
@ -155,17 +157,15 @@ cc1020_set_rx(void)
// configure driver
cc1020_rxlen = 0; // receive buffer position to start
cc1020_rxstate = CC1020_RX_SEARCHING; // rx state machine to searching mode
cc1020_state = CC1020_RX; // driver state to receive mode
//cc1020_rxstate = CC1020_RX_SEARCHING; // rx state machine to searching mode
CC1020_SET_OPSTATE(CC1020_RX | CC1020_RX_SEARCHING); // driver state to receive mode
// configure radio
ENERGEST_ON(ENERGEST_TYPE_LISTEN);
cc1020_wakeupRX(RX_CURRENT);
cc1020_setupRX(RX_CURRENT);
LNA_POWER_ON(); // enable amplifier
ENERGEST_OFF(ENERGEST_TYPE_TRANSMIT);
ENERGEST_ON(ENERGEST_TYPE_LISTEN);
// activate
IE1 |= URXIE0; // enable interrupt
}
@ -176,6 +176,7 @@ cc1020_set_tx(void)
int s;
// configure radio rx
ENERGEST_OFF(ENERGEST_TYPE_LISTEN);
LNA_POWER_OFF(); // power down LNA
s = splhigh();
DISABLE_RX_IRQ();
@ -183,17 +184,15 @@ cc1020_set_tx(void)
splx(s);
// configure radio tx
ENERGEST_ON(ENERGEST_TYPE_TRANSMIT);
cc1020_wakeupTX(TX_CURRENT);
cc1020_setupTX(TX_CURRENT);
P3SEL |= 0x0C; // select Tx line and clk
U0CTL |= SWRST; // UART to reset mode
IFG1 &= ~UTXIFG0; // Reset IFG.
ENERGEST_OFF(ENERGEST_TYPE_LISTEN);
ENERGEST_ON(ENERGEST_TYPE_TRANSMIT);
// configure driver
cc1020_state = CC1020_TX;
CC1020_SET_OPSTATE(CC1020_TX);
}
void
@ -218,8 +217,10 @@ cc1020_send(const void *buf, unsigned short len)
return -1;
/* Previous data hasn't been sent yet. */
if (cc1020_txlen > 0)
return -1;
if (cc1020_txlen > 0) {
printf("data in buffer");
return -1;
}
/* The preamble and the sync word are already in buffer. */
cc1020_txlen = PREAMBLESIZE + SYNCWDSIZE;
@ -235,7 +236,8 @@ cc1020_send(const void *buf, unsigned short len)
cc1020_txbuf[cc1020_txlen++] = TAIL;
cc1020_txbuf[cc1020_txlen++] = TAIL;
process_poll(&cc1020_sender_process);
//process_poll(&cc1020_sender_process);
process_post_synch(&cc1020_sender_process, PROCESS_EVENT_POLL, NULL);
return len;
}
@ -269,6 +271,7 @@ cc1020_set_receiver(void (*recv)(const struct radio_driver *))
int
cc1020_on(void)
{
cc1020_state &= ~CC1020_TURN_OFF;
// Switch to receive mode
cc1020_set_rx();
@ -280,21 +283,22 @@ cc1020_off(void)
{
int s;
// Discard the current read buffer when the radio is shutting down.
cc1020_rxlen = 0;
LNA_POWER_OFF(); // power down lna
s = splhigh();
cc1020_rxstate = CC1020_OFF;
DISABLE_RX_IRQ();
cc1020_state = CC1020_OFF;
splx(s);
cc1020_setupPD(); // power down radio
cc1020_state = CC1020_OFF;
ENERGEST_OFF(ENERGEST_TYPE_LISTEN);
ENERGEST_OFF(ENERGEST_TYPE_TRANSMIT);
if (cc1020_state & CC1020_RX_SEARCHING) {
// Discard the current read buffer when the radio is shutting down.
