PS/2 library receives data partially by interrupt

example_keyboards
tmk 14 years ago
parent 0632618d29
commit 04f351b802

181
ps2.c

@ -1,5 +1,5 @@
/* /*
Copyright (c) 2010 Jun WAKO <wakojun@gmail.com> Copyright (c) 2010,2011 Jun WAKO <wakojun@gmail.com>
This software is licensed with a Modified BSD License. This software is licensed with a Modified BSD License.
All of this is supposed to be Free Software, Open Source, DFSG-free, All of this is supposed to be Free Software, Open Source, DFSG-free,
@ -36,12 +36,13 @@ POSSIBILITY OF SUCH DAMAGE.
*/ */
#include <stdbool.h> #include <stdbool.h>
#include <avr/io.h> #include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h> #include <util/delay.h>
#include "ps2.h" #include "ps2.h"
#include "print.h"
#include "debug.h" #include "debug.h"
static uint8_t recv_data(void);
static inline void clock_lo(void); static inline void clock_lo(void);
static inline void clock_hi(void); static inline void clock_hi(void);
static inline bool clock_in(void); static inline bool clock_in(void);
@ -52,6 +53,8 @@ static inline uint16_t wait_clock_lo(uint16_t us);
static inline uint16_t wait_clock_hi(uint16_t us); static inline uint16_t wait_clock_hi(uint16_t us);
static inline uint16_t wait_data_lo(uint16_t us); static inline uint16_t wait_data_lo(uint16_t us);
static inline uint16_t wait_data_hi(uint16_t us); static inline uint16_t wait_data_hi(uint16_t us);
static inline void idle(void);
static inline void inhibit(void);
/* /*
@ -79,38 +82,38 @@ http://www.mcamafia.de/pdf/ibm_hitrc07.pdf
} \ } \
} while (0) } while (0)
#define WAIT_NORETRY(stat, us, err) do { \
if (!wait_##stat(us)) { \
ps2_error = err; \
return 0; \
} \
} while (0)
uint8_t ps2_error = PS2_ERR_NONE; uint8_t ps2_error = PS2_ERR_NONE;
void ps2_host_init(void) void ps2_host_init(void)
{ {
/* inhibit */ #ifdef PS2_INT_ENABLE
clock_lo(); PS2_INT_ENABLE();
data_hi(); idle();
#else
inhibit();
#endif
} }
uint8_t ps2_host_send(uint8_t data) uint8_t ps2_host_send(uint8_t data)
{ {
bool parity = true; bool parity;
RETRY:
parity = true;
ps2_error = 0; ps2_error = 0;
/* request to send */ /* terminate a transmission if we have */
clock_lo(); inhibit();
_delay_us(100); _delay_us(100);
/* start bit [1] */ /* start bit [1] */
data_lo(); data_lo();
clock_hi(); clock_hi();
WAIT(clock_lo, 15000, 1); WAIT(clock_lo, 15000, 1);
/* data [2-9] */ /* data [2-9] */
for (uint8_t i = 0; i < 8; i++) { for (uint8_t i = 0; i < 8; i++) {
_delay_us(15);
if (data&(1<<i)) { if (data&(1<<i)) {
parity = !parity; parity = !parity;
data_hi(); data_hi();
@ -121,44 +124,145 @@ uint8_t ps2_host_send(uint8_t data)
WAIT(clock_lo, 50, 3); WAIT(clock_lo, 50, 3);
} }
/* parity [10] */ /* parity [10] */
_delay_us(15);
if (parity) { data_hi(); } else { data_lo(); } if (parity) { data_hi(); } else { data_lo(); }
WAIT(clock_hi, 50, 4); WAIT(clock_hi, 50, 4);
WAIT(clock_lo, 50, 5); WAIT(clock_lo, 50, 5);
/* stop bit [11] */ /* stop bit [11] */
_delay_us(15);
data_hi(); data_hi();
/* ack [12] */ /* ack [12] */
WAIT(data_lo, 50, 6); WAIT(data_lo, 50, 6);
WAIT(clock_lo, 50, 7); WAIT(clock_lo, 50, 7);
/* wait for idle state */
WAIT(clock_hi, 50, 8); WAIT(clock_hi, 50, 8);
WAIT(data_hi, 50, 9); WAIT(data_hi, 50, 9);
/* inhibit device to send */ uint8_t res = ps2_host_recv_response();
