71M6521DE Datasheet, PDF (14/101 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/71M6521FE
Energy Meter IC
DATASHEET
JANUARY 2008
Real-Time Monitor
The CE contains a Real-Time Monitor (RTM), which can be programmed through the UART to monitor four selectable CE
DRAM locations at full sample rate. The four monitored locations are serially output to the TMUXOUT pin via the digital output
multiplexer at the beginning of each CE code pass. The RTM can be enabled and disabled with RTM_EN. The RTM output is
clocked by CKTEST. Each RTM word is clocked out in 35 cycles and contains a leading flag bit. See the Functional
Description section for the RTM output format. RTM is low when not in use.
Pulse Generator
The chip contains two pulse generators that create low-jitter pulses at a rate set by either CE or MPU. The function is
distinguished by EXT_PULSE (a CE input variable in CE DRAM):
• If EXT_PULSE = 1, APULSEW*WRATE and APULSER*WRATE control the pulse rate (external pulse generation)
• If EXT_PULSE is 0, APULSEW is replaced with WSUM_X and APULSER is replaced with VARSUM_X (internal pulse
generation).
The I/O RAM bits DIO_PV and DIO_PW, as described in the Digital I/O section, can be programmed to route WPULSE to the
output pin DIO6 and VARPULSE to the output pin DIO7. Pulses can also be output on OPT_TX (see OPT_TXE[1:0] for
details).
During each CE code pass, the hardware stores exported sign bits in an 8-bit FIFO and outputs them at a specified interval.
This permits the CE code to calculate all of the pulse generator outputs at the beginning of its code pass and to rely on
hardware to spread them over the MUX frame. The FIFO is reset at the beginning of each MUX frame. PLS_INTERVAL controls
the delay to the first pulse update and the interval between subsequent updates. Its LSB is four CK_FIR cycles, or 4 * 203ns. If
PLS_INTERVAL is zero, the FIFO is deactivated and the pulse outputs are updated immediately. Thus, NINTERVAL is
4*PLS_INTERVAL.
For use with the standard CE code supplied by TERIDIAN, PLS_INTERVAL is set to a fixed value of 81. PLS_INTERVAL is
specified so that all of the pulse updates are output before the MUX frame completes.
On-chip hardware provides a maximum pulse width feature: PLS_MAXWIDTH[7:0] selects a maximum negative pulse width to
be ‘Nmax’ updates per multiplexer cycle according to the formula: Nmax = (2*PLS_MAXWIDTH+1). If PLS_MAXWIDTH = 255,
no width checking is performed.
Given that PLS_INTERVAL = 81, the maximum pulse width is determined by:
Maximum Pulse Width = (2 * PLS_MAXWIDTH +1) * 81*4*203ns = 65.9µs + PLS_MAXWIDTH * 131.5µs
If the pulse period corresponding to the pulse rate exceeds the desired pulse width, a square wave with 50% duty-cycle is
generated.
The CE pulse output polarity is programmable to be either positive or negative. Pulse polarity may be inverted with PLS_INV.
When this bit is set, the pulses are active high, rather than the more usual active low.
CE Functional Overview
The ADC processes one sample per channel per multiplexer cycle. Figure 4 shows the timing of the samples taken during one
multiplexer cycle.
The number of samples processed during one accumulation cycle is controlled by the I/O RAM registers PRE_SAMPS
(0x2001[7:6]) and SUM_CYCLES (0x2001[5:0]). The integration time for each energy output is
PRE_SAMPS * SUM_CYCLES / 2520.6, where 2520.6 is the sample rate [Hz]
For example, PRE_SAMPS = 42 and SUM_CYCLES = 50 will establish 2100 samples per accumulation cycle. PRE_SAMPS = 100
and SUM_CYCLES = 21 will result in the exact same accumulation cycle of 2100 samples or 833ms. After an accumulation
cycle is completed, the XFER_BUSY interrupt signals to the MPU that accumulated data are available.
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