cc1020_rxlen = 0;
LNA_POWER_OFF(); // power down lna
s = splhigh();
//cc1020_rxstate = CC1020_OFF;
DISABLE_RX_IRQ();
cc1020_state = CC1020_OFF;
splx(s);
cc1020_setupPD(); // power down radio
ENERGEST_OFF(ENERGEST_TYPE_LISTEN);
cc1020_state = CC1020_OFF;
} else {
cc1020_state |= CC1020_TURN_OFF;
}
return 1;
}
@ -328,8 +332,7 @@ interrupt(UART0RX_VECTOR) cc1020_rxhandler(void)
} shiftbuf;
static unsigned char pktlen;
switch (cc1020_rxstate) {
case CC1020_RX_SEARCHING:
if (cc1020_state & CC1020_RX_SEARCHING) {
shiftbuf.b1 = shiftbuf.b2;
shiftbuf.b2 = shiftbuf.b3;
shiftbuf.b3 = shiftbuf.b4;
@ -364,10 +367,9 @@ interrupt(UART0RX_VECTOR) cc1020_rxhandler(void)
}
// Update RSSI.
rssi = cc1020_read_reg(CC1020_RSS);
cc1020_rxstate = CC1020_RX_RECEIVING;
CC1020_SET_OPSTATE(CC1020_RX | CC1020_RX_RECEIVING);
break;
case CC1020_RX_RECEIVING:
} else if( cc1020_state & CC1020_RX_RECEIVING ) {
if (syncbs == 0) {
cc1020_rxbuf[cc1020_rxlen] = RXBUF0;
} else {
@ -387,13 +389,15 @@ interrupt(UART0RX_VECTOR) cc1020_rxhandler(void)
pktlen = ((struct cc1020_header *)cc1020_rxbuf)->length;
if (pktlen == 0 || pktlen > sizeof (cc1020_rxbuf)) {
cc1020_rxlen = 0;
cc1020_rxstate = CC1020_RX_SEARCHING;
CC1020_SET_OPSTATE(CC1020_RX | CC1020_RX_SEARCHING);
//cc1020_rxstate = CC1020_RX_SEARCHING;
}
} else if (cc1020_rxlen > HDRSIZE) {
if (cc1020_rxlen == pktlen) {
// disable receiver
DISABLE_RX_IRQ();
cc1020_rxstate = CC1020_RX_PROCESSING;
CC1020_SET_OPSTATE(CC1020_RX | CC1020_RX_PROCESSING);
//cc1020_rxstate = CC1020_RX_PROCESSING;
// call receiver to copy from buffer
if (receiver_callback != NULL) {
@ -402,18 +406,21 @@ interrupt(UART0RX_VECTOR) cc1020_rxhandler(void)
// reset receiver
cc1020_rxlen = 0;
cc1020_rxstate = CC1020_RX_SEARCHING;
ENABLE_RX_IRQ();
CC1020_SET_OPSTATE(CC1020_RX | CC1020_RX_SEARCHING);
//cc1020_rxstate = CC1020_RX_SEARCHING;
if ((cc1020_state & CC1020_TURN_OFF) && (cc1020_txlen == 0)) {
cc1020_off();
} else {
ENABLE_RX_IRQ();
}
}
}
break;
default:
break;
}
}
}
PROCESS_THREAD(cc1020_sender_process, ev, data)
{
int i;
PROCESS_BEGIN();
dma_subscribe(0, &cc1020_sender_process);
@ -423,9 +430,9 @@ PROCESS_THREAD(cc1020_sender_process, ev, data)
cc1020_set_rx();
if (cc1020_rxstate != CC1020_RX_SEARCHING) {
if((cc1020_state & CC1020_RX_SEARCHING) == 0) {
// Wait until the receiver is idle.
PROCESS_WAIT_UNTIL(cc1020_rxstate == CC1020_RX_SEARCHING);
PROCESS_WAIT_UNTIL(cc1020_state & CC1020_RX_SEARCHING);
// Wait for the medium to become idle.
while (cc1020_carrier_sense());
@ -437,17 +444,31 @@ PROCESS_THREAD(cc1020_sender_process, ev, data)
// Switch to transceive mode.
cc1020_set_tx();
#if 0
U0CTL &= ~SWRST;
for (i = 0; i < cc1020_txlen; i++) {
UART0_TX = cc1020_txbuf[i];
UART0_WAIT_TX();
}
#endif
// Initiate radio transfer.