clock_lo(); if (res == 0xFE && data != 0xFE)
goto RETRY;
return 1; inhibit();
return res;
ERROR: ERROR:
/* inhibit device to send */ inhibit();
data_hi();
clock_lo();
return 0; return 0;
} }
uint8_t ps2_host_recv(void) /* receive data when host want else inhibit communication */
uint8_t ps2_host_recv_response(void)
{ {
uint8_t data = 0; uint8_t data = 0;
bool parity = true;
ps2_error = 0;
/* terminate a transmission if we have */ /* terminate a transmission if we have */
clock_lo(); inhibit();
_delay_us(100); _delay_us(100);
/* release lines(idle state) */ /* release lines(idle state) */
clock_hi(); idle();
data_hi();
/* wait start bit */
wait_clock_lo(2000);
data = recv_data();
inhibit();
return data;
}
#ifndef PS2_INT_VECT
uint8_t ps2_host_recv(void)
{
return ps2_host_recv_response();
}
#else
/* ring buffer to store ps/2 key data */
#define PBUF_SIZE 8
static uint8_t pbuf[PBUF_SIZE];
static uint8_t pbuf_head = 0;
static uint8_t pbuf_tail = 0;
static inline void pbuf_enqueue(uint8_t data)
{
if (!data)
return;
uint8_t next = (pbuf_head + 1) % PBUF_SIZE;
if (next != pbuf_tail) {
pbuf[pbuf_head] = data;
pbuf_head = next;
} else {
print("pbuf: full\n");
}
}
static inline uint8_t pbuf_dequeue(void)
{
uint8_t val = 0;
uint8_t sreg = SREG;
cli();
if (pbuf_head != pbuf_tail) {
val = pbuf[pbuf_tail];
pbuf_tail = (pbuf_tail + 1) % PBUF_SIZE;
}
SREG = sreg;
return val;
}
/* get data received by interrupt */
uint8_t ps2_host_recv(void)
{
return pbuf_dequeue();
}
ISR(PS2_INT_VECT)
{
PORTC = 0xFF;
/* interrupt means start bit comes */
pbuf_enqueue(recv_data());
/* release lines(idle state) */
idle();
_delay_us(5);
PORTC = 0x00;
}
#endif
/*
static void ps2_reset(void)
{
ps2_host_send(0xFF);
if (ps2_host_recv_response() == 0xFA) {
_delay_ms(1000);
ps2_host_recv_response();
}
}
*/
/* send LED state to keyboard */
void ps2_host_set_led(uint8_t led)
{
#ifdef PS2_INT_DISABLE
PS2_INT_DISABLE();
#endif
ps2_host_send(0xED);
ps2_host_recv_response();
ps2_host_send(led);
ps2_host_recv_response();
#ifdef PS2_INT_ENABLE
PS2_INT_ENABLE();
idle();
#endif
}
/* called after start bit comes */
static uint8_t recv_data(void)
{
uint8_t data = 0;
bool parity = true;
ps2_error = 0;
/* start bit [1] */ /* start bit [1] */
WAIT(clock_lo, 2000, 1); // How long should we wait? WAIT(clock_lo, 1, 1);
WAIT(data_lo, 1, 2); WAIT(data_lo, 1, 2);
WAIT(clock_hi, 50, 3); WAIT(clock_hi, 50, 3);
@ -185,18 +289,11 @@ uint8_t ps2_host_recv(void)
WAIT(data_hi, 1, 9); WAIT(data_hi, 1, 9);
WAIT(clock_hi, 50, 10); WAIT(clock_hi, 50, 10);
/* inhibit device to send */
clock_lo();
return data; return data;
ERROR: ERROR:
/* inhibit device to send */
data_hi();
clock_lo();
return 0; return 0;
} }
static inline void clock_lo() static inline void clock_lo()
{ {
PS2_CLOCK_PORT &= ~(1<<PS2_CLOCK_BIT); PS2_CLOCK_PORT &= ~(1<<PS2_CLOCK_BIT);
@ -252,3 +349,17 @@ static inline uint16_t wait_data_hi(uint16_t us)
while (!data_in() && us) { asm(""); _delay_us(1); us--; } while (!data_in() && us) { asm(""); _delay_us(1); us--; }
return us; return us;
} }
/* idle state that device can send */
static inline void idle(void)
{
clock_hi();
data_hi();
}
/* inhibit device to send */
static inline void inhibit(void)
{
clock_lo();
data_hi();
}

