Diff: main.cpp

Revision:

0:c988614df67a

Child:

1:2a347c40b1da

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp	Sun May 26 03:05:25 2013 +0000
@@ -0,0 +1,53 @@
+//
+//  5MHZ CLOCK OUT USING PWM1 PIN
+//  (ONLY LPC1768-mbed)
+//
+// Code and Comment is from
+// http://mbed.org/forum/mbed/topic/733/
+//
+// The LPC1768 clocks at 96MHz. 
+// 96e6 / 5e6 gives 19.2. 
+// The PWM block counts off of the system clock,
+// so your period counter would have to be either 19 or 20. 
+// 19 gives you a frequency of just over 5.052MHz, 
+// and 20 gives you 4.8 MHz. (Note, to get these frequencies, you have to bypass the library).
+//
+// Assuming that the 5.052andabit is good enough, 
+// here's how you get there. I'll use the library object as a starting point,
+// as it does a load of the pin connection setup,
+// enabling the block and all that for us.
+//
+// I'll just go and trample over some of its settings afterwards.
+// I'm going to use pin 21 of the mbed, which is PWM output 6.
+//
+// The PWM block has a match register 0, 
+// which is used as the period counter. 
+// Then, each of the PWM outputs (1 through to 6) have their own match counters,
+// which are used to determine when to change the output level.
+// We're using the block in a simple "output high, until match counter matched, then output low" mode.
+// Thus our value of MR0 is going to be 18 (we count from zero, match at 18),
+// and our value of MR6 (if using p21) is going to be 9.
+// These counters are derived from the system clock, through a peripheral clock divider and a prescaler.
+// So what we need to do is:
+//
+//    1)Reset the PWM block
+//    2)Set the peripheral clock divider to /1 (the library sets the prescaler to /1 for us)
+//    3)Load the match registers
+//    4)Tell the block to start at next match
+//    5)Re-start the PWM block
+
+#include "mbed.h"
+
+PwmOut fmclck(p21);
+
+int main() {        
+    LPC_PWM1->TCR = (1 << 1);               // 1)Reset counter, disable PWM
+    LPC_SC->PCLKSEL0 &= ~(0x3 << 12);  
+    LPC_SC->PCLKSEL0 |= (1 << 12);          // 2)Set peripheral clock divider to /1, i.e. system clock
+    LPC_PWM1->MR0 = 18;                     // 3)Match Register 0 is shared period counter for all PWM1
+    LPC_PWM1->MR6 = 9;                      // 3)Pin 21 is PWM output 6, so Match Register 6
+    LPC_PWM1->LER |= 1;                     // 4)Start updating at next period start
+    LPC_PWM1->TCR = (1 << 0) || (1 << 3);   // 5)Enable counter and PWM
+
+    while(1);
+}
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