dma_transfer(&TXBUF0, cc1020_txbuf, cc1020_txlen);
// wait for DMA0 to finish
PROCESS_WAIT_UNTIL(ev == dma_event);
ENERGEST_OFF(ENERGEST_TYPE_TRANSMIT);
RIMESTATS_ADD(lltx);
// clean up
cc1020_txlen = 0;
cc1020_set_rx();
if (cc1020_state & CC1020_TURN_OFF) {
CC1020_SET_OPSTATE(CC1020_RX | CC1020_RX_SEARCHING);
cc1020_off();
} else {
cc1020_set_rx();
}
}
PROCESS_END();

152
platform/msb430/dev/msb430-uart1.c
View file

@ -32,11 +32,19 @@ whether in contract, strict liability, or tort (including negligence
or otherwise) arising in any way out of the use of this software, even
if advised of the possibility of such damage.
This implementation was developed by the CST group at the FUB.
For documentation and questions please use the web site
http://scatterweb.mi.fu-berlin.de and the mailinglist
scatterweb@lists.spline.inf.fu-berlin.de (subscription via the Website).
Berlin, 2007
*/
/**
* \file UART interface
* \author Michael Baar <baar@inf.fu-berlin.de>
* @file ScatterWeb.Uart.c
* @addtogroup interfaces
* @brief UART interface
* @author Michael Baar <baar@inf.fu-berlin.de>
*
* UART switch for RS232 and SPI protocols on UART1 written for
* ScatterWeb MSB boards. Compatible to ScatterWeb EOS,
@ -54,36 +62,38 @@ if advised of the possibility of such damage.
#define U1ME ME2
#endif
static void uart_configure(unsigned char mode);
static void uart_set_mode(unsigned char mode);
void _uart_configure(unsigned char mode);
void _uart_set_mode(unsigned char mode);
static volatile unsigned char uart_mode = UART_MODE_RESET;
static volatile unsigned char uart_lockcnt;
volatile unsigned char uart_mode = UART_MODE_RESET;
volatile unsigned char uart_lockcnt = 0;
volatile uint8_t uart_edge = 0;
static unsigned char uart_speed_br0[UART_NUM_MODES];
static unsigned char uart_speed_br1[UART_NUM_MODES];
static unsigned char uart_speed_bmn[UART_NUM_MODES];
static fp_uart_handler uart_handler[UART_NUM_MODES];
static unsigned char _uart_speed_br0[UART_NUM_MODES];
static unsigned char _uart_speed_br1[UART_NUM_MODES];
static unsigned char _uart_speed_bmn[UART_NUM_MODES];
static fp_uart_handler _uart_handler[UART_NUM_MODES] = {NULL, NULL};
/*---------------------------------------------------------------------------*/
void
uart_set_speed(unsigned char mode, unsigned char ubr0,
unsigned char ubr1, unsigned char umctl)
{
// store setting
uart_speed_br0[mode] = ubr0; // baudrate
uart_speed_br1[mode] = ubr1; // baudrate
uart_speed_bmn[mode] = umctl; // modulation
_uart_speed_br0[mode] = ubr0; // baudrate
_uart_speed_br1[mode] = ubr1; // baudrate
_uart_speed_bmn[mode] = umctl; // modulation
// reconfigure, if mode active
if (uart_mode == mode)
uart_configure(mode);
_uart_configure(mode);
}
/*---------------------------------------------------------------------------*/
void
uart_set_handler(unsigned char mode, fp_uart_handler fpHandler)
{
// store setting
uart_handler[mode] = fpHandler;
_uart_handler[mode] = fpHandler;
if (mode == uart_mode) {
if (fpHandler == NULL)
IE2 &= ~URXIE1; // Disable USART1 RX interrupt
@ -91,28 +101,30 @@ uart_set_handler(unsigned char mode, fp_uart_handler fpHandler)
IE2 |= URXIE1; // Enable USART1 RX interrupt
}
}
/*---------------------------------------------------------------------------*/
int
uart_lock(unsigned char mode)
{
// already locked?