10
ps2.h

@ -1,5 +1,5 @@
/* /*
Copyright (c) 2010 Jun WAKO <wakojun@gmail.com> Copyright (c) 2010,2011 Jun WAKO <wakojun@gmail.com>
This software is licensed with a Modified BSD License. This software is licensed with a Modified BSD License.
All of this is supposed to be Free Software, Open Source, DFSG-free, All of this is supposed to be Free Software, Open Source, DFSG-free,
@ -66,11 +66,13 @@ POSSIBILITY OF SUCH DAMAGE.
extern uint8_t ps2_error; extern uint8_t ps2_error;
/* host side */ /* host role */
void ps2_host_init(void); void ps2_host_init(void);
uint8_t ps2_host_send(uint8_t); uint8_t ps2_host_send(uint8_t data);
uint8_t ps2_host_recv_response(void);
uint8_t ps2_host_recv(void); uint8_t ps2_host_recv(void);
void ps2_host_set_led(uint8_t usb_led);
/* TODO: device side */ /* device role */
#endif #endif

@ -23,14 +23,38 @@
# define MOUSEKEY_DELAY_TIME 255 # define MOUSEKEY_DELAY_TIME 255
#endif #endif
/* PS/2 mouse */ /* PS/2 lines */
#define PS2_CLOCK_PORT PORTD #define PS2_CLOCK_PORT PORTD
#define PS2_CLOCK_PIN PIND #define PS2_CLOCK_PIN PIND
#define PS2_CLOCK_DDR DDRD #define PS2_CLOCK_DDR DDRD
#define PS2_CLOCK_BIT 6 #define PS2_CLOCK_BIT 3
#define PS2_DATA_PORT PORTD #define PS2_DATA_PORT PORTD
#define PS2_DATA_PIN PIND #define PS2_DATA_PIN PIND
#define PS2_DATA_DDR DDRD #define PS2_DATA_DDR DDRD
#define PS2_DATA_BIT 7 #define PS2_DATA_BIT 7
/* External interrupt for PS/2 clock line (optional) */
#define PS2_INT_ENABLE() do { \
EIMSK |= (1<<INT1); \
EICRA |= ((1<<ISC11) | (0<<ISC10)); \
EIFR |= (1<<INTF1); \
} while (0)
#define PS2_INT_DISABLE() do { \
EIMSK &= ~(1<<INT1); \
} while (0)
#define PS2_INT_VECT INT1_vect
/* Pin Change interrupt for PS/2 clock line (optional)
#define PS2_INT_ENABLE() do { \
PCMSK2 |= (1<<PCINT22); \
PCICR |= (1<<PCIE2); \
PCIFR |= (1<<PCIF2); \
} while (0)
#define PS2_INT_DISABLE() do { \
PCMSK2 &= ~(1<<PCINT22); \
PCICR &= ~(1<<PCIE); \
} while (0)
#define PS2_INT_VECT PCINT2_vect
*/
#endif #endif

@ -29,6 +29,7 @@ typedef struct {
} report_mouse_t; } report_mouse_t;
extern uint8_t host_keyboard_led;
void host_keyboard_send(report_keyboard_t *report); void host_keyboard_send(report_keyboard_t *report);
void host_mouse_send(report_mouse_t *report); void host_mouse_send(report_mouse_t *report);

