Lpc2000

---- Original Message ----
From: "Andrew Berney" <amb@...>
To: <lpc2000@yahoogroups.com>
Sent: Tuesday, May 23, 2006 5:26 PM
Subject: [lpc2000] Timer as counter...

> Just a quick question to see if there's anyway of using the inbuilt
> timer functionality of a LPC2119 as a counter. Ie to count the number
> pulses (rising or falling edge) from an external source over a given
> time frame. Basically we have a pot that changes a source input
> frequency from 36khz to around 48khz and driving an IRQ for each
> pulse doesn't strike me as being a good idea. All I need to
> realistically know is roughly what frequency it's being driven at.

Being able to count rising and/or falling edges on the CAPx.x pins was added
to LPC213x and newer.  The timer hardware of the LPC2119 cannot.

Connecting your signal to an interrupt pin, and incrementing a counter in
the interrupt handler will take a few percent performance.

You may be able to misuse some other peripherals to get a lower interrupt
rate.  For example, configure SPI to slave mode, and connect your signal to
SCKx, and you'll get an interrupt every eighth clock.

Or maybe configure a UART to a suitably high baudrate and FIFO with trigger
level 14, and connect your signal to RxD.  The UART should detect every
falling edge as the start of a new incoming byte, and at every 14th edge,
you should get a FIFO full interrupt.  The baudrate should not be so high
that you get character timeout interrupts.

You can also connect your signal to both a CAPx.0 and a CAPx.1 pin, and
capture CR0 on rising edges and CR1 on falling edges.  You can then always
check the half-period time by looking at CR0 and CR1.  This requires no
interrupts at all.

Karl Olsen