if ((mode != uart_mode) && (uart_lockcnt)) {
return FALSE;
return 0;
} else {
// increase lock count
uart_lockcnt++;
// switch mode (if neccessary)
_uart_set_mode(mode);
return 1;
}
// increase lock count
uart_lockcnt++;
// switch mode (if neccessary)
uart_set_mode(mode);
return TRUE;
}
/*---------------------------------------------------------------------------*/
int
uart_unlock(unsigned char mode)
{
/* Strict checking. */
if (mode != uart_mode)
return FALSE;
/*
Do we wan't strict checking?
if( (uart_lockcnt == 0) || (mode != uart_mode) )
return false;
*/
// decrement lock
if (uart_lockcnt > 0) {
@ -120,28 +132,28 @@ uart_unlock(unsigned char mode)
// if no more locks, switch back to default mode
if (uart_lockcnt == 0) {
uart_set_mode(UART_MODE_DEFAULT);
_uart_set_mode(UART_MODE_DEFAULT);
}
return TRUE;
return 1;
}
return FALSE;
return 0;
}
static void
uart_configure(unsigned char mode)
/*---------------------------------------------------------------------------*/
void
_uart_configure(unsigned char mode)
{
int s;
s = splhigh();
_DINT(); // disable interrupts
UART_WAIT_TXDONE();
// wait till all buffered data has been transmitted
// configure
if (mode == UART_MODE_RS232) {
P5OUT |= 0x01;
// unselect SPI
P3SEL |= 0xC0;
// select rs232 to RS232 mode
// select rs232
// to RS232 mode
UCTL1 = SWRST | CHAR; // 8-bit character
UTCTL1 |= SSEL1; // UCLK = MCLK
// activate
@ -159,48 +171,56 @@ uart_configure(unsigned char mode)
}
// restore speed settings
UBR01 = uart_speed_br0[mode]; // set baudrate
UBR11 = uart_speed_br1[mode];
UMCTL1 = uart_speed_bmn[mode]; // set modulation
UBR01 = _uart_speed_br0[mode]; // set baudrate
UBR11 = _uart_speed_br1[mode];
UMCTL1 = _uart_speed_bmn[mode]; // set modulation
UCTL1 &= ~SWRST; // clear reset flag
splx(s);
_EINT(); // enable interrupts
}
static void
uart_set_mode(unsigned char mode)
/*---------------------------------------------------------------------------*/
void
_uart_set_mode(unsigned char mode)
{
// do nothing if mode already set
if (mode == uart_mode )
if (mode == uart_mode)
return;
IE2 &= ~(URXIE1 | UTXIE1); // disable irq
uart_configure(mode); // configure uart parameters
_uart_configure(mode); // configure uart parameters
uart_mode = mode;
if (uart_handler[mode] != NULL)
if (_uart_handler[mode] != NULL)
IE2 |= URXIE1; // Enable USART1 RX interrupt
}
interrupt(UART1RX_VECTOR) uart_rx(void)
{
fp_uart_handler handler = uart_handler[uart_mode];
/* Check status register for receive errors. - before reading RXBUF since
it clears the error and interrupt flags */
if (!(URCTL1 & RXERR) && handler != NULL) {
if(handler(UART_RX)) {
LPM_AWAKE();
}
} else {
// read out the char to clear the I-flags, etc.
UART_RX;
}
}
/*---------------------------------------------------------------------------*/
int
uart_get_mode(void)
{
return uart_mode;
}
/*---------------------------------------------------------------------------*/
interrupt(UART1RX_VECTOR)_uart_rx(void)
{
fp_uart_handler handler = _uart_handler[uart_mode];
int c;
if (!(IFG2 & URXIFG1)) {
// If start edge detected, toggle & return
uart_edge = 1;
U1TCTL &= ~URXSE;
U1TCTL |= URXSE;
_BIC_SR_IRQ(LPM3_bits);
return;
}
uart_edge = 0;
if (!(URCTL1 & RXERR)) {
c = UART_RX;
if (handler(c)) {
_BIC_SR_IRQ(LPM3_bits);
}
} else {
// read out the char to clear the I-flags, etc.
c = UART_RX;
}
}