@ -6,7 +6,7 @@
#include "host_vusb.h" #include "host_vusb.h"
#define KBUF_SIZE 8 #define KBUF_SIZE 16
static report_keyboard_t kbuf[KBUF_SIZE]; static report_keyboard_t kbuf[KBUF_SIZE];
static uint8_t kbuf_head = 0; static uint8_t kbuf_head = 0;
static uint8_t kbuf_tail = 0; static uint8_t kbuf_tail = 0;
@ -14,12 +14,18 @@ static uint8_t kbuf_tail = 0;
void host_vusb_keyboard_send() void host_vusb_keyboard_send()
{ {
while (usbInterruptIsReady() && kbuf_head != kbuf_tail) {
usbSetInterrupt((void *)&kbuf[kbuf_tail], sizeof(report_keyboard_t));
kbuf_tail = (kbuf_tail + 1) % KBUF_SIZE;
}
/*
if (kbuf_head != kbuf_tail) { if (kbuf_head != kbuf_tail) {
if (usbInterruptIsReady()) { if (usbInterruptIsReady()) {
usbSetInterrupt((void *)&kbuf[kbuf_tail], sizeof(report_keyboard_t)); usbSetInterrupt((void *)&kbuf[kbuf_tail], sizeof(report_keyboard_t));
kbuf_tail = (kbuf_tail + 1) % KBUF_SIZE; kbuf_tail = (kbuf_tail + 1) % KBUF_SIZE;
} }
} }
*/
} }
void host_keyboard_send(report_keyboard_t *report) void host_keyboard_send(report_keyboard_t *report)
@ -28,14 +34,20 @@ void host_keyboard_send(report_keyboard_t *report)
if (next != kbuf_tail) { if (next != kbuf_tail) {
kbuf[kbuf_head] = *report; kbuf[kbuf_head] = *report;
kbuf_head = next; kbuf_head = next;
print("kbuf: "); phex(kbuf_head); phex(kbuf_tail); print("\n");
} else {
print("kbuf: full\n");
// hmm...
/*
matrix_init();
kbuf_head = 0;
kbuf_tail = 0;
*/
} }
} }
void host_mouse_send(report_mouse_t *report) void host_mouse_send(report_mouse_t *report)
{ {
// dirty hack to send twice a loop :(
//while (!usbInterruptIsReady3()) usbPoll();
if (usbInterruptIsReady3()) { if (usbInterruptIsReady3()) {
usbSetInterrupt3((void *)report, sizeof(*report)); usbSetInterrupt3((void *)report, sizeof(*report));
} else { } else {
@ -46,30 +58,51 @@ void host_mouse_send(report_mouse_t *report)
static struct {
uint16_t len;
enum {
NONE,
SET_LED
} kind;
} last_req;
uint8_t host_keyboard_led = 0;
static uchar idleRate;
static uchar idleRate; /* repeat rate for keyboards, never used for mice */
usbMsgLen_t usbFunctionSetup(uchar data[8]) usbMsgLen_t usbFunctionSetup(uchar data[8])
{ {
usbRequest_t *rq = (void *)data; usbRequest_t *rq = (void *)data;
print("Setup: "); //print("Setup: ");
if((rq->bmRequestType & USBRQ_TYPE_MASK) == USBRQ_TYPE_CLASS){ /* class request type */ if((rq->bmRequestType & USBRQ_TYPE_MASK) == USBRQ_TYPE_CLASS){ /* class request type */
/*
print("CLASS: "); print("CLASS: ");
phex(rq->bRequest); phex(rq->bRequest); print(" ");
phex16(rq->wValue.word); print(" ");
phex16(rq->wIndex.word); print(" ");
phex16(rq->wLength.word); print(" ");
*/
if(rq->bRequest == USBRQ_HID_GET_REPORT){ if(rq->bRequest == USBRQ_HID_GET_REPORT){
print("GET_REPORT"); print(" GET_REPORT");
/* we only have one report type, so don't look at wValue */ /* we only have one report type, so don't look at wValue */
usbMsgPtr = (void *)keyboard_report; usbMsgPtr = (void *)keyboard_report;
return sizeof(*keyboard_report); return sizeof(*keyboard_report);
}else if(rq->bRequest == USBRQ_HID_GET_IDLE){ }else if(rq->bRequest == USBRQ_HID_GET_IDLE){
print("GET_IDLE: "); print(" GET_IDLE: ");
phex(idleRate); phex(idleRate);
usbMsgPtr = &idleRate; usbMsgPtr = &idleRate;
return 1; return 1;
}else if(rq->bRequest == USBRQ_HID_SET_IDLE){ }else if(rq->bRequest == USBRQ_HID_SET_IDLE){
idleRate = rq->wValue.bytes[1]; idleRate = rq->wValue.bytes[1];
print("SET_IDLE: "); print(" SET_IDLE: ");
phex(idleRate); phex(idleRate);
}else if(rq->bRequest == USBRQ_HID_SET_REPORT){
//print(" SET_REPORT: ");
if (rq->wValue.word == 0x0200 && rq->wIndex.word == 0) {
last_req.kind = SET_LED;
last_req.len = rq->wLength.word;
}
return USB_NO_MSG; // to get data in usbFunctionWrite
} }
print("\n"); print("\n");
}else{ }else{
@ -79,6 +112,26 @@ usbRequest_t *rq = (void *)data;
return 0; /* default for not implemented requests: return no data back to host */ return 0; /* default for not implemented requests: return no data back to host */
} }
uchar usbFunctionWrite(uchar *data, uchar len)
{
if (last_req.len == 0) {
return -1;
}
switch (last_req.kind) {
case SET_LED:
//print("SET_LED\n");
host_keyboard_led = data[0];
last_req.len = 0;
return 1;
break;
case NONE:
default:
return -1;
break;
}
return 1;
}
PROGMEM uchar keyboard_hid_report[] = { PROGMEM uchar keyboard_hid_report[] = {
0x05, 0x01, // Usage Page (Generic Desktop), 0x05, 0x01, // Usage Page (Generic Desktop),

@ -4,4 +4,3 @@
void host_vusb_keyboard_send(void); void host_vusb_keyboard_send(void);
#endif #endif

@ -1,12 +1,30 @@
#include "usb_keycodes.h" #include "usb_keycodes.h"
#include "host.h" #include "host.h"
#include "ps2.h"
#include "usb.h"
#include "keyboard.h" #include "keyboard.h"
#include "print.h"
static report_keyboard_t report0; static report_keyboard_t report0;
static report_keyboard_t report1; static report_keyboard_t report1;
static report_keyboard_t *report = &report0; static report_keyboard_t *report = &report0;
static report_keyboard_t *report_prev = &report1; static report_keyboard_t *report_prev = &report1;
void keyboard_set_led(uint8_t usb_led)
{
uint8_t ps2_led = 0;
if (usb_led & (1<<USB_LED_SCROLL_LOCK))
ps2_led |= (1<<PS2_LED_SCROLL_LOCK);
if (usb_led & (1<<USB_LED_NUM_LOCK))
ps2_led |= (1<<PS2_LED_NUM_LOCK);
if (usb_led & (1<<USB_LED_CAPS_LOCK))
ps2_led |= (1<<PS2_LED_CAPS_LOCK);
print("ps2_led: "); phex(ps2_led); print("\n");
ps2_host_set_led(ps2_led);
}
void keyboard_send(void) void keyboard_send(void)
{ {
host_keyboard_send(report); host_keyboard_send(report);

@ -6,6 +6,7 @@
#include "host.h" #include "host.h"
void keyboard_set_led(uint8_t led);
void keyboard_send(void); void keyboard_send(void);
bool keyboard_has_key(void); bool keyboard_has_key(void);
void keyboard_add_mod(uint8_t mod); void keyboard_add_mod(uint8_t mod);

@ -28,9 +28,13 @@
#include "host_vusb.h" #include "host_vusb.h"
#include "timer.h" #include "timer.h"
#define DEBUGP_INIT() do { DDRC = 0xFF; } while (0)
#define DEBUGP(x) do { PORTC = x; } while (0)
static uint8_t last_led = 0;
int main(void) int main(void)
{ {
DEBUGP_INIT();
wdt_enable(WDTO_1S); wdt_enable(WDTO_1S);
/* Even if you don't use the watchdog, turn it off here. On newer devices, /* Even if you don't use the watchdog, turn it off here. On newer devices,
* the status of the watchdog (on/off, period) is PRESERVED OVER RESET! * the status of the watchdog (on/off, period) is PRESERVED OVER RESET!
@ -60,10 +64,12 @@ int main(void)
uint8_t fn_bits = 0; uint8_t fn_bits = 0;
while (1) { /* main event loop */ while (1) { /* main event loop */
DEBUGP(0x01);
wdt_reset(); wdt_reset();
usbPoll(); usbPoll();
host_vusb_keyboard_send(); host_vusb_keyboard_send();
DEBUGP(0x02);
matrix_scan(); matrix_scan();
fn_bits = 0; fn_bits = 0;
keyboard_swap_report(); keyboard_swap_report();
@ -94,10 +100,16 @@ int main(void)
} }
} }
} }
DEBUGP(0x03);
layer_switching(fn_bits); layer_switching(fn_bits);
if (matrix_is_modified()) { if (matrix_is_modified()) {
keyboard_send(); keyboard_send();
} }
mousekey_send(); mousekey_send();
if (last_led != host_keyboard_led) {
keyboard_set_led(host_keyboard_led);
last_led = host_keyboard_led;
}
} }
} }

@ -60,8 +60,6 @@ static bool matrix_has_ghost_in_row(uint8_t row);
#endif #endif
static void matrix_make(uint8_t code); static void matrix_make(uint8_t code);
static void matrix_break(uint8_t code); static void matrix_break(uint8_t code);
static void ps2_reset(void);
static void ps2_set_leds(uint8_t leds);
inline inline
@ -79,19 +77,7 @@ uint8_t matrix_cols(void)
void matrix_init(void) void matrix_init(void)
{ {
ps2_host_init(); ps2_host_init();
_delay_ms(1000);
// flush LEDs
/*
ps2_set_leds(1<<PS2_LED_NUM_LOCK);
_delay_ms(100);
ps2_set_leds(1<<PS2_LED_NUM_LOCK|1<<PS2_LED_CAPS_LOCK);
_delay_ms(100);
ps2_set_leds(1<<PS2_LED_NUM_LOCK|1<<PS2_LED_CAPS_LOCK|1<<PS2_LED_SCROLL_LOCK);
_delay_ms(300);
ps2_set_leds(0x00);
*/
// initialize matrix state: all keys off // initialize matrix state: all keys off
for (uint8_t i=0; i < MATRIX_ROWS; i++) matrix[i] = 0x00; for (uint8_t i=0; i < MATRIX_ROWS; i++) matrix[i] = 0x00;
@ -190,10 +176,7 @@ uint8_t matrix_scan(void)
} }
uint8_t code; uint8_t code;
code = ps2_host_recv(); while ((code = ps2_host_recv())) {
if (code == 0x00) return 0;
//while ((code = ps2_host_recv())) {
//phex(code); print(" ");
switch (state) { switch (state) {
case INIT: case INIT:
switch (code) { switch (code) {
@ -350,26 +333,7 @@ uint8_t matrix_scan(void)
default: default:
state = INIT; state = INIT;
} }
//}
//print("|");
// handle LED indicators
/*
static uint8_t prev_leds = 0;
if (prev_leds != usb_keyboard_leds) {
uint8_t leds = 0;
if (usb_keyboard_leds&(1<<USB_LED_SCROLL_LOCK))
leds |= (1<<PS2_LED_SCROLL_LOCK);
if (usb_keyboard_leds&(1<<USB_LED_NUM_LOCK))
leds |= (1<<PS2_LED_NUM_LOCK);
if (usb_keyboard_leds&(1<<USB_LED_CAPS_LOCK))
leds |= (1<<PS2_LED_CAPS_LOCK);
ps2_set_leds(leds);
prev_leds = usb_keyboard_leds;
} }
*/
return 1; return 1;
} }
@ -479,19 +443,3 @@ static void matrix_break(uint8_t code)
//print("matrix_break: "); phex(code); print("\n"); //print("matrix_break: "); phex(code); print("\n");
} }
} }
static void ps2_reset(void)
{
ps2_host_send(0xFF);
if (ps2_host_recv() != 0xFA) return;
_delay_ms(1000);
if (ps2_host_recv() != 0xAA) return;
}
static void ps2_set_leds(uint8_t leds)
{
ps2_host_send(0xED);
if (ps2_host_recv() != 0xFA) return; // 0xFA
ps2_host_send(leds);
if (ps2_host_recv() != 0xFA) return; // 0xFA
}

@ -121,7 +121,7 @@ section at the end of this file).
* The value is in milliamperes. [It will be divided by two since USB * The value is in milliamperes. [It will be divided by two since USB
* communicates power requirements in units of 2 mA.] * communicates power requirements in units of 2 mA.]
*/ */
#define USB_CFG_IMPLEMENT_FN_WRITE 0 #define USB_CFG_IMPLEMENT_FN_WRITE 1
/* Set this to 1 if you want usbFunctionWrite() to be called for control-out /* Set this to 1 if you want usbFunctionWrite() to be called for control-out
* transfers. Set it to 0 if you don't need it and want to save a couple of * transfers. Set it to 0 if you don't need it and want to save a couple of
* bytes. * bytes.

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